cambridge igcse chemistry study and revision guide
TRANSCRIPT
Author’sdedicationToMartha,SarahandJoseph.Thankyouforeverything.
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®IGCSEistheregisteredtrademarkofCambridgeInternationalExaminations.Thequestions,exampleanswers,marksawardedand/orcommentsthatappearinthisbookwerewrittenbytheauthor.Inexamination,thewaymarkswouldbeawardedtoanswerslikethesemaybedifferent.
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Contents
Introduction
1Theparticulatenatureofmatter
2Elements,compoundsandexperimentaltechniques
3Atomicstructure,bondingandstructureofsolids
4Stoichiometry:chemicalcalculations
5Electricityandchemistry
6Chemicalenergetics
7Chemicalreactions
8Acids,basesandsalts
9ThePeriodicTable
10Metals
11Airandwater
12Sulfur
13Inorganiccarbonchemistry
14Organicchemistry1
15Organicchemistry2
16Experimentalchemistry
Answerstoexam-stylequestions
Index
Introduction
WelcometotheCambridgeIGCSE®ChemistryStudyandRevisionGuide.ThisbookhasbeenwrittentohelpyoureviseeverythingyouneedtoknowforyourChemistryexam.FollowingtheChemistrysyllabus,itcoversallthekeycontentaswellassamplequestionsandanswers,practicequestionsandexaminertipstohelpyoulearnhowtoanswerquestionsandtocheckyourunderstanding.
HowtousethisbookKeyobjectivesThekeyskillsandknowledgecoveredinthechapter.Youcanalsousethisasachecklisttotrackyourprogress.
KeytermsDefinitionsofkeytermsyouneedtoknowfromthesyllabus.
Sampleexam-stylequestionsExam-stylequestionsforyoutothinkabout.
Student’sanswers
Typicalstudentanswerstoseehowthequestionmighthavebeenanswered.
Examiner’scommentsFeedbackfromanexaminershowingwhatwasgood,andwhatcouldbeimproved.
Examiner’stipsAdvicetohelpyougivetheperfectanswer.
Commonerrors•Mistakesstudentsoftenmakeandhowtoavoidthem.
ExtendedContentfortheextendedsyllabusisshadedgreen.
Exam-stylequestionsPracticequestionsforyoutoanswersothatyoucanseewhatyouhavelearned.
HowtoreviseThisbookisnotintendedtogivedetailedinformationaboutthechaptersyouarerequiredtostudyfortheIGCSEChemistrycourse.Insteaditismeanttogiveconciseinformationconcerningthethingsthatyouarelikelytocomeacrossinyourexaminations.Youhaveprobablybeenusingamoredetailedtextbookoverthetwoyearsofyourcourse.Thisbookisintendedforuseoverthesixweeksjustbeforetheexaminations.Notwopeoplereviseinthesameway.Itwouldbefoolishtogiveprecise
instructionstoanyoneabouthowtheyshouldprepareforexaminations.However,Iintendtomakesomesuggestionsaboutthedifferentapproachesthatareavailable,sothatyoucanchoosethemethodsthataremostsuitableforyou.TheonlythingIwouldstronglyrecommendaboutrevisionisthatitshould
involvewritingaswellasreading.Thosewhoreadthroughnotesorbooksastheironlymeansofrevisingoftenfindthattheybecomedistractedandstartthinkingaboutotherthings.Writingthingsdownhelpsyoutofocusonwhatyouaretryingtolearn.Anotherwaytohelpyoulearnistohighlightkeywordsandphrasesthatyou
wishtodrawattentionto.Highlightingmakesyoufocusonthingsthatyoumayhavehadproblemswithuptonow.Afterhighlighting,youcouldrewritethehighlightedparts,leavingouttheless
importantparts.Itmayalsobeagoodideatoleaveoutthosepartsthatyoualreadyknow.Ifyouknowthattransitionelementsareallmetals,thereisnopointinwritingitdown,becauseyoudonotneedtoreviseit.Justfocusonthepartsthatyouarenotsofamiliarwith.Yournoteswillbemoreconciseandmorepersonalthantheinformationinthebook.Youmayprefertowritedowntheinformationinamoreeye-catchingform,suchasinadiagram.Theimportantthingisthatitispersonaltoyouandhelpsyourrevision.Whenyouhavemadenotesofthistype,trywritingthemout.Againjustfocus
ontheimportantkeywordsandphrases.Whenyoucanwritethemoutwithoutlookingatyournotes,youmaybeconfidentthatyouhavelearnedthisparticularchapter.Youcantestyourselfbyansweringthe‘Exam-stylequestions’sectionsofthebookaswellaslookingatpastpaperquestionsorquestionsyouhaveattemptedduringthecourse.Ifyoucomeacrossaproblem,alwaysgobacktoyournotesandothersourcesofinformation.Thereisnoneedtoattempttimedquestionsunderexamconditionsuntilmuchnearertheexam.
ExamtechniqueThemostimportantpiecesofadvicethatanyonecangiveabouthowtoapproachanexamarenotnew.However,itiswellworthrepeatingthem,becausethepointsmadebelowarestillmajorcausesofavoidablelossesofmarks.
•Writeclearly.Ifexaminerscannotreadwhatyouhavewritten,theycannotawardmarks,despitetheirbesteffortstodecipheryourwork.Inthistechnologicalage,whenhandwritingisnotusedasmuchasitwasinthepast,itisessentialthatyouranswerscanberead.
•Usecorrectspellings.TechnicaltermsusedinChemistrymustbewrittencorrectly.Incorrectspellingsdomorethancreateabadimpressionofacandidate’swork.Ifawordisincorrectlyspelt,itoftenchangesthewholemeaningoftheword.Anexampleisusingtheword‘alkane’insteadof‘alkene’.Thisisnotonlyaspellingmistake,butitreplacesonewordwithanentirelydifferentwordwhichhasanentirelydifferentmeaning.
•Readthequestionscarefully.Donotjustglanceataquestionandpickoutafewwords.Readthewholequestionandwhenyouhavereadit,readitagain.Aquestionmaylooklikeanotheroneyouhaveseenduringyourrevision,butifyoureaditmorecarefully,youmayrealisethattherearedifferenceswhichmeanthatadifferentapproachisrequiredtoanswerthequestion.
•Checkyouranswers.Whenyouhavefinishedeachquestion,readthroughittomakesureitmakessenseandthatitanswersthequestion.
•Donotspendtoolongonanyquestions.Ifyouspendtoolongonsomequestions,youmayfindyoudonothavethenecessaryamountoftimetoanswersomeoftheothers.Itisimportanttoanswerallthequestions.
Somecommonphrasesthatyouwillseeinquestions,andtheirmeanings,include:
•Definetheterm/whatismeantbythetermmeansgiveadefinitionofawordorphrasewhichonlyappliestothatwordorphrase.Forexample,‘Definethetermisotope’meansgiveastatementthattellssomeoneexactlywhatanisotopeis.Definitionsarefoundinthe‘Keyterms’sectionatthestartofeachchapter.
•Statemeansgiveabriefstatement.Noexplanationofthestatementisrequired.Forexample,‘Statethenameoftheacidthatisusedtomake
magnesiumsulfate’.•Explain/giveareasonorreasons.Thissometimesfollowsthecommandword‘state’,i.e.‘stateandexplain’or‘stateandgiveareason’.Thismeansyoushouldgiveapieceofinformationfollowedbyabriefexplanationofwhyyouchosethisinformation.
•Outlinemeansabriefdescriptionisrequired.•Predictmeansyouaremeanttomakeaprediction,notbasedonanyknowledgethatyouhaveremembered,butbymakingalogicalconnectionbetweenotherpiecesofinformationreferredtointhequestion.
•Deducealsomeansyouarenotrequiredtogiveananswerbasedonwhatyouhaveremembered,buttosuggestalogicalconnectionbasedoninformationinthequestion.
•Suggestmaymeanthereismorethanonepossibleanswer.ItmayalsomeanyouarerequiredtoapplyyourknowledgeofChemistrytoa‘novel’situation,e.g.anexperimentorareactionyouhavenotcomeacrossbefore.Insuchexamples,youwillbesuppliedwithsufficientinformationtomakeareasonablesuggestion.
•Calculate/determinemeanscarryoutacalculationbasedondatathatisprovided.
•Howwouldyou…?meansgiveabriefdescriptionofanexperimentthatyouwouldcarryout.Manyexamcandidatesanswerthistypeofquestionwithtoomuchtheoreticalinformationratherthanbriefexperimentaldetail.
1Theparticulatenatureofmatter
KeyobjectivesBytheendofthissection,youshouldknowthedifferentpropertiesofsolids,liquidsandgases•beabletodescribethestructureofsolids,liquidsandgasesintermsofparticleseparation,arrangementandtypesofmotion•knowwhatismeantbymelting,boiling,evaporation,freezing,condensationandsublimation•beabletodescribetheeffectoftemperatureonthemotionofgasparticles•haveanunderstandingofBrownianmotion•beabletodescribeandexplaindiffusion
•beabletoexplainchangesofstateintermsofthekinetictheory•beabletodescribeandexplainBrownianmotionintermsofrandommolecularbombardmentandstateevidenceforBrownianmotion•beabletodescribeandexplaindependenceofrateofdiffusiononrelativemolecularmass.
KeytermsMelting TheprocessthatoccurswhenasolidturnsintoaliquidMeltingpoint Thetemperatureatwhichasubstancemelts.Eachsubstancehasa
specificmeltingpointBoiling TheprocessthatoccurswhenaliquidturnsintoagasBoilingpoint Thetemperatureatwhichasubstanceboils.Eachsubstancehasa
specificboilingpointEvaporation Theprocessthatoccursatthesurfaceofaliquidasitturnsintoa
gas.Evaporationcanoccurattemperatureslowerthantheboilingpointofaliquid
Freezing TheprocessthatoccurswhenaliquidturnsintoasolidFreezingpoint
Thetemperatureatwhichasubstancefreezes.Thishasthesamevalueasthemeltingpoint
CondensationTheprocessthatoccurswhenagasturnsintoaliquidSublimation Theprocessthatoccurswhenasolidturnsintoagaswithoutfirst
turningintoaliquidBrownianmotion
Therandommotionofvisibleparticlescausedbybombardmentbymuchsmallerparticles
Diffusion Theprocessthatoccurswhenparticlesmovefromaregionofhighconcentrationtoaregionoflowconcentration
Solids,liquidsandgasesDifferencesbetweensolids,liquidsandgasesareshowninFigure1.1.
Examiner’stipWhenaskedtodrawdiagramsofthearrangementofparticlesinsolids,liquidsandgases,solidsandgasesareusuallydrawnquitewell,buttheparticlesinliquidsareusuallydrawntoofarapart.Inreality,themajorityofparticlesinaliquidaretouching.
Thedifferencesbetweenthepropertiesofsolids,liquidsandgases,alongwiththereasons(basedonkinetictheory)forthedifferences,areshowninTable1.1.
ChangesofstateFigure1.2summarisesthechangesinstatethatoccurbetweensolids,liquidsandgases.
Commonerror•Thereisoftenconfusionbetweenboilingandevaporation.Boilingonlytakesplaceattheboilingpointofaliquid,butevaporationoccursattemperaturesbelowtheboilingpoint.Puddlesofwaterevaporateonasunnyday.Thismeansthatthewaterturnsintowatervapourattemperatureswellbelowtheboilingpointofwater.Thewaterinthepuddlesdoesnotreach100°C!
HeatingandcoolingcurvesAheatingcurveshowsthechangesofstateoccurringwhenthetemperatureoficeisgraduallyincreased.Asimilar(butnotthesame)curveresultswhenagasiscooledgraduallyuntilitformsasolid.Thisisknownasacoolingcurve.
Theprocessbeginswithiceatatemperaturebelow0°C.Thetemperaturegraduallyincreasesuntilitreaches0°C,whichisthemeltingpointofice.Atthispointiceandwaterexisttogether.Thetemperaturedoesnotchangeuntilalltheicehaschangedintowaterwhichiswhythelineishorizontal.Asharpmeltingpoint(atonespecifictemperature)isanindicationthatanysolidispure.Thetemperaturethenbeginstoincreaseagainuntilitreaches100°Cwhichis
theboilingpointofwater.Thetemperaturedoesnotchangeuntilallthewaterhaschangedintowatervapourwhichiswhythelineishorizontalforasecondtime.Whenallthewaterhasboiled,thetemperaturebeginstoriseagainastheparticlesinthegaseousstategainmoreenergy.
KinetictheoryWhenheatenergyisgiventoasolid,theheatenergycausestheparticlestovibratefasterandfasteraboutafixedpositionuntiltheparticleshavesufficientenergyformeltingtooccur.Atthe
meltingpointtheenergygainedbytheparticlesissufficienttoovercometheattractionbetweenparticlesinthesolid.Theorderedarrangementofparticlesthenbreaksdownasthesolidturnsintoaliquid.Asthisisoccurring,thereisnofurtherincreaseintemperatureuntiltheorderedarrangementhascompletelybrokendownandallthesolidhasturnedintoaliquid.Theenergygiventotheparticlesthencausesthemtomovefasterfromplacetoplaceuntiltheyhavesufficientenergyforboilingtooccur.Attheboilingpointtheenergygainedbytheparticlesissufficienttocompletelyovercometheattractionbetweenthemintheliquidstate.Theparticlesthenmoveasfarawayfromeachotheraspossibleastheforcesofattractionbetweenthemarealmostcompletelyovercome.Againthereisnoincreaseintemperatureuntiltheliquidhasturnedcompletelyintoagas.Inthegaseousstate,thegasparticlesgainmoreandmoreenergyandmoveatincreasingspeeds.
BrownianmotionWhenRobertBrownusedamicroscopetoobservepollengrainsonthesurfaceofwaterin1827,henoticedthatthepollengrainsmovedinarandommanner.ThisrandommovementisknownasBrownianmotion.Thesamethingcanbeobservedifsmokeparticlesinairareobservedthroughamicroscope.
Brownianmotioniscausedbythelargerparticles(pollengrainsorsmokeparticles)beingbombardedbysmallerparticles(watermoleculesorairmolecules).Thesmallerparticlesmoveinstraightlinesuntiltheycollidewiththelargerparticles.Becausemoreofthesmallerparticlesmaycollideononesideofthelargerparticlesthantheother,themovementofthelargerparticlesisrandomandunpredictable.
DiffusionParticlesinsolidsdonotmovefromoneplacetoanother.However,particlesinliquidsmoveslowlyandparticlesingasesmovemuchfaster.Movementofparticlesfromaregionofhighconcentrationtoaregionoflow
concentrationisknownasdiffusion.Itcanbedemonstratedexperimentallyinliquidsandingases.
DiffusioninliquidsIfcrystalsofacolouredsolid,suchasnickel(II)sulfate,areplacedinaliquidsuchaswater,thecolourofthenickel(II)sulfatespreadsthroughouttheliquidinamatterofdays,producingasolutionwithauniformgreencolour.
DiffusioningasesIfbromineliquidisplacedinthebottomofagasjarwithanothergasjarontop,theliquidevaporatesandthebrowncolourofbrominegasfillsbothgasjarsafterashorttime.
Whengaseousmoleculesdiffuse,therateatwhichtheydiffuseisinverselyrelatedtotherelativemolecularmassofthegas.Therefore,moleculesingaseswithlowerrelativemolecularmasswilldiffusefasterthanmoleculesingaseswithhigherrelativemolecularmass.Thisisbecauselightermoleculesmovefasterthanheaviermolecules.
Sampleexam-stylequestions1Acompoundhasameltingpointof−30°Candaboilingpointof85°C.Whatisitsphysicalstateat25°C?Explainyouranswer.
Student’sanswer
1Liquid.Themeltingpointisbelow25°Candtheboilingpointisabove25°C.
Examiner’scommentsTherearetwocommonerrorsinaquestionofthistype.
•Somestudentsignorethenegativesignin−30°C,whichgivesthemtheimpressionthat−30°Cishigherthan25°C,whichmeanstheythinkthecompoundisasolid.
•Somestudentsknowthatthesubstanceisaliquid,butonlystatethatthemeltingpointisbelow25°Cwithoutmentioningtheboilingpoint.Suchcandidatesgetsomecreditbutnotmaximumcredit.
2WhentheapparatusshowninFigure1.7issetup,concentratedammoniareleasesammoniagas,NH3,andconcentratedhydrochloricacidreleaseshydrogenchloridegas,HCl.
Whenammoniagasreactswithhydrogenchloridegas,awhitesolidisproducedaccordingtotheequation:
aWhatisthenameofthewhitesolid?bNametheprocessbywhichthetwogasesmovethroughtheglasstube.cExplainwhythewhitesolidformsnearertheconcentratedhydrochloric
acidendoftheglasstuberatherthantheammoniasolutionend.
Student’sanswer
2aAmmoniumchloridebDiffusioncMoleculesofammoniaandhydrogenchloridediffusethroughtheglass
tube.Becauseammonia,NH3,hasalowerrelativemolecularmass(17)thanhydrogenchloride,HCl(36.5),ammoniamoleculesdiffusefasterthanhydrogenchloridemolecules.Therefore,thegasesmeetandreactnearerthehydrochloricacidend.
Examiner’scommentsaAmmoniumcompoundsareoftenmistakenlyreferredtoasammoniacompounds.Similarlyammoniaisoftenreferredtoasammonium.Studentsshouldmakesurethattheyareawareofthedifferencebetweenammonia,NH3,andtheammoniumion,NH4+,whichispartofallammoniumsaltssuchasammoniumchloride.
bThisisthecorrectanswer.cItiscommontoseestatementsaboutammoniamovingfasterthanhydrogenchloridebecauseammoniaislighterthanhydrogenchloride.Thiswouldgainverylittle(ifany)credit.Answersmustrefertoammoniaandhydrogenchloridemoleculesandalsostatethatammoniahasasmallerrelativemolecularmassthanhydrogenchloridewhichiswhyammoniamoleculesdiffusefaster.
Studentsshouldcalculaterelativemolecularmasses,usingrelativeatomicmassesinthePeriodicTable,iftheyarenotprovidedinthequestion.
Exam-stylequestions1Asubstancehasameltingpointof85°Candaboilingpointof180°C.Whatisthephysicalstateofthesubstanceat50°C?Explainyouranswer.
[Total:2marks]2UsethelettersA,B,CandDtoanswerthequestionsunderthetable.
WhichsubstanceoutofA,B,CandDisaasolid[1]
baliquid[1]
cagas[1]
dunlikelytorepresentarealsubstance?[1]
[Total:4marks]
2Elements,compoundsandexperimentaltechniques
KeyobjectivesBytheendofthissection,youshouldknowthedifferencebetweenelements,mixturesandcompounds•knowthatelementscanbeclassifiedasmetalsandnon-metals•knowandunderstandhowtousethefollowingmethodsofseparationandpurification
•chromatography(seealsoChapter15)•dissolving,filtrationandcrystallisation(seealsoChapter8)•(simple)distillation•fractionaldistillation(seealsoChapter14)
•beabletointerpretsimplechromatograms,includinguseoflocatingagents(whichdonotneedtobenamed)anduseofRfvalues(seeChapter15).
KeytermsElement Asubstancethatcannotbedecomposedintoanythingsimplerby
chemicalmeans.Itisasubstancemadeupofatomsallofwhichhavethesameatomicnumber(seealsoChapter3)
CompoundAsubstancewhichcontainstwoormoreelementschemicallycombinedinfixedproportionsbymass
Mixture Containstwoormoresubstances(elementsorcompounds)whichcanbepresentinvariableproportions
Substance Ageneraltermthatreferstoelements,mixturesandcompoundsSolution AliquidwhichcontainsasubstanceorsubstancesdissolvedinitSolvent ApureliquidSolute Thedissolvedsubstanceinasolution
ElementsTheperiodictableconsistsofelementsonly.Eachelementhasachemicalsymbol.Elementsareclassifiedasmetalsandnon-metalsasshowninTable2.1below.
Table2.1Classificationofelements
Property Metal Non-metalPhysicalstateatroomtemperature
Solid(exceptmercury)
Solid,liquid(bromineonly)orgas
Malleability Good Poor,usuallysoftorbrittleDuctility Good Poor,usuallysoftorbrittleAppearance Shiny(lustrous) UsuallydullMeltingpoint/boilingpoint Usuallyhigh UsuallylowDensity Usuallyhigh UsuallylowConductivity(electricalandthermal)
Good Poor(exceptgraphite)
Examiner’stipElementscannotbedecomposedintoanythingsimplerbychemicalmeans.Studentssometimesusethewordsmallerinsteadofsimpler,whichisanerror.Forexample,apieceofsulfurcanbebrokenwithahammerintoseveralsmallerpiecesofsulfur,butthisisnotbreakingitintoanythingsimpler.Theactofbreakingwithahammerisaphysicalprocessandnotachemicalprocess.Thussulfurisanelement.
CompoundsCompoundshaveachemicalformulawhichshowsthemtocontaintwoormoreelementswhicharechemicallycombined.Examplesofcompoundsare:
sodiumchloride,NaCl•carbondioxide,CO2copper(II)nitrate,Cu(NO3)2.
Acompound,iron(II)sulfide,hastheformulaFeS.Therelativeatomicmasses(Ar)ofiron(Fe)andsulfur(S)are56and32,respectively.Therefore,therelativemolecularmass(Mr)ofiron(II)sulfideis56+32=88.Thepercentagesofironandsulfuriniron(II)sulfideare:
Examiner’stipIfacompoundispresentinanaqueoussolution,theaqueoussolutionisamixturebecauseitcontainstwosubstanceswhicharenotchemicallycombined.Forexample,sodiumhydroxidesolution(alsoreferredtoasaqueoussodiumhydroxide)isamixture,notacompound.Itcontainssodiumhydroxideandwater,twosubstancesthatarenotchemicallycombined.
Thismeansthatallsamplesofiron(II)sulfidecontain63.6%ironand36.4%sulfurbymass.Thisiswhatismeantbythestatementthatcompoundscontainelements
chemicallycombinedinfixedproportionsbymass.
Examiner’stipStudentsmaystatethatcompoundscontaintwoormoreelements,butoftendonotmentionthattheelementsarechemicallycombined(whichmeansthattheelementsarejoinedbyionicorcovalentbonds).
MixturesMixturescontaintwoormoreelementsand/orcompoundsinvariableproportions.Mixturesdonothaveachemicalformula.Airisanexampleofamixture.Aircontainsnitrogenandoxygenwithsmaller
amountsofothergases,suchaswatervapour,carbondioxideandargon.Pollutedairmayalsocontainothergasessuchascarbonmonoxide,sulfurdioxideandnitrogendioxide.Airhasdifferentpercentagesofitsconstituentgasesindifferentplacesdueto,
forexample,theamountsofpollutantgaseswhicharelowerinthecountrysidethaninindustrialareas.However,thedifferentsamplesareallcalled‘air’,thusshowingthatthecompositionofamixturecanvary.Airdoesnothaveachemicalformulabecauseitcontainsseveralchemical
substancesasopposedtoonesubstance.
Examiner’stipManystudentsareundertheimpressionthatamixturecontainingtwosubstances,suchassaltandsand,mustcontainequalamountsofeachsubstance.Thisisnotthecase.Ifwehadamixtureofsaltandsandwhichcontainedequalamountsofeachsubstanceandweaddedmoresalttoit,itwouldstillbecalledamixtureofsaltandsand.Therefore,amixtureofsaltandsandcancontainmoresaltthansand,ormoresandthansaltorequalamountsofsaltandsand.Thisisdifferenttothecompositionofacompoundasshowninthecaseofiron(II)sulfideabove.
SeparationofmixturesChromatographyChromatographycanbeusedtoseparatethecomponentsofsolutionswhichcontainseveraldissolvedsubstances.Thesubstancesareoftencoloured,butmaybecolourless.Paperchromatographycanbeusedtoseparatethedyesinink.
•Aspotoftheinkisplacedonthechromatographypaper.•Thepaperisplacedinasuitablesolventinabeaker.Ifthesolventisvolatile(vaporiseseasily)itisnecessarytoputalidonthebeakertopreventthevapourfromescaping.
•Asthesolventrises,thedyesintheinkseparate.
Chromatographycanalsobeusedtoidentifythecomponentsofamixtureaswellasseparatethem.
•AmixtureofdyesisplacedonchromatographypaperinthepositionmarkedX.
•FourdyeswhoseidentitiesareknownareplacedinpositionsmarkedA,B,CandD,asshown(Figure2.2).Thesefourdyesarereferredtoasstandards.
•Chromatographyisthencarriedoutandthechromatographypaper(alsoknownasachromatogram)isremovedfromthebeakeranddried.
•ThepaperisthenlabelledtoshowwhatmixtureXcontains,asdescribedbelow.
Theresultsoftheexperimentshow:Xiscomposedofthreedyesbecausethemixturehasbeenseparatedintothree.ThethreedyesareA,BandD.WeknowthisbecausethethreedyesinmixtureXhavetravelledthesamedistancesasthethreestandardsA,BandDwhoseidentitiesareknown.WecanalsoconcludethatmixtureXdoesnotcontaindyeC,becausenoneofthecomponentsofXtravelledthesamedistanceasdyeC.
Chromatographycanalsobeusedtoidentifycolourlesssubstances.Theexperimentaltechniqueisthesame,butbecausethecomponentsofthemixturearecolourless,thespotsonthechromatographypaperareinvisible.Afterdrying,thepaperissprayedwithalocatingagentwhichreactswiththecomponentsofthemixturetoproducecolouredspots.InChapter15whichlooksattheseparationofaminoacidsbychromatography,thechromatographypaperissprayedwithninhydrinwhichisalocatingagentthatproducesbluecolouredspotswithaminoacids.Insteadofusingstandardsasdescribedintheaboveexperiment,components
ofamixturecanbeidentifiedbytheirRfvalues.Chromatographyiscarriedoutandafterthechromatographypaperisdried,thedistancethatthesolventhastravelledandthedistancethatthecomponentofthemixturehastravelledarebothmeasuredasshowninFigure2.4.
Examiner’stipWhenaskedwhatismeantbyRfvalue,studentsareadvisedtowritedowntheequationgivenhere,ratherthantryingtoexplainRfvalueintheformofasentence,whichismuchmoredifficult.Thecorrectequationwouldscorealltheavailablemarks.
Thisillustratesthefactthatanswersneednotalwaysbeexpressedinwords,sentencesandparagraphsalonetogetfullcredit.Diagrams,equations,sketchgraphsandformulaeareoftenmuchmoreappropriatethansentences.
WhentheRfvalueiscalculated,thecomponentofthemixturecanbeidentified
bycomparisonwithRfvaluesinadatabook.Rfvaluescanbedeterminedforallthecomponentsofthemixture.
Dissolving,filtrationandcrystallisationDissolving,filtrationandcrystallisationaremethodsusedtoseparateamixtureoftwosolids,oneofwhichissolubleinagivensolventandtheotherofwhichisinsoluble.Thismethodcanbeusedtoseparateamixtureofcommonsaltandsandand
producepuresamplesofbothsolids.
•Ifthemixtureisnotpowdereditshouldbegroundintoapowderusingamortarandpestle.Thepowdershouldbeaddedtowaterinabeaker.Thecommonsaltdissolvesandthesandremainsundissolved.
•Themixtureisthentransferredtothefiltrationapparatus.Thesand(residue)remainsinthefilterpaperandthesaltsolution(filtrate)passesthroughintotheconicalflask.Thisprocessiscalledfiltration.
•Toobtainpuresand,distilledwatershouldbepassedthroughthefilterpaper(thisisknownaswashingtheresidue)andthenthefilterpapershouldberemovedanddriedinalowovenoronawarmwindowsill.
Thefinaltwostagesareknownascrystallisation:Toobtainsaltcrystals,thesaltsolutionshouldthenbeheatedinanevaporatingdishuntilabouthalfofthewaterhasbeenremoved(alternatively,whencrystalsformonaglassrodplacedinthehotsolutionandwithdrawn,itistimetostopheating).Thehotsaturatedsaltsolutionshouldthenbeallowedtocooldownslowly.Crystalsofsaltshouldthenform.Ifthereisanyliquidleft,itshouldthenbeseparatedbyfiltration.Thesaltcrystalsshouldthenbewashedwithasmallamountofcolddistilledwaterandthendriedinalowovenoronawarmwindowsill.
CommonerrorsFiltration(oftenspeltwronglyasfilteration)canalsobecalledfiltering.Thereareothercommonerrorswhendescribingtheprocess:
Thewordsresidueandfiltrateareoftenusedthewrongwayround.Filtrateisoftenusedasanincorrectalternativetofilteredasin‘hefiltratedthesolution’.Thewordfiltrateddoesnotexist!Thefiltrateshouldnotbeheateduntilallthewaterevaporatestodryness.Thisdoesnotleadtotheproductionofgoodcrystals.Inaddition,somecrystalscontainwaterofcrystallisationwhichwouldbedrivenoffbytoomuchheat(seeChapter8).Ifthecrystalsaredriedwithtoomuchwatertheywilldissolve,whichdefeatsthepurpose.Thewatershouldideallybeicecoldtominimisetheamountthatdissolves.
(Simple)distillation(Simple)distillationisamethodofseparatingapureliquidfromasolution.
Theflaskisheated.Thewaterinthesodiumchloridesolutionevaporatesandwatervapour/steamenterstheLiebigcondenser,whereitcondensesaswater.ThewaterdripsoutoftheendoftheLiebigcondenserandcollectsinthebeaker.Thewaterispureandcanbecalleddistilledwater.Sodiumchloridedoesnotvaporiseorevenmeltbecauseithasaveryhighmeltingpoint,andthereforeitremainsinthedistillationflask.
FractionaldistillationFractionaldistillationisamethodofseparatingtwo(ormore)miscibleliquidswithdifferentboilingpoints.Itcanbecarriedoutinthelaboratoryoronanindustrialscaleasinthefractionaldistillationofliquidair(seeChapter11)orfractionaldistillationofpetroleum(seeChapter14).Inthelaboratoryethanolandwatercanbeseparatedbyfractionaldistillation
usingtheapparatusshowninFigure2.7.
Ethanolhasaboilingpointof78°Candwaterhasaboilingpointof100°C.
Ethanolhasaboilingpointof78°Candwaterhasaboilingpointof100°C.Theflaskisheatedandethanolvapourentersthefractionatingcolumn.
However,somewateralsoevaporates(belowitsboilingpoint)andentersthefractionatingcolumnaswatervapour/steam.Thewatervapour/steamcondensesinthefractionatingcolumnanddripsbackdownintothedistillationflask.Whenthetemperaturereaches78°C,theethanolvapourreachesthetopofthefractionatingcolumnandenterstheLiebigcondenserwhereitcondenses.Finally,liquidethanolcollectsasthedistillateandallthewaterremainsinthedistillationflask.
Summary:MethodsofseparationofmixturesMethodofseparation Exampleofmixture
thatisseparatedPropertythatthemethoddependson
Paperchromatography Dyesinink Adsorptionbypaper/solubilityinsolvent
Dissolving,filtrationandcrystallisation
Sandandsalt Solubility
(Simple)distillation Sodiumchloridesolution BoilingpointFractionaldistillation Ethanolandwater Boilingpoint
Sampleexam-stylequestion1Statewhetherthefollowingareelements,mixturesorcompounds.asilverbbronzecseawaterdwaterebauxitefaluminiumoxide
Student’sanswer
1aelementbmixturecmixturedcompoundemixturefcompound
Examiner’scommentaMetalscanbeelements,butalloysaremixturesofmetals.Silverisanelement.Itistheelementwithatomicnumber47inthePeriodicTable.IfyouarenotsureifasubstanceisanelementyoushouldknowthatthePeriodicTableonlycontainselements.
bBronzeisanalloy(seeChapter10)and,assuch,itisamixtureofmetals.Bronzecontainscopper,tinandothermetalsinvariableproportions.
cSeawateriswater(whichisacompound)containingmanysubstances,invariableproportions,dissolvedinit.
dWaterhastheformulaH2O.Anysubstancewithaformulathatshowsmorethanoneelementisacompound.Althoughwaterisfoundinmanyformssuchastapwater,seawateranddistilledwater,thetermwaterreferstothepurecompound.
eBauxiteisametallicorefromwhichaluminiumisextracted(seeChapter5).Thewordorereferstoanimpuresubstance.Metallicoresaremixtures.
fAluminiumoxidehastheformulaAl2O3.Bauxitecontainsthecompoundaluminiumoxidewithimpurities.
Exam-stylequestions1Statethenameoftheprocess(es)thatyouwouldusetoobtainasugarcrystalsfromamixtureofsugarandsandbpurewaterfroman
aqueoussolutionofcopper(II)sulfatecliquidoctane(boilingpoint126°C)fromamixtureofliquidoctaneandliquiddecane(boilingpoint174°C)dpuresilverchloridefromtheprecipitateformedwhenaqueoussilvernitrateisaddedtodilutehydrochloricacid.
Insomecases,onlyoneprocessisrequired,butothersmayrequiremorethanone.
[Total:8marks]2Astudentwastoldtomakepurecrystalsofcopper(II)sulfatefromanaqueoussolutionofcopper(II)sulfate.Describehowthestudentshouldcarrythisout.
[4marks]
[4marks]3Astudentisgivenamixtureoftwoaminoacids.Theaminoacidsarebothcolourlesssolidsthataresolubleinwater.Givefullexperimentaldetailsofhowyouwouldseparateandidentifytheaminoacidspresentinthemixtureusingpaperchromatography.Youareprovidedwithallthenecessaryapparatusandasuitablelocatingagent.
[5marks]
3Atomicstructure,bondingandstructureofsolids
KeyobjectivesBytheendofthissection,youshould
•knowtherelativechargesandapproximaterelativemassesofprotons,neutronsandelectrons
•knowthedefinitionofprotonnumber(atomicnumber)andnucleonnumber(massnumber)
•knowthatprotonnumberisthebasisofthePeriodicTable•knowthedefinitionofisotopesandthatisotopesareradioactiveandnon-radioactive
•knowonemedicalandoneindustrialuseofradioactiveisotopes•knowthebuildupofelectronsinshellsforthefirst20elementsinthePeriodicTable
•knowthedifferencebetweenmetalsandnon-metals•knowthationsareformedwhenatomsloseandgainelectrons•beabletodescribetheformationofionicbondsbetweenelementsfromGroupsIandVII
•beabletodescribetheformationofsinglecovalentbondsinH2,Cl2,H2O,CH4,NH3,HCl
•beabletodescribethedifferenceinvolatility,solubilityandelectricalconductivitybetweenioniccompoundsandcovalentsubstanceswithbothgiantstructuresandsimplemolecularstructures
•knowthegiantcovalentstructuresofgraphiteanddiamond•relatethestructuresofgraphiteanddiamondtotheiruses
•knowwhyisotopeshavethesamechemicalproperties
•beabletodescribetheformationofionicbondsbetweenothermetallicandnon-metallicelements
•beabletodeterminetheformulaeofioniccompoundsfromthechargesontheionspresent
•knowthatagiantioniclatticeisaregulararrangementofpositiveandnegativeions
•beabletodescribetheformationofmorecomplexcovalentmolecules,suchasN2,C2H4,CH3OHandCO2
•beabletoexplainthedifferencesinmeltingpointandboilingpointbetweenioniccompoundsandcovalentsubstanceswithbothgiantstructuresandsimplemolecularstructuresintermsofattractiveforcesbetweenparticles
•knowthestructureofsilicon(IV)oxide(silicondioxide)•beabletodescribethesimilarityinpropertiesbetweendiamondandsilicon(IV)oxide(silicondioxide)relatedtotheirstructures
•beabletodescribemetallicbondingasaforceofattractionbetweenpositiveionsandamobileseaofelectrons
•knowhowthestructureofmetalscanbeusedtoexplainmalleabilityandconductionofelectricity.
KeytermsProtonnumber(atomicnumber)
Thenumberofprotonsinoneatomofanelement
Nucleonnumber(massnumber)
Thesumofthenumberofprotonsandneutronsinoneatomofanelement
Isotopes Atomsofthesameelementcontainingthesamenumberofprotonsbutdifferentnumbersofneutrons,orAtomsofthesameelementwiththesameprotonnumber(atomicnumber)butdifferentnucleonnumber(massnumber)
Lattice Aregulararrangementofparticlespresentinasolid.Theparticles(atoms,moleculesorions)arearrangedinarepeatedpattern
AtomicstructureAtomsaremadefromsmallerparticlescalledprotons,neutronsandelectrons.
Table3.1Thepropertiesofprotons,neutronsandelectrons
Particle Relativemass/atomicmassunits RelativechargeProton 1 +1Neutron 1 0Electron 1/1837(negligible) −1
Examiner’stipMakesureyoulearntheinformationinTable3.1.Youneedtoknowthedifferencesbetweenrelativemassandrelativechargeofaproton,neutronandelectron.
Theprotonsandneutronsexistinthecentreoftheatominadenseregioncalledthenucleus.Theelectronsmovearoundthenucleusandexistinelectronshellsatincreasingdistancesfromthenucleus.
Examiner’stipInsomebooks,thetwonumbersmaybereversed.Itisagoodideatorememberthatthenucleonnumberisalwayshigherthantheprotonnumber(withtheexceptionofhydrogen,inwhichcasebothnumbersare1inthemostabundantisotope).
AtomsareoftenrepresentedasshowninFigure3.1.
Theprotonnumberisthenumberofprotonsinoneatomoftheelement.Becauseatomsdonothaveacharge,thenumberofprotonsinanatomisalwaysequaltothenumberofelectrons.
equaltothenumberofelectrons.Thenucleonnumberisthenumberofneutronsandprotonsaddedtogetherin
oneatomofanelement.Therefore
IntheexamplegiveninFigure3.1:
Numberofprotons=protonnumber
Numberofelectrons=numberofprotons
Numberofneutrons=(nucleonnumber−protonnumber)
15 15 31−15=16
Isotopesareatomsofthesameelementcontainingthesamenumberofprotonsbutdifferentnumbersofneutrons.ExamplesofisotopesofargonareshowninTable3.2.
Someisotopesareradioactiveandsomearenon-radioactive.Radioactiveisotopesdecay(whichmeansgiveoffradiation)andusuallychangeintootherelements.Radioactiveisotopesarealsoknownasradioisotopes.Radioactiveisotopes/radioisotopescanbeused
•inmedicine(e.g.cobalt-60isusedinradiotherapytreatment)•inindustry(e.g.uranium-235isusedasasourceofpowerinnuclearreactors).
Commonerror•Manystudentsthinkofisotopesasreferringonlytoradioactiveisotopes,butsomearealsonon-radioactive.
ThearrangementofelectronsinatomsElectronsarearrangedinelectronshellsatincreasingdistancesfromthenucleus.Theseshellscanholduptoamaximumnumberofelectrons,asshowninTable3.3.
Table3.3Maximumnumberofelectronspershellnumber
Shellnumber Maximumnumberofelectrons1 22 83 8*
*Shell3can,infact,holdupto18electrons,butthisdoesnotneedtobeconsideredatthislevelandonlybecomesrelevantinhigher-levelcourses.
SomeexamplesofarrangementofelectronsinshellsareshownTable3.4.
Table3.4Arrangementofelectronsinshells
Element Numberofelectronsinoneatom
Arrangementofelectronsinshells
Helium,He 2 2Carbon,C 6 2,4Phosphorus,P
15 2,8,5
Potassium,K
19 2,8,8,1
Studentsareexpectedtobeabletowritedownanddrawtheelectronarrangementofthefirst20elementsinthePeriodicTable.
Isotopesofthesameelementallhavethesamenumberofelectrons,andthereforeallhavethesamenumberofelectronsintheiroutershells.Thismeansthatisotopesofthesameelementallhavethesamechemicalproperties.Thechemicalpropertiesofelementsdependonthenumberofelectronsin
theoutershelloftheiratoms.
theoutershelloftheiratoms.
ThePeriodicTableElementsinthePeriodicTablearearrangedinorderofincreasingprotonnumber.Thismeansthataswemovefromoneelementtothenextelement,theatomshaveoneextraprotoninthenucleusandoneextraelectron.Theextraelectrongoesintotheoutershelluntiltheoutershellisfull.Thenextshellthenbeginstofillup.Elementsinthesamegroupallhavethesamenumberofelectronsintheouter
shelloftheiratoms.Thisappliesbeyondthefirst20elements.Examplesare:
•AllGroup(I)elementshave1electronintheoutershell.•AllGroup(II)elementshave2electronsintheoutershell.•AllGroup(VII)elementshave7electronsintheoutershell.•AllGroup(0)elementshaveafulloutershell.•Mostmetallicelementshave1,2or3electronsintheiroutershell.•Mostnon-metallicelementshave5,6or7electronsintheiroutershell(orafulloutershellinthecaseofNoblegases).
IonicbondingIonicbondingoccursincompoundscontainingmetallicelementscombinedwithnon-metallicelements.Metalatoms(with1,2or3electronsintheiroutershells)loseanelectronor
electronsinordertoachieveafulloutershellandformpositiveions(cations).Non-metalatoms(with5,6or7electronsintheiroutershells)gainan
electronorelectronsinordertoachieveafulloutershellandformnegativeions(anions).Anexampleoccursinsodiumchloride.Sodiumatomscontain11protonsand11electronsandchlorineatomscontain
17protonsand17electrons.Becausebothcontainequalnumbersofprotonsandelectronsbothatomsareuncharged.Anelectronmovesfromtheoutershellofasodiumatomtotheoutershellof
achlorineatomsothatbothatomsachieveafulloutershell.
Afterthetransferofelectrons,sodiumformsapositivesodiumionandchlorineformsanegativechlorideion.
Thesodiumionstillhas11protonsbutonly10electrons,thereforeithas1+chargeandiswrittenNa+.Thechlorideionstillhas17protonsbutnowhas18electrons;thereforeithas1−chargeandiswrittenCl−.Insodiumchloride,theratioofsodiumionstochlorideionsis1:1andthus
theformulaofsodiumchlorideisNaCl.InallexamplesinwhichGroup(I)elementscombinewithGroup(VII)elements,theratioofionsisalways1:1.Otherexamplesoccurinwhichatomsdonotcombineintheratio1:1.This
applieswhenthenumberofelectronslostbyonemetalatomisnotequaltothenumberofelectronsgainedbyonenon-metallicatom.
Examiner’stipRememberthatatomshaveequalnumbersofprotonsandelectrons,andarethereforeuncharged.
Positiveions(cations)havemoreprotonsthanelectronsandarethereforepositivelycharged.
Negativeions(anions)havemoreelectronsthanprotonsandarethereforenegativelycharged.
negativelycharged.
Becausetheratioofmagnesiumionstofluorideionsis1:2,theformulaofmagnesiumfluorideisMgF2.
TheformulaeofioniccompoundsTheformulaeofioniccompoundscanbededucedfromknowledgeofthe
chargesontheions.ExamplesofcommonionsareshowninTable3.5.
Boldtypedenotespolyatomicions.Theseareionswhichhavemorethanonecapitalletterintheformula(seerule4below).
Examiner’stipTable3.6showsthechargesonionsindifferentgroupsinthePeriodicTable.
Table3.6
Group ChargeonionI 1+II 2+III 3+V 3−VI 2−VII 1−
Inothercases,e.g.thetransitionelements,itisnotpossibletousethePeriodicTabletodeducethechargesonions.Insuchcases,thechargesmustbe
learnedbyheart.
ExamplesofhowtodetermineformulaeofioniccompoundsThemostimportantthingtoknowisthatallcompoundshavenooverallcharge;thereforeinthecaseofioniccompoundsthenumberofpositivechargesisequaltothenumberofnegativecharges.Toworkouttheformulaofacompoundyoushould:
1Writedowntheformulaeofthepositiveandthenegativeions.2Countthenumberofpositivechargesandthenumberofnegativecharges.3Ifthechargesarenotequal,addmorepositiveions,morenegativeionsorbothuntilthechargesareequal.
4Ifmorethanoneofapolyatomicionisrequired,thewholeformulaoftheionmustgoinabracketandthenumberofionsgoesoutsidethebracketasasubscript,e.g.(NO3)2.
Writedowntheformulaeofthefollowingcompounds.
CommonerrorsSomecommonincorrectanswersusingtheformulaofiron(III)sulfateasanexampleare
•FeSO4:Thenumberofchargeshasnotbeenmadeequal.(Thisisthecorrectformulaofiron(II)sulfate.)
•Fe2(SO)3:The4isleftoutoftheformulaofsulfate.•(Fe)2(SO4)3:AbracketisnotrequiredaroundFeasitonlyhasonecapitalletter.
•Fe2(SO)4:The4isleftoutoftheformulaofsulfateandplacedincorrectlyoutsidethebracket.
Examiner’stipYoushouldbeabletowriteformulaeofcompoundscontainingallpossiblecombinationsofpositiveandnegativeionsinTable3.5.
CovalentbondingCovalentbondingoccursinelementsandcompoundscontainingnon-metallicelementsonly.Covalentbondsareformedwhenpairsofelectronsareshared.Asharedpair
ofelectronsisknownasasingle(covalent)bond.
Doublebonds(twosharedpairsofelectrons)andtriplebonds(threesharedpairsofelectrons)alsoexist.
Atomswhichformacovalentbondjointogethertoformunchargedmolecules.Alltheatomsinvolvedachieveafulloutershellofelectrons.Examplesofcovalentmoleculescontainingsinglebondsonlyareshownin
Figure3.5.Onlytheouterelectronshellsareshown.
ExamplesofcovalentmoleculescontainingdoubleandtriplebondsareshowninFigure3.6.Onlytheouterelectronshellsareshown.
StructureofsolidsSolidshavefourdifferenttypesofstructuresasshownbelow.
GiantionicstructureSodiumchlorideisanexampleofagiantionicstructure.Itisheldtogetherbystrongforcesofattractionbetweenoppositelychargedsodiumionsandchlorideions(calledionicbonds)whicharepresentinagiantioniclattice.
Giantcovalentstructure(macromolecularstructure)Diamondisanexampleofagiantcovalentstructure.Itisheldtogetherbystrongcovalentbondsbetweencarbonatoms.Graphiteisanotherexampleofsubstancewithagiantcovalentstructure,
althoughithasmanydifferenceswhencomparedtodiamond(seepages23–24).
Silicon(IV)oxide,SiO2,isanotherexampleofagiantcovalentstructure.Ithassimilarpropertiestodiamondduetoasimilarstructure.
GiantmetallicstructuresAllmetallicelementshavegiantmetallicstructures.Theycontainpositiveionssurroundedbyamobileseaofelectrons.Metalsareheldtogetherbythestrongforcesofattractionbetweenpositiveionsandthemobileseaofelectronsknownasmetallicbonds.
Examiner’stipInexamquestionswhichaskforthemeaningofmetallicbonding,students
Inexamquestionswhichaskforthemeaningofmetallicbonding,studentsusuallydescribethegiantmetallicstructurebutdonotmentionthestrongforcesofattractionbetweenpositiveionsandthemobileseaofelectrons,knownasmetallicbonds.
SimplemolecularstructuresIodineisanexampleofasubstancewithasimplemolecularstructure.Ithasstrongcovalentbondsbetweentheatomswithinthemolecules(intramolecular)butweakintermolecularforcesofattractionbetweenthemolecules.Thepropertiesofdifferenttypesofsolid,relatedtotheirstructures,areshown
inTable3.7.
Commonerrors•Inexamquestionswhichaskwhysubstanceswithsimplemolecularstructureshavelowmeltingpointsandboilingpoints,itisverycommonlysaidthatthisisbecausecovalentbondsareweak.Thisisabaderror.Allcovalentbondsarestrongbonds.
•Thecorrectansweristhatintermolecularforcesareweakwhichiswhysubstanceswithsimplemolecularsubstancesareeithersolidswithlowmeltingpointssuchasiodine,liquidssuchaswaterorgasessuchascarbondioxide.
DiamondandgraphiteBothdiamondandgraphitehavegiantcovalent(macromolecular)structures,butbecausetherearedifferencesintheirstructureandbonding,theseleadtodifferencesinpropertiesanduses.
Table3.8Differencesinstructureandbondingbetweendiamondandgraphite
Diamond GraphiteNumberofcarbonatomscovalentlybondedtoeachcarbonatom
4 3
atomArrangementofatoms Tetrahedrally Inlayers(madeofrings
containing6carbonatoms)Bonding Allcovalent Covalentbetweenatomswithin
layersWeakvanderWaalsforcesbetweenlayers
Mobileelectrons None.Alloutershellelectronsusedinbonding
Oneelectronfromeachatomexistsinthespacesinbetweenthelayersasmobileelectrons
Hardness Hardbecauseallbondsarestronganddirectional
SoftbecauseweakvanderWaalsforcesbetweenlayersallowlayerstoslideovereachother
Conductionofelectricity
Non-conductorbecausetherearenomobileelectrons
Goodconductorsduetomobileelectronsbetweenlayers
Use Incuttingtoolsduetohighstrength
AsalubricantbecauselayerscanslideAsaconductorinmotors
Silicon(IV)oxide(silicondioxide)Silicon(IV)oxide(silicondioxide)hasagiantcovalent(macromolecular)structure.Eachsiliconatomiscovalentlybondedtofouroxygenatoms.Thebondsare
directedtetrahedrally.Eachoxygenatomiscovalentlybondedtotwosiliconatoms.Allthebondsinsilicon(IV)oxidearestrongcovalentbonds.Thereareno
mobileelectronspresent.Becauseofitsstructureandbonding,silicon(IV)oxideisstrong,hard,hasahighmeltingandboilingpointandisanon-conductorofelectricity.Thesepropertiesarelikethoseofdiamondwhichhasaverysimilarstructureandthesamebonding.
Exam-stylequestions1Completethefollowingtable
[Total:5marks]2Drawdotandcrossdiagramsshowingthearrangementofoutershellelectronsinthefollowingcovalentmolecules.
aoxygen,O2
bmethanol,
chydrogencyanide,
dphosphine,
ecarbonylchloride,
[Total:5marks]3Deducetheformulaeofthefollowingioniccompounds.amagnesiumhydroxidebcalciumchloridecammoniumphosphatedlithiumsulfideeleadnitratefcalciumcarbonategaluminiumnitratehpotassiumsulfiteizincsulfatejammoniumsulfate
[Total:10marks]4Usethetabletoanswerthequestionsthatfollow.Considerroomtemperaturetobe25°C.
aWhichsubstanceorsubstancesaresolidatroomtemperature?[1mark]
bWhichsubstanceorsubstancesareliquidatroomtemperature?[1mark]
[1mark]cWhichsubstanceorsubstancesaregaseousatroomtemperature?
[1mark]dWhichsubstancecouldhaveagiantmetallicstructure?
[1mark]eWhichsubstancehasagiantionicstructure?
[1mark]fWhichsubstancehasagiantmolecularstructure?
[1mark][Total:6marks]
4Stoichiometry:chemicalcalculations
KeyobjectivesBytheendofthissection,youshould
•beabletoconstructwordequationsandsimplebalancedequations•beabletodefinerelativeatomicmass,Ar
•beabletodefineandcalculaterelativemolecularmass,Mr
•beabletoconstructequationswithstatesymbols,includingionicequations•beabletodeduceabalancedequationforachemicalreactiongivenrelevantinformation•beabletodefinethemoleandtheAvogadroconstantandbeabletousethemolargasvolume(takenas24dm−3atroomtemperatureandpressure)•beabletocalculatereactingmasses,volumesofgasesandsolutionsandconcentrationsofsolutions•defineandcalculateempiricalformulaeandmolecularformulae•beabletousetheideaoflimitingreactants•calculatepercentageyieldandpercentagepurity.
KeytermsStoichiometry Thecalculationoftherelativequantitiesofreactantsand
productsinachemicalreactionRelativeatomicmass
Theaveragemassofoneatomofanelementonascalewhereone12Catomhas12unitsofmassexactly,Ar
Relativemolecularmass
Thesumoftherelativeatomicmasses,Mr
Empiricalformula
Thesmallestwholenumberratiooftheatomsofeachelementinacompound
Molecularformula
Thenumberofatomsofeachelementinonemoleculeofasubstance
Mole Thesamenumberofparticlesasthereareatomsin12gofthecarbon-12isotope
Avogadroconstant
Thenumberofparticlesinonemoleofasubstance.Itisequalto6.02×1023particles
StoichiometryStoichiometrymeansthecalculationoftherelativequantitiesofreactantsandproductsinachemicalreaction.
WordequationsWordequationsgivethenamesofthereactantsandproductswhichtakepartinachemicalreaction.Whenhydrogenburnsinoxygentoformwatertheword
equationis:
SymbolequationsSymbolequationsgivethecorrectformulaeofthereactantsandproductsinareaction.Symbolequationsarebalancedwhenthenumberofatomsofeachelementisthesameonbothsidesoftheequation.
Stepsforwritingbalancedequations1Writedownthewordequation(thiscanbeomittedwithexperience).2Writedownthecorrectformulaeofreactantsandproducts.3Countthenumberofatomsofeachelementonbothsides.4Ifthenumberofatomsofeachelementonbothsidesisnotthesame,putnumbersinfrontoftheformulaesothatthenumberofatomsofeachelementonbothsidesisthesame.
5Putstatesymbolsaftertheformulae,i.e.(s)=solid,(l)=liquid,(g)=gas,(aq)=aqueoussolution.Thiscanbedoneaftersteps2or3ifpreferred.
WorkedExample
Becausethenumberofatomsofoxygenisnotthesameonbothsides,thefirststepistoput2infrontofH2O.Thismultiplieseverythingthatcomesafterit.
Inbalancingtheoxygen,wehaveunbalancedthehydrogen.Thereforeweneedtoputa2infrontofH2.Theequationisthenbalanced.Statesymbolscanbeinserted.
Commonerrors•Studentsoftenuseincorrectformulae,e.g.HinsteadofH2orOinsteadofO2,orchangeformulaesuchaschangingH2OintoH2O2.Thiswouldmakethenumberofatomsofeachelementthesameonbothsides,butH2O2isnotthecorrectformulaforwater(infact,itistheformulaforhydrogenperoxide).Theonlywaytobalanceequationsistoputnumbersinfrontoftheformulae.
WorkedExample
Thealuminiumisbalanced.Tobalancethechlorine,weput2infrontofAlCl3and3infrontofCl2.
Thealuminiumisnowunbalanced,thereforewemustputa2infrontofaluminium.
IonicequationsStartingfromabalancedequationwithstatesymbols,anionicequationcanbewrittenusingthefollowingsteps.1Anythingwith(aq)asastatesymbolshouldbewrittenasionsifitis
aadiluteacid,e.g.HCl(aq)iswrittenasH+(aq)andCl–(aq)
bametalliccompound,e.g.CuSO4(aq)iswrittenCu2+(aq)andSO42–(aq)
canammoniumsalt,e.g.(NH4)2SO4(aq)iswritten2NH4+(aq)andSO42–(aq).
2Numbersinfrontofformulaeinequationsmeanthateverythingafterthenumberismultiplied,e.g.2HNO3(aq)iswrittenas2H+(aq)and2NO3−(aq).
3Theformulaeofanysubstancewithstatesymbols(s),(l),(g)arenotwrittenasions,thusarenotchangedinanionicequation.
4Anyionswhicharethesameonbothsides,knownasspectatorions,arecancelled.
Sampleexam-stylequestionWritethefollowingbalancedequationasanionicequation.
Student’sanswer
SO42-(aq)arethesameonbothsidesandarecancelledout.
ChemicalcalculationsCalculatingrelativemolecularmass,MrUsethefollowingrelativeatomicmasses,Ar,tocalculatetherelativemolecularmassesofthecompoundsshown.
H=1C=12N=14O=16Al=27S=32Pb=207
•CO2=12+(16×2)=44•N2O=(14×2)+16=44•C4H10=(12×4)+(1×10)=58•Pb(NO3)2.Therecommendedmethodistomultiplyoutthebrackets,i.e.
PbN2O6=207+(14×2)+(16×6)=331•Al2(SO4)3=Al2S3O12=(27×2)+(32×3)+(16×12)=342
Howtocalculatemoles
Molesfrommasses
Massof1molemeansrelativeatomicmassofanysubstancewhichonlycontainsatoms.Relativemolecularmassorrelativeformulamassshouldbeusedforallothersubstances.Rearrangingformass:
Rearrangingformassofonemole:
Pleasenotethatthemassmustbeingrams(g).Ifthemassisgivenin
Pleasenotethatthemassmustbeingrams(g).Ifthemassisgiveninkilograms(kg),itmustbemultipliedby1000orifitisgivenintonnes,itmustbemultipliedby1000000toconvertitintograms.
MolesfromgasvolumesThevolumeofonemoleofanygasis24dm3atroomtemperatureandpressure.
Rearrangingforvolume:
Thevolumeofthegasandthevolumeofonemoleofgasmustbeinthesameunitswhenusingtheseequations.
Molesfromvolumesandconcentrationofsolutions
Correctunitsareveryimportantintheseequations.Becausesolutionsaremeasuredoutusingburettesandpipetteswhicharegraduatedincm3,theequationsbelowmaybemoreuseful.
Examiner’stipItisagoodideatorememberthatbothoftheseexpressionshavemoles×1000onthetopline.
MolecalculationsusingequationsThefollowingcalculationsshouldbeapproachedinthefollowingorder.aCalculateanyrelativemolecularmasses,Mr,thatarerequired.
bCalculatethenumberofmolesofthesubstancewheresufficientinformationisgiventodoso.
cUsethemoleratiointheequationtocalculatethenumberofmolesoftheothersubstance.
dUseyouranswertoctocalculateeitherthe•massor•volumeofgasor•volumeofsolutionor•concentrationofsolution.
Examiner’stipItisextremelyimportanttoshowalltheworkingoutincalculations.Ifsomecorrectworkingoutisshownandthefinalanswerisincorrect,youwillstillbeawardedaconsiderableamountofcredit.
Workedexamples1Calciumcarbonatedecomposeswhenitisheatedaccordingtotheequation
Calculatethemassofcalciumoxide,CaO,thatisproducedwhen20.0gofcalciumcarbonate,CaCO3,isheateduntilthereisnofurtherchange.
[3marks]Relativeatomicmasses,Ar:C=12,O=16,Ca=40
a
bMolesofCaCO3=20÷100=0.20moles[1mark]
cMoleratiofromtheequation1moleCaCO3:1moleCaO
0.20molesCaCO3:0.20molesofCaO
[1mark]d
Examiner’stips1Becausethequestiondoesnotaskaboutcarbondioxide,CO2,thereisnoneedtocalculatetherelativemolecularmass,Mrofcarbondioxide.
2Thefinalanswershouldalwaysbeexpressedusingcorrectunits.
2Calculatethevolumeofcarbondioxideatroomtemperatureandpressurethatisproducedbyheating2.1gofsodiumhydrogencarbonate,NaHCO3,accordingtotheequation
Thevolumeofonemoleofanygasis24dm3atroomtemperatureandpressure.
[4marks]aMr:NaHCO3=23+1+12+(16×3)=84
[1mark]bMolesofNaHCO3=2.1÷84=0.025moles[1mark]
cMoleratiofromtheequation2moleNaHCO3:1moleCO20.025molesNaHCO3:0.025÷2=0.0125molesofCO2
[1mark]d
Examiner’stipsThequestionasksforthevolumeofcarbondioxide.Itisaverycommonerrortocalculatethemassinstead.Thosewhodothiscanachievethefirstthreemarksaslongastheworkingoutisclearlyshown.
Itisverycommonforstudentsnottousethemoleratiointheequationortouseitthewrongwayround,i.e.1:2insteadof2:1.Againitispossibletoscorethreemarksoutoffourunderthesecircumstancesdependingonhowmuchcorrectworkingoutisshown.
3Calculatethevolumeofaqueoussodiumhydroxide,NaOH(aq),ofconcentration0.20moldm–3whichwouldberequiredtoneutraliseexactly25.0cm3ofdilutesulfuricacid,H2SO4(aq),ofconcentration0.25moldm−3accordingtotheequation
aTherearenomassesinvolvedinthequestion,sonoMrvalueshavetobecalculated.
bMolesof
cMoleratioinequation
d
Examiner’stipsThisquestionasksyoutocalculatethevolumeofasolution.Manycandidatesusethevalueof24dm3becausetheyconfusethevolumeofasolutionwiththevolumeofagas.
Manystudentscalculaterelativemolecularmasses,althoughthereisnomentionofmassinthequestion.
Whencalculatingthenumberofmolesofasolution,manyuse
theequation
whichtheyoftenlearnas
Thisequationcanonlybeusedifthevolumeisindm3,butinthiscasethevolumeisincm3whichmeansthefactorof1000mustbeused.
4240dm3ofnitrogen,N2(g),reactswithexcesshydrogen,H2(g),accordingtotheequation
aWhatwouldbethevolumeofammonia,NH3(g),produced?
bWhatvolumeofhydrogen,H2(g),wouldreactwiththenitrogen?
Examiner’stipsThereisamuchquickerwayofdoingthiscalculation.Forgasesonly,thevolumeisdirectlyproportionaltothenumberof
moles,whichmeans
Moleratio1:3:2
Volumeratio1:3:2
Therefore,240dm3N2(g)reactswith720dm3H2(g)toproduce480dm3NH3(g).
Allvolumesaremeasuredatroomtemperatureandpressure.Thevolumeofonemoleofanygasis24dm3atroom
temperatureandpressure.aAmmonia•Therearenomassesinvolvedinthequestion,sonoMr
valueshavetobecalculated.•Molesofnitrogen=240÷24=10.0•Moleratio1moleofnitrogen:2molesofammonia10molesofnitrogen:2×10=20molesofammonia•Volumeof20molesofammonia=20×24=480dm3
bHydrogen•Moleratio1moleofnitrogen:3molesofhydrogen10molesofnitrogen:3×10=30molesofhydrogen•Volumeof30molesofhydrogen=30×24=720dm3
EmpiricalformulaeTheempiricalformulaisthesmallestwholenumberratiooftheatomsofeachelementinacompound.Theempiricalformulaofacompoundcanbecalculatedifthe
massesoftheelementsthatcombinetogetherareknown.Thesemassescanbeexpressedinunitsofmass(usuallygrams)orpercentagesbymass.
WorkedexampleAcompoundcontainsthefollowingpercentagecompositionbymass:26.7%carbon,2.2%hydrogenand71.1%oxygen.
Percentagecompositionbymassmeansthat100gofthecompoundcontains26.7gofcarbon,2.2gofhydrogenand71.1gofoxygen.
MethodCalculatethenumberofmolesofatomsofeachelement.
•Carbon,C=26.7÷12=2.225•Hydrogen,H=2.2÷1=2.2•Oxygen,O=71.1÷16=4.44375
Dividealltheabovebythesmallest
•C,2.225÷2.2=1•H,2.2÷2.2=1•O,4.44375÷2.2=2
Writedowntheempiricalformula=CHO2
Examiner’stipIfdividingbythesmallestdoesnotproduceawholenumberineachcase,multiplyallthenumbersby2.Ifthisstilldoesnotproduceawholenumberineachcase,multiplyallthenumbersby3.Continueuntilawholenumberratioisobtained.
CommonerrorsCommonerrorsindeterminingthemolecularformulaeare:
•UsingMrinsteadofAre.g.usingO2=32insteadofO=16
whencalculatingmolesofatoms.•UsingatomicnumberinsteadofArwhencalculatingmolesofatoms.
•Overapproximatione.g.ifacompoundcontainsmanganese,Mn,andoxygen,O,andthenumberofmolesofatomsis
Dividingbothbythesmallest
•Somecandidatesdecidethat1.5isapproximately1andwritetheempiricalformulaasMnO.Thisisincorrect.
•Somecandidatesdecidethat1.5isapproximately2andwritetheempiricalformulaasMnO2.Thisisincorrect.
•Thecorrectmethodistomultiplyboth×2,i.e.1×2=2and1.5×2=3andtheempiricalformulaisMn2O3.
•Anumberwouldhavetobeveryclosetoawholenumber(say0.1away)ifsuchanapproximationistobemade.
MolecularformulaeThemolecularformulaisthenumberofatomsofeachelementinonemoleculeofasubstance.ExamplesofmolecularandempiricalformulaeareshowninTable
4.1.
Table4.1Examplesofmolecularandempiricalformulae
Name Molecularformula EmpiricalformulaButane C4H10 C2H5Hydrogenperoxide H2O2 HOGlucose C6H12O6 CH2OBenzene C6H6 CHMethane CH4 CH4
DeterminationofmolecularformulaefromempiricalformulaeItispossibletodeterminethemolecularformulaofasubstancefromitsempiricalformulaalone,butonlyiftheMrofthesubstanceisalsoknown.IftheempiricalformulaofacompoundisCH2,themolecular
formulaofthecompoundcanbeexpressedas(CH2)n,wherenisawholenumber.IftheMrofthecompoundis70,theMrofCH2=12+(1×2)=14
Therefore,n=70÷14=5andthemolecularformulaisCH2×5=C5H10.
LimitingreactantsWhentwosubstancesaremixed,studentsusuallyassumethatbothsubstanceswillreactcompletelyandthatneitherisleftover.Thisispossible,butitisalsopossiblethattoomuchofeithersubstanceisused,inwhichcaseoneofthetwosubstanceswillbeleftoverattheendofthereaction.Thesubstancethatisallusedupiscalledthelimitingreactantandtheothersubstanceissaidtobeinexcess.
Workedexample5.6gofiron,Fe,and4.0gofsulfur,S,aremixedtogetherandheated.
TheequationisDeducewhichsubstanceisthelimitingreactant.
Therefore,0.10moleofFereactswith0.10moleofS.However,thereare0.125molesofS.0.125isgreaterthan0.10,
thereforesomeSisleftover.SisinexcessandFeisthelimitingreactant.
PercentageyieldIfthereactantsshowninanequationareconvertedcompletelyintotheproducts,wesaythatthepercentageyieldis100%.However,insomecircumstances,yieldsarelessthan100%.
Workedexample0.60gofmagnesiumribbon,Mg,wereburnedinexcessoxygen,O2,accordingtotheequation
Themassofmagnesiumoxide,MgO,thatwasproducedwasfoundtobe0.80g.Calculatethepercentageyield.
Therefore,0.025moleofMg:0.025moleofMgOMassofMgO=0.025×40=1.00gThus,iftheyieldwas1.00g,thepercentageyieldwouldbe100%.
However,theyieldisonly0.80g.
PercentagepurityNaturallyoccurringsubstancesareimpureandcontainlessthan100%ofacompound.Anexampleislimestonewhichcontainslessthan100%ofcalciumcarbonate,CaCO3(s).Thepercentagebymassofcalciumcarbonateinlimestoneisknownasthepercentagepurity.
Workedexample1.00goflimestoneisaddedto100cm3of0.200moldm–3hydrochloricacid(anexcess)(seeequation1).
Equation1
Theleftoveracidwastitratedandfoundtobeneutralisedby24.8cm3,0.100moldm–3ofsodiumhydroxidesolution,NaOH(seeequation2).
Equation2
Moleratioinequation21moleNaOHreactswith1moleHCl
2.48×10–3molesofNaOHreactwith2.48×10–3molesof
HClMolesofHClthatreactedwithcalciumcarbonate,CaCO3MolesofHCladded−molesHClleftover
Moleratioinequation22molesHClreactwith1moleCaCO3
0.01752molesHClreactwith
Exam-stylequestionsThevolumeofonemoleofanygasis24dm3atroomtemperatureandpressure.
1Whatmassofhydrogengasisproducedwhen8.1gofaluminiumpowderreactswithexcessdilutehydrochloricacidaccordingtotheequation
[Total:3marks]2Whatvolumeofoxygengas,O2(g),isproducedatroomtemperatureandpressurewhen0.142gofpotassiumsuperoxide,KO2(s),reactswithexcesscarbondioxide,CO2(g),accordingtotheequation
[Total:3marks]
3Whatmassofcalciumcarbide,CaC2(s),isrequiredtoproduce120cm3ofethynegas,C2H2(g),byreactionwithexcesswateraccordingtotheequation
[Total:3marks]
420.0cm3ofaqueousKOHneutralised35.0cm3ofdilutesulfuricacid,H2SO4(aq),whoseconcentrationwas0.20moldm−3.Theequationis
Calculatetheconcentrationoftheaqueouspotassiumhydroxide,KOH(aq),in
amoldm−3
bgdm−3
[Total:4marks]5Acompoundhascompositionbymasswhichis54.5%carbon,9.1%hydrogenand36.4%oxygen.
TheMrofthecompound=44.Calculatethe
aempiricalformulabmolecularformulaofthecompound.[Total:4marks]
6When0.38goftitanium(IV)chloride,TiCl4(s),reactedwithexcesssodium,thereactionproduced0.024goftitanium,Ti(s).Theequationis
Calculatethepercentageyieldoftitanium.[Total:4marks]
5Electricityandchemistry
KeyobjectivesBytheendofthissection,youshould
•beabletodefineelectrolysis,electrolyteandelectrode•beabletodescribetheproductsofelectrolysisandstatetheobservationsmadewhenvariouselectrolytesconductelectricity•beabletodescribetheelectroplatingofmetalsandoutlinetheusesofelectroplating•beabletodescribeandexplainthereasonswhycopperand(steel-cored)aluminiumareusedincablesandwhyplasticsareusedasinsulators
•beabletoconstructionichalf-equationsforreactionsatthecathode•understandthereasonswhydifferentsubstancesareconductors,insulatorsandelectrolytes•beabletodescribeproductionofenergyfromsimplecells•beabletodescribeinoutlinethemanufactureofaluminiumfrompurealuminiumoxideinmoltencryolite(seeChapter10)•beabletodescribeinoutlinetheproductionofchlorine,hydrogenandsodiumhydroxidefromconcentratedaqueoussodiumchloride•knowwhattheproductsofelectrolysisofaqueouscopper(II)sulfatearewithbothcarbonelectrodesandcopperelectrodes,andrelatethistotherefiningofcopper(seeChapter10).
KeytermsElectrolysisTheprocessbywhichanioniccompound,whenmoltenorin
aqueoussolution,ischemicallychangedbythepassageofanelectriccurrent
Electrolyte AliquidwhichischemicallychangedbyanelectriccurrentElectrodes Theconductingrodsbywhichtheelectriccurrententersand
leavestheelectrolyteAnode Thepositive(+)electrodeCathode Thenegative(−)electrode
Substancesthatconductelectricitycanbesubdividedintoconductorsandelectrolytes(Table5.1).
Table5.1Differencesbetweenconductorsandelectrolytes
Conductors ElectrolytesPhysicalstate
Solid Liquid
Differences Conductelectricity,butarenotchemicallychangedbytheelectriccurrent.Theonlychangeundergonebyconductorsisthattheybecomehot,whichisaphysicalchange
Conductelectricityandarechemicallychangedbytheelectriccurrent.Theproductsofchemicalchangeareformedattheelectrodes
Examples AllmetallicelementsandalloysGraphiteandgraphene
MoltenioniccompoundsAqueoussolutionscontainingions
Particlesresponsibleforconduction
Moving(mobile)electrons Movingions
Examiner’stipThetermselectrolysis,electrolyteandelectrodeallbeginwiththesameletter.Makesureyouknowthedifferencebetweentheseterms.
ElectrolytesElectrolytesmustbeintheliquidstate.Solidioniccompounds,e.g.sodiumchloride,donotconductelectricityinthesolidstatebecausealthoughtheycontainions,theionsareheldtogetherbystrongforcesofattractioninthegiantioniclattice.Becausetheionsarenotmoving,solidsodiumchloridedoesnotconductelectricity.Therearetwowaystomakeionicsolidsintoelectrolytes:
Heatuntiltheionicsolidmelts.Thisrequiresalargeamountofheatenergy,becauseioniccompoundshavehighmeltingpoints(seeChapter3).Moltenioniccompoundsareelectrolytesbecausetheionsaremovingintheliquidstate.
Dissolvetheionicsolidinwater.Anaqueoussolutionofanioniccompoundalsocontainsmovingions.
Whenmoltenioniccompoundsandaqueoussolutionsofioniccompoundsconductelectricity,thepositiveions(cations)movetothecathode(−)andthenegativeions(anions)movetotheanode(+).Atthecathode,positiveionsgainelectronsandarereduced(seeChapter7),
forexampleAttheanode,negativeionsloseelectronsandareoxidised(seeChapter7),for
exampleThus,theproductsofelectrolysisareformedattheelectrodes.Whenionslosetheirchargetoformatomsormoleculestheyaresaidtobe
discharged.
PracticalelectrolysisElectrolysiscanbecarriedoutinschoollaboratoriesusingtheapparatusshowninFigure5.1.
Theelectrolyteisplacedinacrucible,ifitisasolidthathastobeheateduntilitsmeltingpoint,orabeaker,ifitisaliquidatroomtemperature.Figure5.2showshowgaseousproductscanbecollectedduringelectrolysis,
asintheelectrolysisofdilutesulfuricacid.TheproductsthatformduringelectrolysisaresummarisedinTable5.2.
Table5.2Summaryofproductsformedduringelectrolysis
Typeofelectrolyte
Productsatanode(+) Productsatcathode(−)
Moltenioniccompound
Non-metallicelement Metallicelement
Aqueoussolutionscontainingions
Eitheroxygengasorhalogen(chlorine,bromineoriodine)fromanyconcentratedaqueoussolutionofahalide(chloride,bromideoriodide)
Eitherhydrogengasormetallicelementbelowhydrogeninthe
ofahalide(chloride,bromideoriodide)reactivityseries,e.g.copper
Examiner’stipRememberthatmoltenioniccompoundsproduceanon-metallicelementattheanodeandametallicelementatthecathode.
Aqueoussolutionsproduceoxygenorahalogenattheanodeandhydrogenorametalatthecathode.Thehydrogenandoxygencomefromthewaterthatiscontainedintheaqueoussolution.
Someexamplesofproductsoftheelectrolysisofdifferentelectrolytes,usingcarbonorplatinum(inertelectrodes),areshowninTable5.3.
Examiner’stipMakesureyourememberthat
•Veryreactivemetalsthatreactwithcoldwater(suchaspotassium,sodiumandcalcium)cannotbeproducedbyelectrolysisofaqueoussolutions.
•Duringtheelectrolysisofallaqueoussolutionscontainingpositiveionsofametalabovehydrogeninthereactivityseries,hydrogenisproducedatthecathodeasopposedtothemetallicelement.Theextractionofthesemetalsbyelectrolysiscanonlybecarriedoutusingamoltenelectrolyte(seeextractionofaluminium,p.73,inChapter10).
•Aqueoussolutionsofacidsalwaysproducehydrogenatthecathode(−).This
isbecausethepositiveH+ioniscommontoboththeacidicsubstanceandwater.
Electrolysisofcopper(II)sulfateIfaqueouscopper(II)sulfateiselectrolysedusingcarbonorplatinumelectrodes(inertelectrodes),theproductsarecopperatthecathodeandoxygenattheanode(seeTable5.3).However,iftheanodeismadeofcopper,oxygenisnotproducedatthe
anode.Instead,thecopperanodegoesintosolutionaspositiveions:
Eventually,theCu2+(aq)ionsreachthecathodewherethereversereaction
occursandcoppermetalisformed:Thiselectrolyticprocessisusedintherefining(purification)ofcopper
whichiscarriedoutonalargescale(seeChapter10).
•Theanodeisimpurecopper•Theelectrolyteisaqueouscopper(II)sulfate•Thecathodeispurecopper.
ThecopperattheanodegoesintosolutionaspositiveCu2+ions.Impuritieseithergointosolutionaspositiveionsorfallofftheanodeandaredepositedatthebottomofthecontainer.Purecopperformsatthecathode.Noneoftheothermetallicionsaredischargedatthecathode.Purecopperisessentialwhencopperisbeingusedasanelectricalconductor,
asinelectricalwiring.Impuritiesinthecopperdecreaseitselectricalconductivityconsiderably.
ElectrolysisofconcentratedaqueoussodiumchlorideSodiumchloride(commonsalt)isfirstminedandthenelectrolysedasaconcentratedaqueoussolution.Electrolysisofconcentratedaqueoussodiumchloridecanbecarriedoutinthelaboratoryoronanindustrialscale.Theionsthatarepresentinthesolutionare
Attheanode,hydrogengasisproduced2H++2e−→H2.TheNa+(aq)remaininthesolution.Atthecathode,chlorinegasisproduced2Cl−→Cl2+2e.TheOH−(aq)remaininthesolution.BecausetheelectrolytecontainsNa+(aq)andOH−(aq),theelectrolytehas
changedintoaqueoussodiumhydroxide,NaOH(aq).Thethreeproductsofelectrolysishaveseveralindustrialuses.
ElectroplatingElectroplatingisanotherelectrolyticprocessthatcanbecarriedoutinaschoollaboratoryoronalargescale.Electroplatingmeanscoatinganobjectwithathinlayerofametal.Thepurposeis
•toimproveappearance•topreventcorrosion,e.g.rusting.
Electroplatingiscarriedoutusing
•theplatingmetalastheanode•theobjecttobeplatedasthecathode•anaqueoussolutioncontainingionsoftheplatingmetalastheelectrolyte.
IntheexampleofsilverplatingshowninFigure5.3,thesilveranodegoesintosolutionassilverions.
Thesilverionsintheelectrolytearedischargedatthecathode.
Thesilverionsthatarereleasedattheanodereplacethosethataredischarged.Thesilverproducedatthecathodeelectroplatesthespoon.
ExtractionofmetalsReactivemetalsareextractedbyelectrolysisofmoltenioniccompounds(seeextractionofaluminium,pp.73–4,Chapter10).Unreactivemetalsareextractedbyelectrolysisofaqueoussolutions(see
refiningofcopper,pp.75–6,Chapter10).
Usesofmetals,plasticsandceramicsAluminiumisusedinsteel-coredcables,becauseit
•isunreactiveduetoacoatingofaluminiumoxide•isagoodconductorofelectricity•hasalowdensity.Copperisusedinelectriccablesandwiresduetoveryhighelectrical
conductivity.Plasticsandceramicsdonotconductelectricityandthereforeareusedas
insulatorsinthepowersupplyindustry.
CellsElectrolysisuseselectricalenergytocarryoutchemicalreactions.Incells,chemicalreactionsareusedtoproduceelectricalenergy.Iftwo
dissimilarmetalsareplacedinanelectrolyte,electricalenergyisproduced.Thisistheprincipleofthebattery.Suchcellscanbesetupinordertoputmetalsinorderofreactivity(seeChapter10).Fuelcellshaveafuel,suchashydrogenorethanol.Thefuelreactswith
oxygeninordertogenerateelectricity(seeChapter6).
Exam-stylequestions1Completethefollowingtabletoshowtheproductsofelectrolysisusingcarbonelectrodes.
[Total:10marks]2Astudentwantedtoelectroplateaknifewithnickel.Whatshouldthestudentuseas
atheanodebtheelectrolytecthecathode?
[Total:3marks]3Astudentcarriesoutelectrolysisofconcentratedaqueouspotassiumiodideinabeakerusingcarbonelectrodes.
aNametheproductattheanode.[1mark]
bWriteanionichalf-equationforthereactionoccurringatthecathode.[1mark]
cStatethetypeofreactionoccurringattheanode.[1mark]
dStatethenameofthesolutionleftinthebeakerwhentheelectrolysishasfinished.
[1mark]eNamethetypeofparticlesthatareresponsiblefortheconductionof
electricityintheconductingwire.
[1mark]fNamethetypeofparticlesthatareresponsiblefortheconductionof
electricityintheelectrolyte.[1mark]
[Total:6marks]
6Chemicalenergetics
KeyobjectivesBytheendofthissection,youshould
•beabletodescribewhatismeantbyexothermicandendothermicreactions•beabletointerpretenergyleveldiagramsshowingexothermicandendothermicreactions•describethereleaseofheatenergybyburningfuels•knowthathydrogenisusedasafuel•knowthatradioactiveisotopes,suchas235U,canbeusedasasourceofenergy
•knowthatbondbreakingisanendothermicprocessandbondformationisanexothermicprocess•beabletodrawandlabelenergyleveldiagramsforexothermicandendothermicreactionsusingdataprovided•beabletocalculatetheenergychangeinareactionusingbondenergies•beabletodescribetheuseofhydrogenasafuelinfuelcells.
KeytermsExothermicreaction
Areactioninwhichthereisanoveralltransferofenergytothesurroundings
Endothermicreaction
Areactioninwhichthereisanoverallgainofenergyfromthesurroundings
Bondenergy Theamountofenergyrequiredtobreakonemoleofcovalentbondsingaseousmolecules
Examplesofexothermicreactionsare
•combustionoffuelssuchasalkanesandhydrogengas;fuelsreleaseheatenergywhentheyburninairoroxygen.
•respiration(seeChapter13).
Examplesofendothermicreactionsare
•photosynthesis(seeChapter13)•thermaldecomposition(seeChapters10and13).
EnergyleveldiagramsExothermicandendothermicreactionscanberepresentedbyenergyleveldiagrams.Thesediagramsshowtheenergyofthereactantsandproductsasareactionprogresses.
ExothermicreactionsThecompletecombustionoffuels,suchasmethane,isanexampleofanexothermicreaction(Figure6.1).Inanyexothermicreaction,theproductshavelessenergythanthereactantsbecauseenergyhasbeengivenouttothesurroundings.
EndothermicreactionsThethermaldecompositionofcalciumcarbonateisanexampleofanendothermicreaction(Figure6.2).Inanyendothermicreaction,theproductshavemoreenergythanthereactantsbecauseenergyhasbeentakeninfromthesurroundings.
FuelsFossilfuels(petroleum,naturalgasandcoal)allreleaseenergywhentheyburn.Uranium235(235U)isasourceofnuclearenergy.Ithastheadvantageofnot
causingglobalwarmingasnogreenhousegasesarereleasedwhenitisusedasafuel,buttherearesafetyissuesconcerningtheadverseeffectsofradiation.Hydrogenalsoreleasesenergywhenitburns.Anadvantageofusinghydrogen
asafuelasanalternativetofossilfuelsisthatwateristheonlyproduct;carbondioxide(agreenhousegas)isnotproduced.Hydrogencanalsobeused,alongwithoxygen,inafuelcelltorelease
electricalenergy.
FuelcellsInhydrogenfuelcells,theoverallreactionisthesameaswhenhydrogenisburnedinairoroxygen.
Fuelcellsoperateinacidicoralkalineconditions.AnalkalinehydrogenfuelcellisshowninFigure6.3.Oneadvantageofcarryingoutthisreactioninafuelcellisthatfuelcellsare
muchmoreefficientthaninternalcombustionengines,whichmeansthereismuchlessenergyloss.
BondenergiesMostchemicalreactionsinvolvebreakingofcovalentbondsinthereactantsandformationofnewcovalentbondsintheproducts.
•Breakingofbondsisanendothermicprocess(energyistakenin).•Formationofbondsisanexothermicprocess(energyisgivenout).
Becausetheamountofenergyputintobreakbondsisveryunlikelytobeequal
Becausetheamountofenergyputintobreakbondsisveryunlikelytobeequaltotheamountofenergygivenoutwhennewbondsareformed,mostreactionsareeitherendothermicorexothermic.Anexampleisthereactionbetweengaseoushydrogenandgaseouschlorine
toformgaseoushydrogenchloride.Theequationis
Bondenergyistheamountofenergyrequiredtobreak1moleofcovalentbondsingaseousmolecules.Itisnumericallyequaltotheamountofenergygivenoutwhennewbondsformingaseousmolecules.Whencovalentbondsinmoleculesarebroken,themoleculeschangeinto
atoms.Theatomsthenjointogethertoformnewmolecules.BondenergiesareshowninTable6.1.
Table6.1Bondenergies(kJ/mole)
Bond Bondenergy(kJ/mole)H–H 435Cl–Cl 242H–Cl 432
Theequationcanbewrittentoshowthestructureofthemolecules:
Energytakenintobreakbonds EnergygivenoutwhenbondsformH–H=435kJ 2×H–Cl=2×432=864Cl–Cl=242kJ Totalenergyputin:435+242=677kJ Totalenergygivenout=864kJ
Because864isalargernumberthan677,thismeansthatmoreenergyisgivenoutwhenthebondsformintheproductsthantheenergythathastobeputintobreakthebondsinthereactants.Therefore,thereactionisexothermicandthe
overallenergychangeiswhichmeansthatwhen1moleofgaseousH2moleculesreactwith1moleof
gaseousCl2moleculestoform2molesofgaseousHClmolecules,187kJofenergyaregivenouttothesurroundings.Thiscanalsobeshownonanenergyleveldiagram.
Exam-stylequestions1Propaneburnsinexcessoxygentoformcarbondioxideandwateraccordingtoequationbelow.(Notethatthetableofbondenergieswillberequiredtoanswerthequestion.)
BondBondenergykJ/molC–C 347C–H 435O=O 497C=O 803O–H 464
Calculatetheoverallenergychangeoccurringwhen1moleofC3H8(g)reactswith5molesofO2(g)toform3molesofCO2(g)and4molesofH2O(g)byusingthefollowingsteps:
aDrawthestructuresofallthemoleculesonbothsidesoftheequation.Showalltheatomsandallthebonds.
[2marks]
bWritedownthenumberofmolesofeachtypeofbondthathavetobebrokeninthereactants.(Remembertoconsiderthenumberofmolesofbothreactantsintheequation.)[2marks]
cCalculatethetotalamountofenergythathastobeputintobreakallthebondsinthereactantsin(b).
[1mark]dWritedownthenumberofmolesofeachtypeofbondthathavetobe
formedintheproducts.(Remembertoconsiderthenumberofmolesofbothproductsintheequation.)[2marks]
eCalculatethetotalamountofenergythatisgivenoutwhenallthebondsintheproductsin(d)areformed.
[1mark]fUseyouranswersto(c)and(e)tocalculatetheoverallenergychangein
thereaction.Youmustshowthecorrectunitsinyouranswerandsaywhetherthereactionisexothermicorendothermic.
[3marks]
7Chemicalreactions
KeyobjectivesBytheendofthissection,youshould
•beabletoidentifyphysicalandchemicalchangesandknowthedifferencesbetweenthem
•describeandexplaintheeffectsofchangingtheconcentrationofaqueoussolutions,particlesize/surfaceareaofsolids,pressureofgases,catalysts(includingenzymes)andtemperatureonratesofreaction
•describetheapplicationoftheabovefactorstothedangerofexplosivecombustionwithfinepowders(e.g.flourmills)andgases(e.g.methaneinmines)
•demonstrateknowledgeandunderstandingofapracticalmethodforinvestigatingtherateofareactioninvolvinggasevolution
•interpretdataobtainedfromexperimentsconcernedwithrateofreaction
•deviseandevaluateasuitablemethodforinvestigatingtheeffectofagivenvariableontherateofareaction
•describeandexplaintheeffectsofchangingtemperatureandconcentrationintermsofcollisionsbetweenparticlesusingtheconceptofactivationenergy
•describeandexplaintheroleoflightinphotochemicalreactionsandtheeffectoflightontherateofthesereactions
•describetheuseofsilversaltsinphotography•describephotosynthesisasareactionbetweencarbondioxideandwaterinthepresenceofchlorophyllandsunlighttoproduceglucoseandoxygen
•understandthatsomechemicalreactionscanbereversedbychangingthereactionconditions,e.g.theeffectofheatonhydratedcopper(II)sulfateandhydratedcobalt(II)chloride
•beabletodemonstrateknowledgeandunderstandingoftheconceptofequilibrium
•beabletopredicttheeffectofchangingtheconditions(concentration,temperatureandpressure)onotherreversiblereactions
•defineoxidationandreductionintermsofoxygenloss/gain
•defineredoxintermsofelectrontransfer•identifyredoxreactionsbychangesinoxidationstateandbythecolourchangesoccurringwhenusingacidifiedpotassiummanganate(VII)andpotassiumiodide
•defineoxidisingagentandreducingagent•identifyoxidisingagentsandreducingagentsfromsimpleequations.
KeytermsCatalyst Asubstancewhichincreasestherateofachemicalreaction.The
catalystischemicallyunchangedattheendofthereactionEnzyme Abiologicalcatalyst.EnzymesareproteinmoleculesOxidationGainofoxygenorlossofhydrogenReductionLossofoxygenorgainofhydrogenOxidisingagent
Asubstancethatoxidisesanothersubstanceinaredoxreaction.Anelectronacceptor
Reducingagent
Asubstancethatreducesanothersubstanceinaredoxreaction.Anelectrondonor
Oxidation LossofelectronsReductionGainofelectrons
Physicalchangesarechangesinwhichnewchemicalsubstancesarenotproduced.Changesinstate,thatismelting,boiling,evaporation,condensation,sublimationandfreezing(seeChapter1),andseparationofmixtures,e.g.filtration,distillation,fractionaldistillation,chromatographyandcrystallisation(seeChapter2),areexamplesofphysicalchanges.
Chemicalchangesorchemicalreactionsarechangesinwhichnewchemicalsubstancesareproduced.Decomposition,electrolysis,respiration,photosynthesis,redox,neutralisation,cracking,addition,substitution,polymerisationandcombustionareexamplesofchemicalchanges.Chemicalequationscanalwaysbeusedtorepresentchemicalchanges.Thetermphysicalpropertiesofasubstancereferstopropertiesofa
substancethatcanbemeasuredandthatinvolvephysicalchanges.Examplesaremeltingpoint,boilingpointanddensity.Thetermchemicalpropertiesofasubstancereferstopropertiesofa
substancewhichinvolvechemicalchanges.Examplesarethethingsthatsubstancesreactwithanddetailsofsuchreactions.Itcanbesaidthataphysicalpropertyofmetalsisthattheyallconduct
electricity,whereasachemicalpropertyofmetalsisthat(someofthem)reactwithacidstoproduceasaltandhydrogen.
Examiner’stip
Studentsshouldensurethattheyareawareofthedifferencesbetweenphysicalpropertiesandchemicalpropertiesandthattheyknowexamplesofbothfordifferenttypesofsubstance.
RateofreactionTherateofachemicalreactioncanbedeterminedbymeasuringeitherhowtheamountofoneofthereactantsdecreaseswithtimeorhowtheamountofoneoftheproductsincreaseswithtime.
Commonerrors•Studentscommonlyrefertocatalystsbeingabletoalterthespeedofachemicalreaction.Thisdoesnotspecificallymeanthatacatalystincreasestherate,becausealteringthespeedsuggeststhatcatalystsmaydecreasetherate,whichisnotthecase.
•Anothercommonerroristosuggestthatcatalystsdonottakepartinareaction.Thisisnotthecase,becauseincreasingtheratesuggeststhatcatalystshaveaconsiderableparttoplay.
•Theterm‘biocatalyst’isnotrecognisedashavingthesamemeaningasbiologicalcatalyst.
ExperimentalinvestigationsReactionsinwhichsolidsreactwithliquidstoproducegasesamongotherproductsarecommonlyinvestigatedinthischapter.
Experiment1Anexampleis
Thereactionbetweenaknownexcessofzincgranulesand50.0cm3of0.10moldm−3dilutesulfuricacidwasinvestigatedbyastudent.TheapparatusshowninFigure7.1wasused.
Thetemperatureiskeptat25°Cbyusingathermostaticallycontrolledwaterbath.Thevolumeofhydrogenproducedcanbemeasuredatregulartimeintervals.Agraphisthenplotted(Figure7.2).
Thegraphissteepestatthestartwhichmeansthatthisiswhentherateofreactionisfastest.Itthenbecomeslesssteepwhichmeansthatthereactionbecomesslower.Eventuallythegraphlevelsout,whichmeansthatnomorehydrogengasisreleasedandtherateofreactioniszero.
CollisiontheoryInanyreactionofthetype
particlesofAandBmustcollidewitheachotheriftheyaretoproduceproductC.Therearetwotypesofcollision:successfulandunsuccessful.Inanunsuccessfulcollision,particlesofAandBmerelybounceoffeach
otherandremainasAandB.However,inasuccessfulcollision,particlesofAandBcollideandchange
intoC.
Collisionsareonlysuccessfulifthereactingparticlescontainatleastaminimumamountofenergycalledtheactivationenergy.Therateofachemicalreactiondependsonthenumberofsuccessful
collisionsoccurringinanygiventime.Ifachangeismadetoincreasethenumberofcollisions,thisautomatically
leadstoanincreaseinthenumberofsuccessfulcollisionsbecauseacertainproportionofallcollisionsarealwayssuccessful.Inthereactioninvestigated
theionicequationis
Thisshowsthatcollisionsmusttakeplacebetweenzincatomsandhydrogenionsforthereactiontooccur.InExperiment1,therateofreactionisfastestatthestartbecausethisiswheretheconcentrationofhydrogenionsishighest,whichmeansthatthenumberofcollisionsbetweenhydrogenionsandzincatomsinanygiventimeismostfrequentatthestart.Therateofreactionthendecreasesbecauseastheconcentrationofhydrogen
ionsdecreases,collisionsoccurlessfrequently.Whenallthesulfuricacidisusedup,theconcentrationofhydrogenions
becomeszero,thereforetherearenomorecollisionsandtheratebecomeszero.
Examiner’stipInreactionsbetweengases,itispossibletomakesimilarstatementsabouttheconcentrationofagas.However,itismorelikelythatgasesarereferredtointermsofpressureratherthanconcentration.Thehigherthepressureexertedbyagas,theclosertogetherarethegaseousmoleculesandthemorefrequentwillbetheircollisionswitheachother.
Thestudentthencarriedoutexperimentsonthesamereaction,changingonlyonevariableineachcase.Thechangedvariableishighlighted.
Experiment2TheconcentrationofsulfuricacidisdoubledinExperiment2butthevolumeofsulfuricacidishalvedwhichmeansthatthenumberofmolesofsulfuricacidisthesame.Figure7.4showsthegraphthatwasplottedinExperiment2withthegraph
forExperiment1.
Becausethegraphissteeperatthestart,thisiswheretherateofreactionisfastest.Therateofreactionincreasesastheconcentrationincreases.Thisisbecause
theconcentrationofhydrogenionsishigher,whichmeansthatthenumberofcollisionsbetweenhydrogenionsandzincatomsisalsohigherinanygiventime.ThegraphsinExperiments1and2(Figure7.4)leveloffatthesamevolume
ofhydrogen,becausethevolumeofhydrogengivenoffisonlydependentonthenumberofmolesofsulfuricacid,whichisthesameinbothexperiments(asitisinallfiveexperiments).InExperiments3,4and5,thegraphwouldbethesameshapeasin
Experiment2,thatis,itwouldbesteeperatthestartandleveloffatthesamevolumeofhydrogen.
Experiment3Usingzincpowderwearedecreasingparticlesize/increasingsurfacearea.Collisionscanonlyoccuronthesurfaceofthezinc.Withasmallerparticlesize,therearemorezincatomsavailabletocollidewiththehydrogenions.Morecollisionsoccurringinanygiventimemeansthatmoresuccessfulcollisionsoccurinanygiventimeandthereforeafasterrateofreactionoccurs.
Decreasedparticlesizeandincreasedrateofreactioncanleadtohazards,forexample,inflourmillsandcoalmines.Flourdustandcoaldust(bothhavingextremelylargesurfaceareas)havebeenknowntoreactexplosivelywithoxygenintheairwhenasparkhasbeencreatedbymachinery.
Experiment4Athighertemperature,thereactingparticleshavemoreenergy.Thismeansthattheparticlesmovefasterandcollidemoreofteninanygiventime.
However,therewillbemoreparticleswithenergygreaterthanorequaltotheactivationenergy.Therefore,morecollisionswillbesuccessfulcollisionsinanygiventime.Thisisthemainreasonwhyratesofreactionarefasterathighertemperatures.
Experiment5Aqueouscopper(II)sulfateactsasacatalystinthisreaction.
Catalystslowertheactivationenergyofareaction.Thismeansthattherearemoreparticleswithenergygreaterthanorequaltotheactivationenergy.Therefore,morecollisionsaresuccessfulcollisionsinanygiventimeandtherateofreactionisfaster.
Activationenergyistheamountofenergythathastobesuppliedtoreactantstomakeareactionoccur.Theloweringofactivationenergyinacatalysedreactioncanbeshownintheenergyprofile(Figure7.5).Ascanbeseen,usingacatalysthasnoeffectontheoverallenergychangeofareactionbutlowerstheactivationenergy,thusincreasingtherateofreaction.
PhotochemicalreactionsPhotochemicalreactionsrequirelightinordertooccur.Furthermore,therateofsuchreactionsincreaseswhenlightintensityincreases.Examplesofphotochemicalreactionsare:
•photosynthesisinwhichcarbondioxideandwaterreactinthepresenceofsunlightandchlorophylltoproduceglucoseandoxygen(seeChapter13)
•substitutionreactionofalkaneswithchlorine(seeChapter14)whichrequiresUVlight
•photochemicaldecompositionofsilversaltsinphotography.Whenaqueoussilvernitrateisaddedtoanaqueoussolutioncontaininghalideions(Cl−,Br−orI−),aprecipitateofasilverhalideoccurs,e.g.
Whenthesilverbromideisexposedtolightitdecomposesintoitselementsandtheprecipitatedarkensasitgraduallydecomposesintosilverandbromine.Silverbromideisusedinphotographicfilmsbecauseitdarkenswhenexposedtolight.
Therateofthisreactionincreasesaslightintensityincreases.
ReversiblereactionsSomereactionscanbereversedbychangingtheconditions.Ifcrystalsofhydratedcopper(II)sulfateandhydratedcobalt(II)chlorideareheated,theychangecolourastheylosetheirwaterofcrystallisationandbecomeanhydroussalts.
However,inbothcases,thereactionscanbemadetoproceedinthereversedirectionsbyaddingwatertotheanhydroussaltsinwhichcasethecrystalsformagain,ascanbeseenbythereversecolourchange.
Thesereactionsarecalledreversiblereactions.Theycanbemadetoproceedinthereversedirectionbychangingtheconditions.
EquilibriumIfreversiblereactionsareallowedtoproceedinaclosedcontainer,theyreachastatethatisknownaschemicalequilibrium.Ifamixtureofhydrogenandiodinegasesisheatedinaclosedcontainer,the
hydrogenreactswiththeiodinetoproducehydrogeniodide:
Thisiscalledtheforwardreaction.Assoonashydrogeniodidemoleculesareformed,theystarttodecompose
intohydrogenandiodine:
Thisiscalledthereverseorbackwardreaction.Therefore,tworeactionsareoccurringinthesamecontaineratthesame
time.Furthermore,onereactionisthereverseoftheother.Thiscanbeshownbythefollowingexpression:
Theforwardreactionstartsoffquicklyandtheratedecreasesastheconcentrationsofhydrogenandiodinedecrease.Thebackwardreactionstartsoffslowlyandtherateincreasesasthe
concentrationofhydrogeniodideincreases.Eventually,bothratesbecomeequal.Thesystemistheninastateof
chemicalequilibrium.Whenthisoccurs,reactantsandproductsareallbeingusedupandproducedatthesamerateandthereforetheirconcentrationsbecomeconstant.
CharacteristicsofequilibriumsystemsEquilibriumcanonlyoccurinaclosedsystem(closedcontainer),inwhichnosubstancescanescapetotheoutsideorenterfromtheoutside.
•Therateoftheforwardreactionisequaltotherateofthereversereaction.•Theconcentrationsofallreactantsandproductsbecomeconstant.
Examiner’stipWhenaskedtodescribethecharacteristicsofanequilibriumsystem,studentsmaketwoverycommonerrors.Theyoftenstate:1Theforwardreactionisequaltothereversereaction.Withoutusingtheword‘rate’,thisisameaninglessstatement.
2aTheamountsofreactantsandproductsbecomeconstant.Inthiscase,theword‘amounts’mustbereplacedby‘concentrations’.
bTheconcentrationsofproductsandreactantsbecomeequal.Thisisincorrect.‘Theconcentrationsofproductsandreactantsbecomeconstant,
whichmeansthattheystopchanging’,isthecorrectstatement.
EffectsofchangingtheconditionsofanequilibriumsystemThefollowingequationrepresentstheequilibriumwhichoccursintheHaberprocess:
Theforwardreactionisexothermic.ThismeansthatN2(g)+3H2(g)→2NH3(g)isanexothermicreaction.Therefore,2NH3(g)→N2(g)+3H2(g)isanendothermicreaction.Theequationshowsthattherearefourgaseousmoleculesontheleft-hand
sideofthe signandtwogaseousmoleculesontheright-handsideofthesign.
Table7.1Theconditionsofthisequilibriumsystemandtheirresults
Change Effectonequilibriumposition Resultinthisexample
Increasetheconcentrationofreactants
Shiftstotheright Concentrationofproductsincreases
Increasetheconcentrationofproducts
Shiftstotheleft Concentrationofreactantsincreases
Increasethetotalpressure
Shiftsinthedirectionoffewermolecules Concentrationofproductsincreases
Increasetemperature
Shiftsinendothermicdirection Concentrationofreactantsincreases
Addacatalyst Increasestherateofbothforwardandreversereactions,butdoesnotshifttheequilibrium
Nochange
Decreasesinconcentrations,pressureandtemperaturehavetheoppositeeffecttoincreases.ThisissummarisedinFigure7.6.
Examiner’stipsWhenstudentsareaskedaboutchangingtemperatureandpressureonanequilibriumsystem,theyoftendemonstrateconfusionbetweenrateofreactionandequilibrium.Thisisshowninthesamplequestionbelow.
Sampleexam-stylequestionWhathappenstothepositionofequilibriumintheHaberprocessreaction,i.e.
N2(g)+3H2(g) 2NH3(g),whenthetemperatureincreases?Theforwardreactionisexothermic.
Student’sanswer
Therateofthereversereactionincreasesbecausetheforwardreactionisexothermic.
Examiner’scommentThestudentshouldnothaveusedtheword‘rate’.Ifthetemperatureofanequilibriumsystemisincreased,therateofbothforwardandreversereactionsareincreased,becauseincreaseintemperatureincreasestherateofallreactions(exceptenzymecatalysedreactions).
Thecorrectanswerisastatementthatsays
‘Theequilibriumshiftstotheleftintheendothermicdirection’
or
‘Theequilibriumshiftstotheleftbecausetheforwardreactionisexothermic’
Studentsareadvisedtotreatequilibriumandrateastwocompletelyseparatethingswhicharenotrelatedtoeachother.
RedoxOxidationwasoriginallydefinedasgainofoxygen.Inthefollowingreaction
magnesiumisoxidisedbecauseitgainsoxygen.Becauseoxygencausestheoxidation,oxygenistheoxidisingagent.Reductionistheoppositeofoxidationandwasoriginallydefinedaslossof
oxygen.Inthefollowingreaction
copper(II)oxideisreducedbecauseitlosesoxygen.Hydrogenisthereducingagent.Inthisreaction,hydrogengainsoxygen,thereforehydrogenisoxidised.Itfollowsthatoxidationandreductionalwaysoccuratthesametime.Areactioninwhichoxidationandreductionbothoccurisknownasaredox
reaction.
Anotherexampleofaredoxreactioninvolvingoxygenis
inwhichiron(II)oxideisoxidisedtoiron(III)oxideusingoxygenasanoxidisingagent.Thisgivesrisetoanothertypeofreactioninwhichanelementisoxidisedfromaloweroxidationstatetoahigheroxidationstateasinironbeingoxidisedfrom+2to+3.However,gainofoxygenand/orlossofhydrogenareverylimiteddefinitions
ofoxidationandreduction,becausemanyredoxreactionsdonotinvolveoxygenorhydrogen.Thefollowingreaction
inwhichiron(II)chlorideisoxidisedtoiron(III)chlorideusingchlorineastheoxidisingagentisanexample.Theionicequationforthisreactionis
Thiscanbebrokendownintotwoionichalf-equations:
•Fe2+isoxidisedtoFe3+bylossofelectrons.Cl2istheoxidisingagent.•Thereforeoxidationiselectronloss.•Oxidisingagentsareelectronacceptors.•Cl2isreducedto2Cl−bygainofelectrons.Fe2+isthereducingagent.•Thereforereductioniselectrongain.•Reducingagentsareelectrondonors.
Examiner’stipInionichalf-equations
•electronsappearontherightforoxidation•electronsappearontheleftforreduction.
Inanyredoxreaction,electronsaretransferredfromthereducingagenttotheoxidisingagent.Thereducingagentisoxidisedandtheoxidisingagentisreduced.
TestingforoxidisingandreducingagentsAqueouspotassiummanganate(VII)isanoxidisingagentwhichcanbeusedtotestforthepresenceofreducingagents.Whenareducingagentisadded,theaqueouspotassiummanganate(VII)changescolourfrompurpletocolourless.
Aqueouspotassiumiodideisareducingagentwhichcanbeusedtotestforthepresenceofoxidisingagents.Whenanoxidisingagentisadded,theaqueouspotassiumiodidechangescolourfromcolourlesstobrown.
Exam-stylequestions1Statewhetherthefollowingchangesarephysicalchangesorchemicalchanges.
aDissolvingsodiumchlorideinwater.[1mark]
bTheelectrolysisofaqueoussodiumchloride.[1mark]
cExposingaprecipitateofsilverchloridetosunlight.[1mark]
dFractionaldistillationofliquidair.[1mark]
eSeparatingthedyesininkbychromatography.[1mark]
[Total:5marks]
2Whenanexcessofmarblechips(calciumcarbonate)isaddedto50cm3of0.10moldm−3hydrochloricacidat25°C,thefollowingreactionoccurs:
Thevolumeofcarbondioxidegaswascollectedinagassyringeandmeasuredatregulartimeintervals.Thisisexperiment1.
Theexperimentwasrepeatedasshowninthetablebelow.Graphswereplottedineachcase(Figure7.7).
Thecalciumcarbonateisinexcessinallfiveexperiments.Copyandcompletethetablebelowtoshowwhichgraphcorrespondstoeachdifferentexperiment.Eachlettermaybeusedonce,morethanonceornotat
all.
[Total:4marks]3Stateinwhichdirections(ifany)thefollowingequilibriummixtureswouldshiftifthepressureonthesystemwasincreased.Explainyouranswerineachcase.
a
[1mark]b
[1mark]
c
[1mark][Total:3marks]
4Stateinwhichdirections(ifany)thefollowingequilibriummixtureswouldshiftifthetemperatureonthesystemwasdecreased.Explainyouranswerineachcase.
a exothermicintheforwarddirection
[1mark]
b endothermicintheforwarddirection
[1mark][Total:2marks]
5Theequationforthereactionbetweenmagnesiumandcopper(II)sulfatesolutionisshown.
aWriteanionicequationforthereaction.[1mark]
bWritetwoionichalf-equationsrepresentingoxidationandreductioninthereaction.
[2marks]cStatetheformulaofthespecieswhichactsasanoxidisingagentinthe
reaction.Explainyouranswer.[2marks]
dStatetheformulaofthespecieswhichactsasareducingagentinthereaction.Explainyouranswer.
[2marks][Total:7marks]
8Acids,basesandsalts
KeyobjectivesBytheendofthissection,youshouldbeableto
•describethecharacteristicpropertiesofacidsintheirreactionswithmetals,basesandcarbonates,andtheireffectonlitmusandmethylorange•describethecharacteristicpropertiesofbasesintheirreactionswithacidsandwithammoniumsaltsandtheireffectonlitmusandmethylorange•describeneutrality,relativeacidityandalkalinityintermsofpHmeasuredusingUniversalindicatorpaper•describeandexplaintheimportanceofcontrollingacidityinsoil•classifyoxidesaseitheracidicorbasicrelatedtometallicandnon-metalliccharacter•suggestmethodsofpreparationofsalts
•defineacidsandbasesintermsofprotontransfer•describethemeaningofweakandstrongacidsandbases•classifyotheroxidesasneutraloramphoteric.
KeytermsAcid AprotondonorBase AprotonacceptorStrongacid
Existscompletelyasionsinaqueoussolution
Weakacid
Onlypartiallyionisedinaqueoussolution
Strongbase
Existscompletelyasionsinaqueoussolution
Salt Anionicsubstanceformedwhenthepositivehydrogenionsinanacidarereplacedbypositivemetallicionsorammoniumions
AcidsAcidsareproton(H+)donors.Thecommonlaboratorystrongacidsaredilutehydrochloric,nitricand
sulfuricacids.Theirformulaearegivenbelow.
•Hydrochloricacid:HCl•Nitricacid:HNO3•Sulfuricacid:H2SO4
Inaqueoussolutionstrongacidsdonotcontainanyparticleswiththeseformulaebecausetheyexistcompletelyasions,i.e.
Weakacids,suchasethanoicacid,CH3COOH(aq),existmainlyascovalentmoleculeswiththeformulaCH3COOH(aq),asmallnumberofwhichdissociateintoions,i.e.
Reactionsofacids
WithmetalsAcidsreactwithmetalsabovehydrogeninthereactivityseries(althoughitwouldbedangeroustouseaGroupImetaloranythingbelowcalciuminGroupIIinareactionwithacids).Thegeneralequationis
Themetaldissolves,bubblesareseenandasolutionofthesaltformswhosecolourdependsonthemetalused.Anexampleis
WithcarbonatesAcidsreactwithbothsolubleandinsolublecarbonates.Thegeneralequationis
Solidcarbonatesdissolve,bubblesareseenandanaqueoussolution(whosecolourdependsonthecarbonateused)ofthesaltforms.Anexampleis
Thistypeofreactionoccurswithcarbonateseitherassolidsorasaqueoussolutions.
WithbasesAcidsreactwithallbasestoformsaltsandwater(inthecaseofammonia,anammoniumsaltistheonlyproduct).Thegeneralequationis
Withinsolublebasesthesoliddissolvesandasolutionforms.Nobubblesareseenbecausenogasisproduced.Anexampleis
Withalkalistherearenoobservationsasacolourlesssolutionisproducedfromtwocolourlesssolutions.Anexampleis
WithammoniaThegeneralequationis
Anexampleis
StrongandweakacidsStrongandweakacidscanbedistinguishedexperimentallybyanyofthefollowingexperimentalmethods.
Strongacid WeakacidAddUniversalindicatorpaper Red
orpH0–2
Orangeyellow
orpHlessthan7andmorethan2
Addmagnesiumribbon Bubblesquickly
Bubblesslowly
Addaninsolublecarbonate,e.g.calciumcarbonate
Bubblesquickly
Bubblesslowly
Setupacircuitwithabulb Bulblightsbrightly
Bulblightsdimly
Examiner’stip•Manystudentshavetheimpressionthatifanacidisweak,moreoftheweakacidisrequiredtoneutralisethesameamountofalkalicomparedtoastrongacid.Theamountofanyacidthatisrequiredtoneutraliseagivenamountofalkalionlydependsonthenumberofmolesoftheacidandnotwhethertheacidisstrongorweak.
•Somequestionsbeginwith,‘Howwouldyoudistinguish…?’Forexample,‘Howwouldyoudistinguishbetweenastrongacidandaweakacid?’Theintentionisthatthestudentgivesbriefexperimentaldetailswithresults,forexample,addmagnesiumribbontobothandbubblesoccurmuchfasterwiththestrongacidthantheweakacid.Insteadstudentsoftenanswerwiththeory,forexample,strongacidsionisecompletelyandweakacidsionisepartially.Althoughthesestatementsarecorrecttheydonotaddressthequestion,‘Howwouldyoudistinguish…?’Itisalsonecessarytostatewhatwouldhappenwithbothsubstancesortogiveacomparison,ratherthansaythestrongacidproducesbubblesrapidlywithoutreferencetotheweakacid.
BasesBasesaremetallicoxidesorhydroxides(orammonia)whichneutraliseacidstoformasaltandwater.
•Basesthataresolubleinwaterarecalledalkalis.•Basesthatdonotdissolveinwaterareknownasinsolublebases.
Alkalisarehydroxidesoroxidesofmetals(orammonia)thatproduceOH−(aq)whendissolvedinwater.Thetwolaboratorystrongalkalisareaqueoussodiumhydroxideand
potassiumhydroxide.Theybothexistcompletelyasionsinaqueoussolution.
Ammoniasolutionisaweakbase.AnaqueoussolutionofammoniaexistsmainlyasNH3molecules,asmallnumberofwhichreactwithwatermoleculestoproduceions.
NH3(aq)acceptsH+fromH2O(l)formingNH4+(aq)whichshowsthatbasesareprotonacceptorsbydefinition.
ReactionsofbasesAsdescribedabove,basesneutraliseacids.
Application:PlantsneedsoiltobeataspecificpHtogrowwell.Soilaciditycanbeneutralisedbytheadditionofasuitablebase,suchascalciumhydroxide,Ca(OH)2(s),alsoknownasslakedlime.
WithammoniumsaltsInsolublebasesandalkalisreactwhenheatedwithammoniumsalts.Ammoniagasisgivenoff.Thegeneralequationis
Anexampleis
Examiner’stipManystudentsdonotapplytherulesforwritingformulaeofioniccompoundswhenwritingequationsofthistype.Inthisexample,theformulaofcalciumchlorideisoftenwrittenasCaCl,withoutconsiderationofthechargesontheionspresent(seeChapter3fordetails).
IndicatorsMethylorangeandlitmuscanbeusedtoindicatewhethersubstancesareacidsoralkalis,butgivenoinformationaboutacidstrength.
Methylorange LitmusColourinacidicsolution Red RedColourinneutralsolution Orange PurpleColourinalkalinesolution Yellow Blue
ThepHscale(Figure8.1)usesnumberstodistinguishbetweenacidsandalkalisofdifferentstrengths.
AqueoussolutionsofacidshavepHlessthan7.AqueoussolutionsofalkalishavepHmorethan7.NeutralsolutionshavepHof7.ThelowerthepHnumbers,thestrongertheacid.ThehigherthepHnumbers,
thestrongerthealkali.StrongacidsareregardedashavingapHof0–2.Strongalkalisareregarded
ashavingapHof12–14.UniversalindicatorhasdifferentcolourstoshowapproximatepHnumbers
asshown.
ApproximatepH ColourofUniversalindicatorLessthan3 Red3–6 Orange–yellow7 Green8–11 BlueMorethan11 Purple
OxidesOxidescanbeputintofourcategories.1Acidicoxidesarenon-metallicoxidesthatneutralisealkalisandformsalts.Examplesarecarbondioxide,CO2,nitrogendioxide,NO2,andsulfurdioxide,SO2.Theseoxidesalldissolveinwaterandreactwiththewatertoformacids.
2Basicoxidesaremetallicoxidesthatneutraliseacidsandformsalts.Examplesaremagnesiumoxide,MgO,calciumoxide,CaO,andcopper(II)oxide,CuO.Somebasicoxidesdissolveinwatertoformalkalinehydroxides,whereasothersareinsolubleinwater.
3Somenon-metallicoxidesareneutraloxideswhichmeansthattheydonotreactwitheitheracidsoralkalis.Anexampleiscarbonmonoxide,CO.
4Somemetallicoxidesareamphotericoxideswhichmeansthattheyreactwithbothacidsandalkalistoformsalts.Examplesarezincoxide,ZnO,andaluminiumoxide,Al2O3.
SaltsSaltsareionicsubstancesformedwhenthepositivehydrogenionsinanacidarereplacedbypositivemetallicionsorammoniumions.Saltscanbemadebydifferentexperimentalmethods,dependingontheir
solubilityinwater.
Solubilityofsalts
PreparationofsaltsTherearethreegeneralmethodsofpreparationofsolidsalts.Inallcases,detailsofcrystallisation,washinganddryingcanbefoundin
Chapter2.Method1:Addinganexcessofaninsolublebaseorinsolublecarbonateor
metaltoadiluteacid.Method2:Titrationusinganacidandanalkaliorasolublecarbonate.Methods1and2canonlybeusedforsaltsthataresolubleinwater.Method3:Mixingtwosolutionstoobtainasaltthatisinsolubleinwaterby
precipitation.
Examiner’stipUsingacidstopreparesalts:
•hydrochloricacid,HCl,isusedtopreparechlorides•nitricacid,HNO3,isusedtopreparenitrates•sulfuricacid,H2SO4,isusedtopreparesulfates(orhydrogensulfates)•thepositiveioninthesaltcomesfromtheinsolublebaseorinsolublecarbonateormetaloralkali.
Examples
Method1Copper(II)sulfatecrystalscanbemadebythismethod.
•Addsolidcopper(II)oxideorcopper(II)hydroxideorcopper(II)carbonatetodilutesulfuricacidinabeaker.
•Stirand/orheatthemixture.•Addthesoliduntilitwillnolongerdissolvewhichmeansalltheacidhasreactedandthesolidisinexcess(ifcopper(II)carbonateisused,therewillbenofurtherbubblingwhenalltheacidhasreacted).
•Filterofftheexcesssolid.•Usingcopper(II)oxidetheequationis
•Makepurecrystalsofcopper(II)sulfatebycrystallisation,washinganddrying(seeChapter2).
Method2Sodiumsulfatecrystalscanbemadebythismethod.
•Carryoutsufficienttitrationstofindouttheexactvolumeofdilutesulfuricacidinaburettethatisrequiredtoneutraliseagivenpipettevolumeofaqueoussodiumhydroxide.Usemethylorangeasasuitableindicator.
•Repeattheprocesswithoutusingtheindicator,butusingthesamevolumeofacidandalkaliasusedinthetitration.
•Makepurecrystalsofsodiumsulfatebycrystallisation,washinganddrying(seeChapter2).
Iftwicethevolumeofthesamedilutesulfuricacidisused,orhalfthevolumeofthesameaqueoussodiumhydroxide,sodiumhydrogensulfate(anacidsalt)isthesaltproduced,accordingtotheequation
Crystalsofsodiumhydrogensulfatecanbemadefromthissolutioninthesameway.
Method3Leadsulfatecanbemadebythismethod.
•Becauseleadnitrateistheonlysolubleleadsalt,leadnitratesolutionmustbeusedandmixedwithanysolublesulfate,suchasaqueoussodiumsulfate(dilutesulfuricacidcouldalsobeusedbecauseitcontainsaqueoussulfateions).
•Theprecipitateofleadsulfatemustberemovedbyfiltrationandthenwashedwithdistilledwateranddriedinalowovenoronawarmwindowsill.
Theequationis
Anionicequationforanyprecipitationreactionshowsthetwoaqueousionsontheleftandthesolidprecipitateontherightinallcases.Inthiscase
Exam-stylequestions1Therearethreegeneralmethodsofpreparationofsolidsalts.Method1:Addinganexcessofaninsolublebaseorinsolublecarbonateormetaltoadiluteacid.
Method2:Titrationusinganacidandanalkaliorasolublecarbonate.Methods1and2canonlybeusedforsaltsthataresolubleinwater.Method3:Mixingtwosolutionstoobtainasaltthatisinsolubleinwaterbyprecipitation.
Foreachofthefollowingsaltpreparations,choosemethod1,2or3,nameanyadditionalreagentwhichisrequiredandwritetheequation.
aCobalt(II)chloridestartingwiththeinsolublecompoundcobalt(II)carbonate.
[3marks]bTheinsolublesaltleadiodide,fromaqueousleadnitrate.
[3marks]cPotassiumnitratefromaqueouspotassiumhydroxide.
[3marks][Total:9marks]
2Givefullexperimentaldetailsofhowyouwouldmakepuredrycrystalsofmagnesiumsulfatestartingwithmagnesiumcarbonate.Youshouldincludeanequationinyouranswer.
[Total:10marks]3Youareprovidedwithamixtureofscandiumoxideandcopper(II)oxidewhicharebothsolids.Scandiumoxideisanamphotericoxideandcopper(II)oxideisabasicoxide.Describehowyoucouldobtainasampleofpure
copper(II)oxidefromthemixture.Bothsolidsareinsolubleinwater.[Total:6marks]
9ThePeriodicTable
KeyobjectivesBytheendofthissection,youshouldbeabletodescribethePeriodicTableasameansofclassifyingelementsanditsusetopredictpropertiesofelements•describethechangefrommetallictonon-metalliccharacteracrossaperiod•describesomepropertiesoftheGroupIelementslithium,sodiumandpotassiumandpredictthepropertiesofotherGroupIelements•describesomepropertiesoftheGroupVIIelementschlorine,bromineandiodineandpredictthepropertiesofotherGroupVIIelements•describethenoblegasesinGroupVIIIor0asbeingunreactive,monatomicgasesandexplainthisintermsofelectronicstructure•beabletodescribesomepropertiesoftransitionelements
•describeandexplaintherelationshipbetweengroupnumber,numberofoutershellelectronsandmetallic/non-metalliccharacter•identifytrendsingroupsgiveninformationabouttheelementsconcerned•knowthattransitionelementshavevariableoxidationstates.
KeytermsPeriodicTable
Containsalltheelementsarrangedinorderofprotonnumber
Groups TheverticalcolumnsinthePeriodicTablePeriods ThehorizontalrowsinthePeriodicTableAlkalimetals
GroupI.Metalswhichreactwithwatertoproducealkalinesolutions(suchaslithium,sodiumandpotassium)
Halogens GroupVII.Non-metallic,diatomicmoleculesNoblegases
Group0.Unreactive,monatomic,colourlessgases
Transitionelements
MetalsfoundintheelongatedsectionofthePeriodicTablebetweenGroupsIIandIII(suchascopper,ironandnickel)
PeriodicTableThePeriodicTablecontainstheelementsarrangedinorderofprotonnumber(atomicnumber).
•Theverticalcolumnsofelementsarecalledgroups.•Thehorizontalrowsofelementsarecalledperiods.
AcrossPeriods2and3,thereisagradualchangefrommetalsontheleft-handsidetonon-metalsontheright-handside.
Commonerror•Studentsoftenthinkthattheelementsarearrangedinorderofmassnumberorrelativeatomicmass.Mostoftherelativeatomicmassesoftheelementsdoincreaseasprotonnumbersincrease,butinsomeplacestherelativeatomicmassdecreases,e.g.argontopotassium.
Atomsofelementsinthesamegrouphavethesamenumberofelectronsintheoutershell.Thenumberofelectronsintheoutershelldeterminesthechemicalpropertiesoftheelement.Thenumberofshellspresentinanatomofanelementisthesameasthe
periodnumberinwhichtheelementisfoundinthePeriodicTable.Potassiumhasprotonnumber19andthereforeitselectronconfigurationis
2,8,8,1.ThereisoneelectronintheoutershellwhichmeanspotassiumisinGroupIofthePeriodicTable.Potassiumhaselectronsinfourshells,whichmeansitisinPeriod4.
GrouppropertiesGroupITheGroupIelementsareknownasthealkalimetalsbecausetheyreactwithwatertoproducealkalinesolutions.TheGroupIelementsareveryreactivemetals.TheGroupIelements,inorderofincreasingprotonnumber,arelithium,
sodium,potassium,rubidium,caesiumandfrancium.Onlylithium,sodiumandpotassiumarefoundinschoollaboratories,becauserubidium,caesiumandfranciumaredangerouslyreactiveandfranciumisalsoradioactive.
Examiner’stipExaminationquestionsoftenaskforobservations,orask,‘Whatwouldyousee…?’inaparticularchemicalorphysicalchange.WhenGroupImetalsreactwithwater,suitableobservationsarethemetaldisappears•themetalmelts•bubbles/fizzing/effervescence(thesealleffectivelymeanthesamething)•themetalfloatsandmovesaroundonthesurface•potassiumandthoseGroupImetalsbelowpotassiumburstintoflames.
However,thefollowingarenotobservations:namesoftheproducts•agasisgivenoff(itisnotpossibletoseeacolourlessgas)•analkalinesolutionforms(itisnotpossibletoseethatasolutionisalkalinebyobservationalone)•colourchangeofanindicator(unlessanindicatorismentionedinthequestion).
TheGroupIelements
•arestoredunderoilbecausetheyreactrapidlywithoxygenintheair•aregoodconductorsofheatandelectricity•canbecutwithaknifebecausetheyaresoft•areshinywhencut,buttarnishrapidlyduetoreactionwithoxygenintheair•havelowdensitiesandlowmeltingpointsandboilingpointscomparedtotransitionmetals.
ThemeltingpointsandboilingpointsofGroupIelementsdecreasedownthegroup.Densitieschangeinanirregularmanner.
ReactionwithwaterAllGroupIelementsreactvigorouslywithwateratroomtemperature.Thereactionsareusuallycarriedoutinaglasstrough.Observationsarethemetalmovesaroundandfloatsonthesurfaceofthewater•thereactionproducesheatwhichcausesthemetaltomeltasitreacts•bubblesofhydrogengasaregivenoff•themetalrapidlydisappears,formingacolourlesssolutionofthealkalinemetalhydroxide.
Theequationforthereactionwithsodiumis
TheequationswithalltheotherGroupImetalswouldbeexactlythesame(includingbalancingnumbers)ifthesymbolsfortheothermetalsreplacedthatofNaintheaboveequation.ReactivityoftheGroupImetalsincreasesdownthegroup.Sodiummoves
aroundthesurfacefasterthanlithiumandalsodisappearsmorerapidly.Potassiumburstsintoalilacflame.Ifrubidiumandcaesiumareaddedtowateranexplosivereactionoccurs,whichiswhytheyarenotkeptinschoollaboratories.
GroupVIITheGroupVIIelementsareknownasthehalogens.TheGroupVIIelementsinorderofincreasingprotonnumberarefluorine,
chlorine,bromine,iodineandastatine.Onlychlorine,bromineandiodinearefoundinschoollaboratories.Fluorineistooreactivetobeusedinschoolsandastatineisradioactive.Theelementsareallnon-metallicandexistasdiatomicmolecules(molecules
containingtwoatoms).TheappearanceofthosefoundinschoolsisshowninTable9.1.
Table9.1Physicalappearanceofchlorine,bromineandiodine
Element AppearanceatroomtemperatureChlorine PalegreengasBromine Red-brownliquidIodine Grey-blacksolid
Thecoloursbecomedarkerasthegroupisdescended.Thechangeinphysicalstatefromgastoliquidtosoliddownthegroupindicatesanincreaseinmeltingpointandboilingpointanddensitydownthegroup(duetoanincreaseinthestrengthofintermolecularforces).
HalogendisplacementreactionsIfhalogens(orsolutionsofthehalogensinwater)areaddedtocolourlessaqueoussolutionsofpotassiumhalides(chlorides,bromidesandiodides),displacementreactionsoccurintheexampleshighlightedinTable9.2.
Anexampleofadisplacementreactioniswhenchlorinedisplacesbrominefromanaqueoussolutionofpotassiumbromide.Theequationis
AscanbeseenfromTable9.2,chlorinedisplacesbromineandiodine•brominedisplacesiodine,butdoesnotdisplacechlorine•iodinedoesnotdisplacechlorineorbromine.
Halogenshigherupthegroupcandisplacethoselowerdown.Alternatively,wecansaythatreactivityincreasesupGroupVII.ThisisoppositetothetrendinreactivityshowninGroupI.Wecanusethisinformationtomakepredictionsabouttheotherhalogensand
halides(seetheExam-stylequestionsattheendofthechapter).
Group0TheGroup0elementsareknownasthenoblegases.TheGroup0elements,inorderofincreasingprotonnumber,arehelium,neon,argon,krypton,xenonandradon.
TheGroup0elements
•areallcolourlessgases•areallmonatomic;thismeanstheyexistasindividualatomsbecausetheiratomsallhaveafulloutershellofelectronsanddonotformcovalentbondswithotherGroup0atomstoformdiatomicmolecules•areveryunreactivebecausetheiratomsallhaveafulloutershellofelectrons,thereforetheydonotneedtoshare,loseorgainelectronstoachieveafulloutershellofelectrons.
UsesofGroup0elements•Heliumisusedinfillingballoonsbecauseofitslowdensity.Itsunreactivenaturemeansthathazardsassociatedwithhydrogenareabsentifheliumisused.
•Argonisusedinlightbulbstopreventthetungstenfilamentfromburning.Thisisbecauseargondoesnotsupportburningduetoitsunreactivity.
TransitionelementsTransitionelementsareallmetals(alsoknownastransitionmetals).TheyarefoundintheelongatedsectionofthePeriodicTablebetweenGroupsIIandIII.Commonexamplesarecopper,ironandnickel.
PhysicalpropertiesTransitionelementsareallmetalsandshowtheusualphysicalpropertiesofmetals(seeChapter10).Inaddition,transitionmetalsareharderandstrongerthantheelementsinGroupI•havehigherdensitiesthantheelementsinGroupI•havehighermeltingpointsthantheelementsinGroupI.
Chemicalproperties•Transitionelementsformcolouredcompounds,e.g.copper(II)sulfatecrystalsareblueandpotassiummanganate(VII)ispurple.
•Theelementsandtheircompoundsshowcatalyticactivity,e.g.ironintheHaberprocessandvanadium(V)oxideintheContactprocess.
•Transitionelementshavevariableoxidationstates,e.g.ironcanformFe2+andFe3+ions.
Exam-stylequestions1ThediagrambelowshowspartofthePeriodicTable.UsethelettersAtoHinclusivetoanswerthequestionsthatfollow.Eachlettermaybeusedonce,morethanonce,ornotatall.Givetheletterthatrepresents
atheGroupIelementthatismostreactive[1mark]
btheGroupVIIelementthatismostreactive[1mark]
catransitionelement[1mark]
danelementinPeriod3[1mark]
eanelementwhoseatomshavefourelectronsintheiroutershell.[1mark]
[Total:5marks]
2Lithiumisaddedtocoldwaterinaglasstrough.aGivethreeobservationsyouwouldexpecttomake.
[3marks]bWriteachemicalequationforthereactionthatoccurs.Includestate
symbols.[3marks]
cIfmethylorangeisaddedtotheliquidinthetroughafterthereaction,whatcolourwoulditturnto?
[1mark][Total:7marks]
3Usethetableofhalogendisplacementreactionstoanswerthefollowingquestions.Writechemicalequations(withstatesymbols)andionicequationsforthereactionsthatoccurbetween
achlorineandaqueouspotassiumiodide[4marks]
bbromineandaqueouspotassiumiodide.[4marks]
[Total:8marks]4UsethePeriodicTabletopredictreactionsthatwouldoccurbetween
afluorineandaqueouspotassiumchloridebastatineandaqueouspotassiumfluoridecbromineandaqueouspotassiumastatidediodineandaqueouspotassiumfluoride.
Ifyoupredictthatareactionwouldoccur,writeachemicalequationforthereaction.Ifyoupredictthatareactionwouldnotoccurwritenoreaction.
[Total:6marks]5Copperandironhavevariableoxidationstates.Statetheformulaeof
acopper(I)oxide[1mark]
bcopper(II)nitrate[1mark]
ciron(II)chloride[1mark]
diron(III)sulfate.[1mark]
[Total:4marks]
10Metals
KeyobjectivesBytheendofthissection,youshouldbeableto
•describethephysicalpropertiesofmetals•describethechemicalpropertiesofmetals•identifyrepresentationsofalloysfromdiagramsofstructureandexplainintermsofpropertieswhyalloysareusedinsteadofpuremetals•placethefollowinginorderofreactivity:potassium,sodium,calcium,magnesium,zinc,iron,(hydrogen)andcopperbyreferencetothereactionswith•waterorsteam
•dilutehydrochloricacidand•thereductionoftheiroxideswithcarbon•deduceanorderofreactivityfromagivensetofexperimentalresults•describetheeaseofobtainingmetalsfromtheiroresbyreferencetothepositionofelementsinthereactivityseries•describeandstatetheessentialreactionsintheextractionofironfromhematite•describetheconversionofironintosteelusingbasicoxidesandoxygen•knowthataluminiumisextractedfromitsorebauxitebyelectrolysis•statetheadvantagesanddisadvantagesofrecyclingiron/steelandaluminium•namesomeusesofaluminium•namesomeusesofcopper
•namesomeusesofmildsteelandstainlesssteel
•describethereactivityseriesasrelatedtothetendencyofametaltoformitspositiveion,illustratedbyreactionwith
•theaqueousions•theoxidesoftheotherlistedmetals
•accountfortheapparentunreactivityofaluminiumintermsoftheoxidelayerwhichadherestothemetal•describeandexplaintheactionofheatonthehydroxides,carbonatesandnitratesofthelistedmetals•describeinoutlinetheextractionofzincfromzincblende•describeinoutlinetheextractionofaluminiumfrombauxite,includingtheroleofcryoliteandthereactionsattheelectrodes•explaintheusesofzincingalvanisingandfor
makingbrass.
PropertiesofmetalsPhysicalpropertiesThephysicalpropertiesofmetalsareshowninTable10.1(seealsoChapter9).
Table10.1Physicalpropertiesofmetals
Property MetallicpropertyPhysicalstateatroomtemperature Solid(exceptmercury)Malleability GoodDuctility GoodAppearance Shiny(lustrous)Meltingpointandboilingpoint UsuallyhighDensity UsuallyhighConductivity(thermalandelectrical) Good
Metalsaremalleable(canbehammeredintodifferentshapes)andductile(canbedrawnintowires).Althoughmetallicbondsarestrong,theyarenotrigid,
whichmeansthattherowsofionsinmetalscanslideovereachotherwhenaforceisapplied(Figure10.1).Thisisbecausetheionsinametallicelementareallthesamesize.Whenametallicobjectisrequiredtobeparticularlystrong,analloyisoften
usedinsteadofametallicelement.Inalloyssuchasbrass,bronzeandsteel,themetallicelementismixedwithsmallamountsofanotherelementorelements.Theionsoratomsoftheotherelementsareadifferentsizetothoseofthemainelement.Thispreventstherowsofmetallicionsfromslidingovereachother.Therefore,analloyretainsitsshapemuchbetterthanapuremetalwhenaforceisapplied.
ReactivityseriesMetalscanbeplacedinorderoftheirreactivitywithotherelements.Thisisknownasthereactivityseries(seeright).
Thepositionsofcarbonandhydrogenareinsertedforreferencetodisplacementreactions.Potassium,sodiumandcalciumreactwithcoldwatertoproduceanaqueoussolutionofanalkalinehydroxideandhydrogengas(seeChapter9),forexample
Magnesium,zincandironreactextremelyslowlywithcoldwater.Theydo
reactmorerapidlywhenheatedwithsteam,forexample
Metalsabovehydrogeninthereactivityseriesreactwithdiluteacidstoproduceasaltandhydrogen(thereactionofpotassiumandsodiumwithacidswouldbetoodangeroustocarryoutinschoollaboratories).
Metalsbelowhydrogeninthereactivityseries,e.g.copper,donotreactwithcoldwater,steamordiluteacids.Oxidesofmetalsbelowcarboninthereactivityseriesarereducedtothemetal
whenheatedwithcarbon,forexample
DisplacementreactionsMetalswilldisplaceothermetalsfromaqueoussolutionsoftheirions,e.g.magnesiumwilldisplacecopperfromanaqueoussolutioncontainingitsionssuchascopper(II)sulfatesolution.
Theionicequationis
Thereactionoccursbecausemagnesiumisamorereactivemetalthancopper.Thismeansthatmagnesiumhasagreatertendencytoformpositiveionsthancopper.Ifcopperwasaddedtoasolutioncontainingmagnesiumions,suchas
aqueousmagnesiumsulfate,noreactionoccurs.Metalshigherupinthereactivityserieswilldisplacethoselowerdown.Asimilarreactionoccursifametaloxideisheatedwithamorereactive
metal.Forexample,ifzincpowderisheatedwithcopper(II)oxide,thefollowingreactionoccurs:
However,ifzincoxideisheatedwithcopper,noreactionoccurs.
However,ifzincoxideisheatedwithcopper,noreactionoccurs.
DeducingorderofreactivityofmetalsToputmetalsinorderofreactivity,reactionscanbeattemptedasin,forexample,addingametaltoanaqueoussolutioncontainingionsofanothermetalorheatingametalwiththeoxideofanothermetal.Anothermethodinvolvestheuseofelectrochemicalcells(seeChapter5)
containingtwodissimilarmetalsinanelectrolyte(Figure10.3).
Inelectrochemicalcellsofthistype,themorereactivemetalisthenegativeterminal,becausethemorereactivemetalistheonewiththegreatertendencytoreleaseelectronsandformpositiveions.Thereadingonthevoltmeter(knownasthecellEMF(electromotiveforce))representsthedifferenceinreactivitybetweenthetwometals.
Examiner’stipIfacellwassetupwithAandBinanelectrolyte
•Awouldbethenegativeelectrode•Bwouldbethepositiveelectrode•thecellEMFwouldbe1.10V+0.46V=1.56V.
Cell1tellsusthatmetalA(−)ishigherinthereactivityseriesthanmetalC(+)and1.10Visameasurementofthedifferenceinreactivity.Cell2tellsusthatmetalC(−)ishigherinthereactivityseriesthanmetalB
Cell2tellsusthatmetalC(−)ishigherinthereactivityseriesthanmetalB(+)and0.46Visameasurementofthedifferenceinreactivity.Therefore,theorderofthethreemetalsinthereactivityseriesis
UnexpectedbehaviourofaluminiumAluminiumappearsbetweenmagnesiumandzincinthereactivityseries.However,aluminiumoftenappearstobemuchlessreactivethanitspositioninthereactivityseriessuggests.Forexample,ifaluminiumisplacedinanaqueoussolutionofcopper(II)
sulfate,thereishardlyanyreactionuntilthelayerofaluminiumoxideisremoved.Thisisbecausealuminiumissoreactivethatitreactswiththeoxygenintheairformingalayerofaluminiumoxidewhichadherestothealuminiumunderneathandprotectsthemetal.Suchalayercanbedeliberatelyplacedontothesurfaceofaluminiummetalbyaprocesscalledanodising.Thismeansthataluminiumcanbeusedforthingswhichwouldnotnormallybeassociatedwithreactivemetals,suchasaeroplanebodies,cookingfoilandpotsandpans.
ThermaldecompositionofmetalliccompoundsTheeasewithwhichmetallichydroxides,nitratesandcarbonatesdecomposewhenheatedisrelatedtothepositionofthemetallicelementinthereactivityseries.Compoundsofmetalsatthetopofthereactivityserieseitherrequirealarge
amountofheatenergytodecomposeortheydonotdecomposeatall.Wesaythattheyarestabletoheat.Asthereactivityofthemetallicelementsdecreases,theircompoundsarelessstabletoheatandaremoreeasilydecomposedbyheat.Inthecaseofnitrates,GroupInitratesdecomposepartially,whereasthe
nitratesoftheotherlistedmetalsdecomposemorecompletely.
HydroxidesHydroxidesofveryreactivemetalsdonotdecomposewhenheated.Thoseoflessreactivemetalsdecomposeintotheiroxidesandgiveoffsteam,for
example,
NitratesAllnitratesdecomposewhenheated,butnitratesdonotalldecomposetoproducesimilarproducts.GroupInitrates(exceptlithiumnitrate)decomposepartiallytoformthe
metallicnitriteandoxygengas,forexample,
Othermetallicnitratesoflessreactivemetalsdecomposemorecompletely,producingthemetaloxideandgivingoffnitrogendioxide,abrowngas,andoxygengas.
CarbonatesGroupIcarbonates(exceptlithiumcarbonate)donotdecomposewhenheated.Thecarbonatesofalltheotherlistedmetalsdecomposeintotheoxideandcarbondioxidegas.Theamountofheatrequiredfordecompositionisgreaterforcarbonatesofveryreactivemetals,suchascalciumcarbonate,
butcarbonatesoflessreactivemetalslikecopper(II)carbonatedecomposeatmuchlowertemperatures
Commonerror•Studentsoftenstatethatmetalshigherupinthereactivityseriesdecomposewithmuchmoredifficultythanthoselowerdown.Itisnotthemetalsthatdecompose;itistheircompounds,namelythehydroxides,nitratesand
carbonates.Metals,beingelements,cannotdecomposebecauseanelementissomethingthatcannotdecomposeintoanythingsimpler.
ExtractionofmetalsMetalscanbeextractedfromtheiroresmoreeasilyaswegodownthereactivityseries.Therearethreegeneralmethodsofextractingmetalsfromtheirores:
1Metalsofaveragereactivity,e.g.ironandzinc,areextractedbychemicalreductionusingcarbon/carbonmonoxideasreducingagents.
2Metalsoflowreactivity,e.g.copper,areextractedbyachemicalreductionusingcarbon/carbonmonoxideasreducingagentsor
belectrolysisofaqueoussolutionscontainingtheirions.3Veryreactivemetals,e.g.potassium,sodium,calcium,magnesiumandaluminium
acannotbeextractedbychemicalreductionbecausetheoresarenotreducedbychemicalreducingagents,suchascarbon,carbonmonoxideorhydrogenbcannotbeextractedbyelectrolysisofaqueoussolutions,becausehydrogenisformedatthecathodeinsteadofthemetal(seeChapter5).
Therefore,thesemetalsareextractedbyelectrolysisofmoltenioniccompounds.
AluminiumAluminiumisextractedfrombauxitewhichisimpurealuminiumoxide,Al2O3.Bauxiteisfirstpurifiedandthenelectrolysisiscarriedout.Aluminiumoxideisnotreducedbycarbonmonoxideoranyothercommonreducingagent,whichmeanselectrolysishastobeused.Thisisexpensiveduetothehighcostofelectricity.Aluminiumoxidehasameltingpointof2017°Cwhichwouldrequirea
largeamountofheatenergytoachieveandthereforewouldfurtherincreasecosts.Instead,thealuminiumoxideisdissolvedinanotheraluminiumcompound,moltencryolite,Na3AlF6.Theadvantagesofdissolvingaluminiumoxideinmoltencryolitearetheelectrolytecanbemaintainedintheliquidstatebetween800°Cand1000°C,atemperatureconsiderablylowerthan2017°C,whichgreatlyreducesenergycosts•cryoliteimprovestheconductivityoftheelectrolyte.
Aluminiumoxideinmoltencryolitebehavesinthesamewayasmoltenaluminiumoxideasfarastheproductsofelectrolysisareconcerned.Electrolysisiscarriedoutinasteeltankusingcarbon(graphite)electrodes.
Theanodesarecarbon(graphite)blocksthatareloweredintotheelectrolyte.Thecathodeisthecarbon(graphite)liningofthetank.Theelectrodereactionsare
Moltenaluminiumcollectsatthebottomofthetankandissiphonedoff.Theoxygenthatisproducedattheanodereacts,atthehightemperatureofthe
cell,withthegraphiteanodesproducingcarbondioxidegaswhichescapes.Thustheanodesburnawayand
havetobereplacedregularly.Thehighcostofelectricityisthelargestexpenseforthisprocess,whichis
carriedoutinregionswherecheapelectricity,e.g.fromhydro-electricpower,isavailable.
IronIronisextractedfromhematite,impureiron(III)oxide,Fe2O3,inablastfurnace.
Hematite,coke,C,andlimestone,CaCO3,arefedintothetopoftheblastfurnace.Ablastofhotairentersnearthebottom.Cokereactswiththeoxygenintheairformingcarbondioxide.Thereactionis
highlyexothermicandprovidesthehightemperaturerequiredfortheotherreactions.
Thecarbondioxidereactswithmorecokehigheruptoproducecarbonmonoxideinanendothermicreaction.
Thecarbonmonoxidereducesthehematitetomolteniron
andthemoltenirontricklestothebottomandistappedoff.Thefunctionofthelimestoneistoremovethemainimpurityintheironore
whichissilicondioxide(silicon(IV)oxide).
Examiner’stipYoushouldknowthatthefunctionofthecokeis
•toactasafuel,becausethehighlyexothermicreactionwithoxygenprovidesahighenoughtemperatureforthereductionofthehematite•toproducecarbonmonoxidewhichreducesthehematitetoiron.
Thelimestonedecomposesatthehightemperatureinsidetheblastfurnace.
Calciumoxidethenreactswithsilicon(IV)oxidetoformcalciumsilicatewhichformsamoltenslagasaseparatelayerabovethemolteniron(itislessdensethaniron).
Slagisusedbybuildersandroadmakersforfoundations.Theironproducedintheblastfurnaceiscalledpigironorcastiron.It
containsabout4%carbonanditsuseislimitedbecauseitisbrittle.Themajorityofpigironisconvertedintosteel.
ConversionofironintosteelPigironcontainsabout4%carbonandothernon-metallicimpurities,suchasphosphorus,siliconandsulfur.Theproductionofsteelinvolvesremovingmostofthenon-metallicimpurities(exceptsmallamountsofcarbon)•addingsmall,controlledamountsoftransitionelements(additives).Thetransitionelementsthatareusedandtheproportionsinwhichtheyareaddeddeterminethepropertiesofthesteelsthatareformed.
Theimpuritiesareremovedbythebasicoxygenprocess:
•Oxygenathighpressureisblownontothesurfaceofthemoltenmetal.•Thiscausesoxidationofsomeofthecarbontocarbondioxide,andsulfurtosulfurdioxide,bothofwhichescapeasgases.
•Siliconisoxidisedtosilicon(IV)oxideandphosphorusisoxidisedtophosphorus(V)oxide,whicharebothsolids.
•Calciumoxide(quicklime)isadded.Thisreactswiththesolidoxidestoproducecalciumsilicateandcalciumphosphatewhichareremovedasmoltenslag.
•Theamountofcarbonremainingcanbecontrolleddependingonthetypeofsteelrequired.Mildsteelcontainsapproximately0.5%carbonand99.5%iron.
•Transitionelementsarethenaddedinexactquantitiestoproducedifferenttypesofsteel.Stainlesssteelcontains18%chromium,8%nickel,aswellas74%iron.
ZincZincisextractedfromzincblende(impurezincsulfide,ZnS)byaprocesssimilartotheblastfurnaceprocess,usingcarbonasthereducingagent.Zincblendeispurified.Zincsulfideisthenheatedverystrongly(roasted)in
acurrentofairtoconvertitintozincoxide.
Thezincoxideisheatedverystronglywithpowderedcokeinafurnace.
Thezincvapourthencoolsandcondenses.Itisremovedasmoltenzinc.
CopperTherefiningofcopperisbasedontheelectrolysisofaqueouscopper(II)sulfateusingcopperelectrodes(seeChapter5).Refiningmeanspurification.Smallamountsofimpuritiesincoppercutdown
itselectricalconductivitynoticeably.Theelectrolysisiscarriedoutusingimpurecopperastheanode(+),pure
copperasthecathode(−)andaqueouscopper(II)sulfateastheelectrolyte.Thecopperfromtheanode(+)goesintosolutionaspositiveions.
Theimpuritieseithergointosolutionaspositiveionsorfallofftheanodeanddepositatthebottomofthecontainer.Theyareremovedfromtimetotime.Coppermetalformsatthecathode(−)
Thuspurecopperisformedonthecathode(whichwasoriginallymadeofpurecopper).Thecathodeisremovedandreplacedfromtimetotime.
Recyclingaluminiumandiron/steelMuchaluminiumisrecycledfromdrinkscans.Ironandsteelarerecycledfromhouseholdgoods.Theadvantagesofrecyclingare
•Naturalresourcesofbauxiteandhematitewilllastlonger.•Drinkscansandotherhouseholdobjectswillnottakeuplargeamountsofroominlandfillsites,wheretheyreactwithoxygenfromtheair(thusremovingit)astheyareoxidised.
•Thecostofrecyclingismuchlowerthanextractingthemetalsfromtheirores,largelyduetoreducedenergycosts.
Thedisadvantagesofrecyclingare
•Thecollectionandsortingofdomesticmaterialstoberecycledcanbeexpensive,timeconsumingandrequireenergy.
•Thepurityofmetalsobtainedbyrecyclingmaynotbeashighasthatobtainedbyextractionfrommetalores.
UsesofmetalsUsesofsomemetals,relatedtotheirproperties,areshowninTable10.2.
Table10.2Usesofsomemetals
Metal Use PropertiesAluminium Manufactureof
aircraftStrengthandlowdensity
Foodcontainers ResistancetocorrosionMildsteel Carbodies Highmalleability,hightensilestrength
Machinery Stainlesssteel
Cutlery Tough,doesnotcorrodeChemicalplants
Copper Electricalwiring HighelectricalconductivityCookingutensils Appearance,highthermalconductivity,high
meltingpointZinc Galvanising Protectsironfromrusting
Makingbrass Brassismoreductileandstrongerthancopper
Exam-stylequestions1Theresultsofsomeexperimentscarriedoutbyaddingametaltoaqueoussolutionscontainingionsofanothermetalareshowninthetablebelow,where✓isdisplacementreactionoccursand✗isnoreactionoccurs.
aPutthefourmetalsinorderofreactivity,startingwiththemostreactivefirst.
[1mark]
[1mark]bWriteachemicalequationforthereactionoccurringwhenmetalBis
addedtoA(NO3)2(aq).
[1mark]cWriteanionicequationforthereactionoccurringwhenmetalCisadded
toD(NO3)2(aq).[1mark]
dIfmetalsAandBarebothplacedinanaqueoussolutionofanelectrolyte,whichmetalwouldbethenegativeelectrode?Explainyouranswer.
[1mark]eWriteanequationforthereactionoccurringwhenmetalBisaddedtothe
oxideofmetalDandthemixtureisheated.[1mark]
[Total:5marks]2Youareprovidedwithamixtureofpowderedcopperandpowderedzinc.Describehowyouwouldobtainasampleofpurecopperfromthemixture.Youshouldgiveallobservationsforanyreactionsthatyoudescribe.
Note:neithermetaldissolvesinwater.[Total:4marks]
3Impurenickelcanberefinedusingamethodsimilartothemethodusedtorefinecopper.Drawadiagramoftheapparatusthatyouwouldusefortherefiningofnickel.Youshouldfullylabelallthechemicalsubstancesinthediagram.
[Total:4marks]4Copper(II)oxide→adddilutenitricacid→bluesolutionB→crystallisation→bluecrystalsB→heatcrystalsstrongly→blacksolidC+browngasD+colourlessgasE
aNameB,C,DandE.[4marks]
bWriteequationsforthereactionsthatoccurwhen
icopper(II)oxideisaddedtodilutenitricacidiibluecrystalsBareheatedstrongly.
[4marks]
[4marks]cWhencopper(II)oxideisreactedwithdilutenitricacid,bluesolutionBis
produced.Givethenamesoftwoothersubstancesthatcouldbeusedinsteadofcopper(II)oxidetoproducebluesolutionBwhenreactedwithdilutenitricacid.
[2marks][Total:10marks]
11Airandwater
KeyobjectivesBytheendofthissection,youshouldbeabletodescribethechemicaltestsforwaterusingcobalt(II)chlorideandcopper(II)sulfate•describeinoutlinethetreatmentofthedrinkingwatersupplyintermsoffiltrationandchlorination•namesomeusesofwaterinindustryandinthehome•statethecompositionofcleandryairasbeingapproximately78%nitrogen,21%oxygenandtheremainderbeingamixtureofnoblegasesandcarbondioxide•namethecommonpollutantsintheairasbeingcarbonmonoxide,sulfurdioxide,oxidesofnitrogenandleadcompoundsandtostatethesourcesofthesepollutants•statetheadverseeffectsofthesecommonpollutantsanddiscusswhythesepollutantsareofglobalconcern•statetheconditionsrequiredfortherustingofiron•describeandexplaincoatingmethodsofrustprevention•describetheneedfornitrogen,phosphorusandpotassiuminfertilisers•describethedisplacementofammoniafromammoniumsalts(seeChapter8)
•describetheseparationofoxygenandnitrogenfromliquidairbyfractionaldistillation•describethecatalyticremovalofoxidesofnitrogenfromcarexhaustgases•describeandexplaingalvanisingandsacrificialprotectionintermsofthereactivityseries(seeChapter10)asamethodofrustprevention•describeandexplaintheessentialconditionsfortheHaberprocessforthemanufactureofammoniaincludingthesourcesofhydrogen(fromhydrocarbonsorsteam)andnitrogen(fromair)(seealsoChapter7onratesandequilibriumandChapter14oncracking).
KeytermsFiltration Atreatmentfordrinkingwaterthatinvolvespassingimpurewater
throughscreenstofilteroutfloatingdebrisChlorinationAtreatmentfordrinkingwaterinwhichsmallamountsofchlorine
areaddedtokillbacteriaFractionaldistillation
Amethodforseparatingthecomponentsofair
Catalyticconverters
Presentincarexhaustsinsomecountriestoremovepollutantgasesandconvertthemintonon-pollutantgases
Rust Ironformsrustwhenitisexposedtooxygenandwater.Rustishydratediron(III)oxide
Fertilisers Substancesaddedtothesoiltosupplynutrientsthatareessentialforthegrowthofplants
WaterTestforwaterTestsforthepresenceofwatercanbecarriedoutusinganhydrouscobaltchlorideoranhydrouscopper(II)sulfate.ThecolourchangesareshowninTable11.1.
Table11.1
Originalcolour FinalcolourAnhydrouscobaltchloride Blue PinkAnhydrouscopper(II)sulfate White Blue
Cobaltchloridepaperiscommonlyusedinsteadoftheanhydroussolid.
Examiner’stipThesesubstancesarenotusedasatestforpurewater.Thecolourchangesshownoccurifwateroranythingcontainingwater(includingallaqueoussolutions)isused.
Purityofwatercanbedeterminedbymeasuringtheboilingpoint,whichis100°Cat1atmospherepressure.
TreatmentofdrinkingwaterImpurewaterismadefitfordrinkingbythefollowingmethods:Filtration:Thisinvolvespassingimpurewaterthroughscreenstofilteroutfloatingdebris.Chlorination:Smallamountsofchlorinegasareaddedtokillbacteria.
Commonerror•Studentsoftenstatethatchlorineisaddedtopurifywater.Watercontainingsmallamountsofaddedchlorineshouldnotbedescribedaspure.
UsesofwaterWaterisusedinindustry
•asasolvent•asacoolant•forcleaning•asachemicalreactant(e.g.inthehydrationofethene;seeChapter15).
Waterisusedinthehome
•incooking•incleaning•fordrinking.
AirAirisamixture,andincommonwithallmixturesitscompositioncanvary,particularlyinindustrialandruralareas.Theapproximatecompositionofcleandryairis
78%nitrogen21%oxygen0.03%carbondioxide•1%argon.
Verysmallamountsofothernoblegasesarealsopresent.
Examiner’stipHydrogeniscommonlyandincorrectlyregardedbystudentsasaconstituentofair.
FractionaldistillationofliquidairInordertoseparatethecomponentsofair,•itiscooledtoremovecarbondioxideandwatervapourassolids•itiscompressedandexpandedcontinuallytoliquefyitat−200°C
•theliquidairisthenseparatedbyfractionaldistillationintoliquidoxygen,liquidnitrogenaswellasliquidGroup0elements.
AirpollutionCommongaseouspollutants,theirsourcesandrelatedpollutionproblemsareshowninTable11.2.
Table11.2
Pollutant Source PollutionproblemsCarbonmonoxide,CO
Incompletecombustionoffossilfuels
Toxic
Sulfurdioxide,SO2 Combustionoffossilfuelscontainingsmallamountsofsulfur
Irritationoftherespiratorysystem
Dissolvesinrainwaterformingacidrainwhichcausesdamagetobuildingsmadeof,e.g.limestoneandmarble
Oxidesofnitrogen(oftenrepresentedasNOxtosignifymorethanoneoxideofnitrogen)includingnitrogendioxide,NO2
Nitrogenandoxygen(bothfromtheair)reacttogetheratveryhightemperaturesincarengines
Productionoflow-levelozone(respiratorysystemirritant)Nitrogendioxide,NO2,dissolvesinrainwaterformingacidrainwhichcausesdamagetobuildingsmadeof,forexample,limestoneandmarblePhotochemicalsmog
Leadcompounds Leadcompoundsinpetrol(onlyincountrieswhereleadedpetrolisstillused)
Toxic
CatalyticconvertersCatalyticconvertersarepresentincarexhaustsinsomecountries.Theirpurposeistoremovepollutantgasesandconvertthemintonon-
pollutantgases.Thecatalystspresentincatalyticconvertersincludealloyscontaining
transitionelements,suchasplatinum,rhodiumandpalladium.Thereareseveralreactionsoccurringinsidecatalyticconvertersincluding
and
Thuspollutantgasesareconvertedintonon-pollutantgases.
CommonerrorsThereareprobablymoremisunderstandingsconcerningatmosphericpollutionthananyotherChapter.Manystudentsthinkthatallatmosphericpollutantsareresponsibleforall
environmentalproblems,particularlyglobalwarming.ItisveryimportantthatstudentsstudyTable11.2carefullyandlearnaboutthesourcesandpollutionproblemscausedbyeachindividualpollutant.Thefollowingpointsneedtoinbenoted:
•Oxidesofnitrogenareproducedbyreactionbetweennitrogenandoxygen,bothofwhichcomefromtheair.Thenitrogenisnotpresentinthefuel.
•Oxidesofnitrogenareoftenmistakenlythoughttobeproducedincarexhaustsasopposedtoincarengines.
•Pollutantsareremovedbycatalyticconvertersincarexhausts.•Ifcarsdonothaveefficientcatalyticconverters,thegaseouspollutantsentertheatmospherethroughthecarexhaust.
•Sulfurdioxideisoftenregardedasbeingproducedbythedeliberateburningofsulfur,asopposedtothesmallamountsofsulfurimpuritiesinfossilfuels.
RustingofironRustcanbedescribedashydratediron(III)oxidewithaformulathatcanberepresentedasFe2O3.xH2O(xisusedbecausetheamountofwaterofcrystallisationvariesfromonesampleofrusttoanother).Irononlyformsrustwhenitisexposedtooxygen(e.g.fromtheair)and
water.
PreventionofrustingRustingcanbepreventedbycoatingtheironwithpaintoilorgreaseplasticothermetals,suchaszinc(whichisknownasgalvanising).
Thesemethodsallworkbypreventingoxygenandwatercomingintocontactwiththeironandthuspreventingareactiontakingplace.
SacrificialprotectionSomemetalswillcontinuetopreventironfromrustingevenifthesurfaceisscratched.Suchmetalsmustbeaboveironinthereactivityseries,butmustnotbesoreactivethattheywillreactrapidlywithwaterthemselves.Zincandmagnesiumarebothusedinthisway.Tin,whichisbelowironinthereactivityseries,willonlyprotectironifitisnotscratched(Figure11.1).Inaddition,barsofzincmaybeattachedtothehullsofshipswithout
attemptingtocoverthesurfaceoftheiron.Rustingwillnotoccurinthesecircumstances(Figure11.2).
Thefirststageofrustingistheoxidationofirontoiron(II)ionsbyoxygeninthepresenceofwater:
Ifzincispresent,evenifthezincisscratched,thezincwillbeoxidisedinpreferencetotheiron.
Thisoccursbecausezincisamorereactivemetalthanironandthereforezincformspositiveionsmorereadilythanirondoes.Theelectronstravelfromthezinctotheiron.Theirondoesnotloseelectrons,
whichmeansthatoxidationofiron(whichisthefirststageofrusting)doesnotoccur.Iftinisusedinsteadofzinc,whenthetinisscratchedtheironwillbeoxidised
inpreferencetothetinbecauseironisamorereactivemetalthantin.Thustinonlypreventsrustingwhenitisnotscratched.
Commonerror•Itisacommonerrorforstudentstostatethatifgalvanisedironorsteelisscratchedandexposedtoairandwater,thezincformsrustinsteadofiron.Ironistheonlymetalthatcanformrust.
NitrogenandfertilisersFertilisersaresubstancesthatareaddedtothesoiltosupplynutrientsthatareessentialforthegrowthofplants.Fertiliserscontainnitrogen,phosphorusandpotassium(NPK).Veryfewplantscanutilisenitrogenfromtheair.Thenitrogenmustbe
suppliedtotheplantsintheformoffertiliserscontainingammoniumsaltsornitrates.
TheHaberprocessNitrogenobtainedfromthefractionaldistillationofliquidair(seeearlierinthisChapter)andhydrogenfromhydrocarbonsbycracking(seeChapter14)orfromsteam,arefirstconvertedintoammoniaintheHaberprocess.Nitrogenandhydrogenreacttogetherinareversiblereactiontoproduce
ammonia.
Thegasesare
•passedoveracatalystofiron•atatemperatureof450°C•atapressureof200atmospheres.
Themixtureemergingfromthecatalystchambercontainsabout15%ammonia.Theammoniaisseparatedfromtheunreactednitrogenandhydrogenbyliquefyingit,andtheunreactednitrogenandhydrogenarepassedoverthecatalystagain.Eventuallyallthenitrogenandhydrogenareconvertedintoammonia.
Theammoniaisusedfor
•themanufactureoffertilisers,suchasammoniumsulfateandammoniumphosphate•themanufactureofnitricacidwhichisconvertedintofertilisers,suchasammoniumnitrate.
Exam-stylequestions1Thepercentageofoxygenintheaircanbedeterminedbypassingairbackwardsandforwardsoverheatedcopper,usingtheapparatusshowninFigure11.3.Thecopperwasinexcess.
Thevolumeofairatthestartwas100cm3.Astheairwaspassedbackwardsandforwards,thevolumeofairdecreased.Thefinalvolume,measuredatroomtemperatureandpressure,was79cm3.
aWhatcolourchangeinvolvingthecopperwouldyouexpecttosee?[2marks]
bWriteachemicalequationforthereactionintheheatedtube.[2marks]
cWhydoesthevolumeofairdecrease?[1mark]
dWhatismeantbythephrase‘thecopperwasinexcess’?[1mark]
eIdentifythemaingaspresentinthe79cm3remainingattheend.[1mark]
[Total:7marks]
2TheequationfortheequilibriumoccurringintheHaberprocessis
Theforwardreactionisexothermic.Thereactioniscarriedoutbypassingnitrogenandhydrogenoveracatalystofironatatemperatureof450°Candapressureof200atmospheres.
aWhatismeantbythetermcatalyst?[2marks]
[2marks]bCopyandcompletethetable.
cAmmoniaisconvertedintofertiliserssuchasammoniumsulfate.
iWhichsubstancewouldbeaddedtoammoniatoproduceammoniumsulfate?
[1mark]iiWriteachemicalequationforthereactionoccurringinquestion(c)(i)
above.[2marks]
dAmmoniaisalsoconvertedintothefertiliserammoniumphosphate.Completetheequationfortheformationofammoniumphosphatefromammonia.(Youmayleaveoutthestatesymbols).
[2marks][Total:13marks]
12Sulfur
KeyobjectivesBytheendofthissection,youshouldbeabletonamesomesourcesofsulfur•knowthatsulfurisusedinthemanufactureofsulfuricacid•knowthatsulfurdioxideisusedtobleachwoodpulpinthemanufactureofpaper,andasafoodpreservative(bykillingbacteria)
•describethemanufactureofsulfuricacidbytheContactprocess,includingessentialconditionsandreactions•describethepropertiesandusesofdiluteandconcentratedsulfuricacid.
KeytermsSulfuricacid
Usedformakingdetergentsandfertilisers,usedastheacidincarbatteriesandusedformakingpaints,dyesandsyntheticfibres
Dilutesulfuricacid
Atypicalstrongacid
Sulfurdioxide
Usedforbleachingofwoodpulpinthemanufactureofpaperandinthepreservationoffoodbykillingbacteria
Sulfurisfoundastheimpureelementinsulfurbedsbelowtheground.Itisalsofoundasmetallicores,mainlysulfides,suchaszincblende(seeChapter10).
ManufactureofsulfuricacidSulfurisusedinthemanufactureofsulfuricacidintheContactprocess.Thesulfurisfirstconvertedintosulfurdioxide,SO2(g),byheatingitstronglyinair.
Thesulfurdioxideisthenmixedwithoxygenandthetwogasesarepassedoveracatalystofvanadium(V)oxide,V2O5(s)atatemperatureof450°Catapressureof1–2atmospheres
producinganequilibriummixturecontainingthetworeactantgasesandsulfurtrioxide,SO3(g).
Sulfurtrioxideisthendissolvedin98%concentratedsulfuricacidtoproduceoleum,H2S2O7(l).
Theoleumisthenaddedtothecorrectamountofwatertoproducesulfuricacidoftherequiredconcentration.
Examiner’stipIfsulfurtrioxideisaddedtowater,areactionoccursandsulfuricacidistheproduct.
However,thisreactionisveryexothermicandtheheatgivenoffissufficienttovaporisethesulfuricacidandathickmistisproducedthatisverydifficulttocollect.Thustheprocessisnotcarriedoutinthisway.
UsesofsulfuricacidSulfuricacidisused
•formakingdetergents•formakingfertilisers(seeChapter11)•astheacidincarbatteries•formakingpaints,dyesandsyntheticfibres.
PropertiesofdilutesulfuricacidDilutesulfuricacidisatypicalstrongacid.Itisadiproticacid,whichmeansthatonemoleculeofsulfuricacidreleases
twoprotonswhenitformsionsinaqueoussolution.
Itcanalsoreleaseoneproton.
Therefore,sulfuricacidcanformsaltscalledsulfatescontainingSO4andacidsaltscalledhydrogensulfates,containingHSO4−(seeChapter8).Itreacts
•withmetalsabovehydrogeninthereactivityseriestoproduceasaltandhydrogen(seeChapters8and10)•withbases(includingalkalis)toformsalts(sulfates)andacidsalts(hydrogensulfates),andwater(seeChapter8)•withcarbonatestoformasalt,waterandcarbondioxide(seeChapter8).
UsesofsulfurdioxideSulfurdioxideisused
•forthebleachingofwoodpulpinthemanufactureofpaper•inthepreservationoffoodbykillingbacteria.
Exam-stylequestions
1SulfuricacidismadeintheContactprocess.Thecatalysedreaction
takesplaceatatemperatureof450°Candapressureof1–2atmospheres.Theforwardreactionisexothermic.aNamethecatalystintheabovereaction.
[1mark]bSuggestwhyatemperaturebelow450°Cisnotused.
[1mark]cWhatwouldhappentotherateatwhichequilibriumwouldbereachedifa
higherpressurewasused?Explainyouranswer.[2marks]
dSuggesttworeasonswhyapressureabove1–2atmospheresisnotused.[2marks]
eHowisthesulfurtrioxideproducedintheabovereactionconvertedintosulfuricacid?Giveequationsforanyreactionsthatyoureferto.
[2marks][Total:8marks]
2Writechemicalequationsforthereactionsbetweendilutesulfuricacidandacopper(II)carbonate[2marks]
baqueoussodiumhydroxideinwhichsodiumsulfateisproduced[2marks]
caqueouspotassiumhydroxideinwhichpotassiumhydrogensulfateis
produced[1mark]
dzinc.[1mark]
[Total:6marks]
13Inorganiccarbonchemistry
KeyobjectivesBytheendofthissection,youshouldbeabletostatethatcarbondioxideandmethanearegreenhousegasesandexplainhowtheymaycontributetoclimatechange•statetheformationofcarbondioxideinavarietyofreactions•statethesourcesofmethane,includingdecompositionofvegetationandwastegasesfromdigestioninanimals•describethemanufactureofquicklime(calciumoxide)fromcalciumcarbonate(limestone)intermsofthermaldecomposition•namesomeusesofquicklimeandslakedlime•nametheusesofcalciumcarbonate
•describethecarboncycleinoutlinetoincludetheprocessesofcombustion,respirationandphotosynthesis.
KeytermsGreenhousegases
GaseswhichabsorbinfraredradiationproducedbythesolarwarmingoftheEarth’ssurface
Greenhouseeffect
GreenhousegasespreventingheatenergyfromescapingfromtheEarth
CarbondioxideandmethaneCarbondioxideandmethanearebothgreenhousegases.ThismeansthattheirbondsabsorbinfraredradiationproducedbythesolarwarmingoftheEarth’ssurface.ThispreventsheatenergyfromescapingfromtheEarth.Thiseffect,knownasthegreenhouseeffect,isthoughttoleadtoglobalwarming.Carbondioxideisformedinlaboratoryreactionsandinindustrialand
environmentalprocessesbycompletecombustionofcarbon-containingsubstancessuchasfossilfuels(seeChapter14)•asaproductofthereactionbetweenanacidandacarbonate(seeChapter8)•fromthethermaldecompositionofcarbonates(seeChapter10)•byrespirationinwhichglucoseisoxidisedtocarbondioxideandwaterinlivingthings.
Carbondioxideisremovedfromtheatmospherebyreactionwithwaterintheleavesofgreenplantscontainingchlorophyll,inthepresenceofsunlight.Thereactionisknownasphotosynthesis.Theproductsareglucoseandoxygen.
Methaneisformedbydecompositionofvegetation•Wastegasesfromdigestioninanimals.
Examiner’stipTheequationsforrespirationandphotosynthesisarethereverseofoneanother.
Thecarboncycleshowshowthepercentageofcarbondioxideintheatmosphereremainsapproximatelyconstantat0.03%duetothevariousprocessesbywhichitisreleasedintoandabsorbedfromtheatmosphere.
CarbonatesQuicklime(calciumoxide)ismanufacturedbythethermaldecompositionofcalciumcarbonate(limestone).Thisisareversiblereaction.
Theprocessiscarriedoutinalimekiln.Adraftofaircarriesoutthecarbondioxideandcausestheequilibriumtoshifttotheright(seeChapter7).Eventuallyallthecalciumcarbonateisconvertedintocalciumoxide.Calciumoxidereactswithwatertoproducecalciumhydroxide(slakedlime).
UsesofquicklimeandslakedlimeBothquicklime,CaO,andslakedlime,Ca(OH)2,canbeusedtoneutraliseacidicsoil(seeChapter8)•neutraliseacidicindustrialwasteproducts,e.g.fluegasdesulfurisation,inwhichsulfurdioxidegasinfactoryemissionsisneutralisedbyquicklimeorslakedlime.
UsesoflimestoneLimestone,CaCO3,isusedinthemanufactureofiron(seeChapter10)•manufactureofcement.
Exam-stylequestions1Statewhyitisnecessarytoneutralisesoilacidityandgivethechemicalnameofacompoundwhichcanbeaddedtosoilinordertoneutraliseit.
[Total:2marks]2aCarbondioxidecanbeproducedbythe
icompletecombustionofoctaneiithermaldecompositionofcalciumcarbonateiiireductionofiron(III)oxidebycarbonmonoxideivreactionbetweencalciumcarbonateanddilutehydrochloricacid.
Writeachemicalequationforeachofthesereactions.Youmayomitstatesymbols.
[Total:7marks]bCarbondioxidecanberemovedfromtheEarth’satmosphereby
photosynthesis.
iNametheotherreactantinphotosynthesis.[1mark]
iiNamethetwoproductsofphotosynthesis.[2marks]
iiiStatetwoconditionsthatarerequiredforphotosynthesis.[2marks]
[Total:12marks]
14Organicchemistry1
KeyobjectivesBytheendofthissection,youshouldbeableto
•nameanddrawthestructuresofmethane,ethane,ethene,ethanol,ethanoicacidandtheproductsofthereactionsstatedinthisChapter
•describetheconceptofahomologousseriesasa‘family’ofsimilarcompoundswithsimilarchemicalpropertiesduetothepresenceofthesamefunctionalgroup
•describethepropertiesofalkanesasbeinggenerallyunreactiveexceptintermsofburning
•describethebondinginalkanes•distinguishbetweensaturatedandunsaturatedhydrocarbons•frommolecularstructures•byreactionwithaqueousbromine•namethefossilfuels:coal,naturalgasandpetroleum•namemethaneasthemainconstituentofnaturalgas•describepetroleumasamixtureofhydrocarbonsanditsseparationbyfractionaldistillation(seeChapter2)
•nametheusesofthefractions•describethepropertiesofmoleculeswithinafraction•describethemanufactureofalkenesandhydrogenbycracking
•nameanddrawthestructuresofunbranchedalkanesandalkenescontaininguptofourcarbonatomspermolecule
•describethegeneralcharacteristicsofhomologousseries•recallthatcompoundsinahomologousserieshavethesamegeneralformula
•describeandidentifystructuralisomerism•describesubstitutionreactionsofalkaneswithchlorine
•describethepropertiesofalkenesintermsofadditionreactionswithbromine,hydrogenandsteam.
KeytermsEmpiricalformula
Thesmallestwholenumberratiooftheatomsofeachelementinacompound
Molecularformula
Thisisthenumberofatomsofeachelementinonemoleculeofasubstance
Displayedformula
Alltheatomsandallthebondsinonemoleculeofthecompound
Structuralformula
Thisshowshowatomsarearrangedingroupsofatoms
Structuralisomerism
Theexistenceofcompoundswiththesamemolecularformulabutdifferentstructuralformulae
Organiccompoundsarecovalentcompoundscontainingcarbonatomsbondedtohydrogen(always),aswellasoxygen,thehalogensandnitrogenatoms.
HomologousseriesOrganiccompoundsbelongto‘families’ofsimilarcompoundsknownashomologousseries,examplesofwhicharealkanes,alkenes,alcohols,carboxylicacidsandesters.
Homologousseriesarecompoundswhichhave
•thesamegeneralformula:thisisbecauseeachmemberdiffersfromthepreviousmemberbya-CH2–groupofatoms
•thesamechemicalproperties:thisisbecauseeachmemberhasthesamefunctionalgroup
•physicalpropertiesthatshowaconstantgradation,e.g.meltingpointsandboilingpointsthatshowalmostconstantincreases.
Afunctionalgroupisagroupofatomsthatallmembersofahomologousserieshaveincommonandwhichisresponsibleforallmembersofahomologousserieshavingthesamechemicalproperties.
FormulaeoforganiccompoundsOrganiccompoundshaveseveraldifferenttypesofformulae(Table14.1).Theseare
•Empiricalformula.Thisisthesmallestwholenumberratiooftheatomsofeachelementinacompound(seeChapter4).
•Molecularformula.Thisisthenumberofatomsofeachelementinonemoleculeofasubstance(seeChapter4).Thisgivesnoinformationabouthowtheatomsarejoinedtogether.
•Displayedformula.Thisshowsalltheatomsandallthebondsinonemoleculeofthecompound.
•Structuralformula.Thisshowshowatomsarearrangedingroupsofatoms.
Examiner’stipTheterm‘displayedformula’isnotalwaysusedinexaminationquestions.Insteadstudentsaremorelikelytobeaskedtodrawthestructureofamoleculeshowingallatomsandallbonds.
StructuralisomerismStructuralisomerismistheexistenceofcompoundswiththesamemolecularformulabutdifferentstructuralformulae(andthereforedifferentdisplayedformulae).
ExampleTherearetwostructuralisomerswiththemolecularformulaC4H10.ThesehavedifferentstructuralanddisplayedformulaeasshowninTable14.2.Becausetheyaretwodifferentcompoundstheyhavetwodifferentnames.Butaneisoftenreferredtoasastraightchainoranunbranchedmolecule,
becausethecarbonatomsarearrangedoneafteranother.2-methylpropaneisoftenreferredtoasabranchedchainmolecule.
CommonerrorsStudentsoftenconfusethetwowordsisotopesandisomers:
•Isotopes(seeChapter3)areatomsofthesameelementwiththesameprotonnumberbutdifferentnucleonnumber.
•Structuralisomersarecompoundswiththesamemolecularformulabutdifferentstructuralformulae.
Allotropeisanothersimilarwordwhichyoumayhavecomeacross.Thisreferstodifferentcrystallineformsofthesameelement,e.g.diamondandgraphite.
•Whendrawingdisplayedformulae,allcarbonatomsmusthavefourbonds(sticks)andallhydrogenatomsmusthaveonebond.Oxygenatomsmusthavetwobonds,halogenatomsonebondandnitrogenatomsthreebonds.
HydrocarbonsHydrocarbonsarecompoundscontainingcarbonandhydrogenonly.Alkanesandalkenesareimportantexamplesofhydrocarbons.
Examiner’stipWhenasked‘Whatismeantbythetermhydrocarbon?’itisimportanttousetheword‘only’,aswellasreferringtocompoundscontainingcarbonandhydrogen.Compoundssuchasethanol,C2H5OH,containcarbonandhydrogen,butbecausetheyalsocontainoxygen,theyarenothydrocarbons.
Alkanes•AlkanesaremembersofahomologousserieswiththegeneralformulaCnH2n+2.
•Thenamesofalkanesallendin-ane.•Alkanesaresaturatedhydrocarbons;thismeansthatalltheirbondsaresinglebonds(eitherC–CorC–H).
•Alkanesdonotcontainafunctionalgroup,becausetheycontainonlyC–CandC–Hbondswhichcanbesaidofallotherhomologousseriesoforganiccompounds.
Examiner’stipThenamesofalkanesareimportantbecauseunbranchedmembersofallotherhomologousseriesarenamedafterthealkanewiththesamenumberofcarbonatoms.Thereforeallorganicmoleculeswith
•1carbonatombeginwithmeth-•2carbonatomsbeginwitheth-•3carbonatomsbeginwithprop-•4carbonatomsbeginwithbut-.
Someexamplesoforganiccompoundswithtwocarbonatomsare
PropertiesofalkanesAlkanesareunreactivecomparedtoalkenes.Thisisbecausethesinglebondsinalkanesneedalotofenergytobreak,butthedoublebondsinalkenesneedlessenergytobeconvertedintosinglebonds(whichiswhathappenstoalkenesinadditionreactions).
CombustionAlkanesundergocombustioninairoroxygenproducingenergy,whichiswhyalkanesareusedasfuels.Completecombustionoccursinexcessoxygen.Theproductsarecarbon
dioxideandwater,forexample
Incompletecombustionofalkanesinalimitedsupplyofairoroxygenleadstotheproductionof(toxic)carbonmonoxideaswellaswater(seeChapter11).
ReactionwithchlorineItisnotpossibletoaddatomstoalkanemoleculeswithoutfirstremovingatoms.Thereforealkanesundergosubstitutionreactionsasopposedtoadditionreactions.Asubstitutionreactionisareactioninwhichoneatomorgroupofatomsis
replacedbyanotheratomorgroupofatoms.Whenmethaneisreactedwithchlorineinthepresenceofultravioletlight,a
substitutionreactionoccursinwhichonechlorineatomreplacesonehydrogenatominmethane.Theorganicproductischloromethane.
Unlessthechlorinesupplyislimited,thereactionshouldnotbeusedasamethodofpreparationofchloromethanebecausethechloromethanereactswithmorechlorine.Allthehydrogenatomsaresubstitutedbychlorineatoms,oneatatime,untilallthehydrogenatomshavebeenreplacedbychlorineatoms.Hydrogenchloridegasisproducedateachstage.
Similarreactionsoccurwithotheralkanesandchlorine.
Alkenes•AlkenesaremembersofahomologousserieswiththegeneralformulaCnH2n.
•AlkenescontainafunctionalgroupwhichisaC==C.•BecauseaC==Cmustbepresentinallalkenes,thereisnoalkenewithonecarbonatom.
•Thenamesofalkenesallendin–ene.•Alkenesareunsaturatedhydrocarbons.•Notallthebondsaresinglebondsinunsaturatedhydrocarbons.Unsaturatedmoleculescontainatleastonecarbon–carbondoublebondorcarbon–carbontriplebond.
TestingforunsaturationAqueousbromine(brominewater)canbeusedtodistinguishbetweensaturatedandunsaturatedsubstances(Table14.4).
Table14.4Usingbrominewatertotestforsaturatedandunsaturatedsubstances
Typeofmolecule Saturated UnsaturatedEffectofaddingaqueousbromine
Nochange(aqueousbromineremainspalebrown)
Aqueousbrominechangesfrompalebrowntocolourless
StructuralisomerisminalkenesThereisonlyonestructureforthealkenescontainingtwoandthreecarbonatoms:etheneandpropene.
Withfourcarbonatoms(C4H8)therearetwounbranchedalkenes,becausethedoublebondcanbeintwodifferentpositionsinthecarbonchain(Table14.5).
Thenumber1inbut-1-enemeansthatthedoublebondisbetweencarbonatoms1and2.Thenumber2inbut-2-enemeansthatthedoublebondisbetweencarbon
atoms2and3.Structuresofunbranchedalkenescontaininguptofourcarbonatomsare
showninTable14.6.
Examiner’stipWhenstudentsareaskedtogivethestructuresoftwoalkeneswithmolecularformulaC4H8,theyoftendraw
Thesearebothbut-1-enedrawndifferently.Oneisthesameastheother,onlyit
Thesearebothbut-1-enedrawndifferently.Oneisthesameastheother,onlyitisdrawnbacktofront.Thedoublebondisbetweencarbonatoms1and2inbothcases.
Isomersmustbedifferentmolecules,notthesamemoleculedrawndifferently.
ReactionsofalkenesItispossibletoaddatomstoalkenemoleculeswithoutfirstremovingatoms.Thereforealkanesundergoadditionreactionsasopposedtosubstitutionreactions.
Additionmeanstwomoleculesjointogethertomakeonemolecule.Intheadditionreactionsofalkenes,thedoublebondbecomesasinglebond
andanatomoragroupofatomsjoinsontobothcarbonatomsthatformedthedoublebond.
WithbromineIfbrominetheelement(Br2(l))oraqueousbromine(Br2(aq))isaddedtoanyalkene,anadditionreactionoccurs.Ifthealkeneisethene,theproductis1,2-dibromoethane.
Withhydrogen(industrial)Ifetheneandhydrogenarepassedoveranickelcatalystat200°C,theproductisethane.Thisadditionreactioniscalledhydrogenation.
Hydrogenationisusedinindustrytoconvert(unsaturated)vegetableoilsto(saturated)margarinebyreactionwithhydrogeninthepresenceofanickelcatalyst.
Withsteam(industrial)
Ethenereactswithsteamusingacatalystofphosphoricacid(H3PO4),at300°Cand60atmospherespressure.Theproductisethanol.
Thisreaction,knownashydrationofethene,isusedtomanufactureethanol(seeChapter15).ThereactionsofethenearesummarisedinFigure14.3.
FuelsandthepetroleumindustryFossilfuelsarefuelsformedbynaturalprocessesovermillionsofyearsasaresultofthedecayofburieddeadorganisms.Examplesarecoal,naturalgasandpetroleum(crudeoil).Fossilfuelsareknownasnon-renewablefuelsbecauseoncetheyrunouttheycannotbereplaced.Theyarealsoknownasafiniteresource.Methane,CH4,isthemainconstituentofnaturalgas.Petroleum(crudeoil)isamixtureofhydrocarbonswhichisseparatedby
fractionaldistillation.Theprocessdoesnotproduceindividualhydrocarbons,butinsteadproducesmixturesofhydrocarbonsknownasfractions.Thesefractions(Figure14.4)aremixturesofhydrocarbonswhichhaveanarrowrangeofboilingpoints.Astheboilingpointrangesincrease,thehydrocarbonscontainanincreasingnumberofcarbonatoms.
ManufactureofalkenesbycrackingFractionaldistillationofpetroleumproduces
•higherboilingpointfractionsthatareinexcessofrequirement•insufficientamountsofthelowerboilingpointfractions,whichareparticularlyindemandas
•fuelsforpetrolengines(alkanesbetweenC5andC10)•monomers(short-chainalkenes)fortheproductionofpolymers•sourcesoforganicchemicals.
Alkenesaremanufacturedbycrackinglong-chainalkanesobtainedfrompetroleum.Crackingreferstodecompositionofalkaneswhichspecificallyinvolvesbreakingcarbon–carbonbondstoformsmallermolecules.Therearetwotypesofcracking:catalyticandthermal.
CatalyticcrackingThealkanemoleculesarepassedovercatalystsknownaszeolitesattemperaturesof500°C.
ThermalcrackingThisusesahighertemperature(800°C)thancatalyticcracking.
Crackingproduces
•short-chainalkenesforproductionofpolymersandorganicchemicals•alkanescontainingbetween5and10carbonatomsasfuelsforpetrolengines•hydrogenwhichisusedtomanufactureammonia.
Ifalong-chainalkaneiscracked,differentmoleculesofthealkanemaybreakindifferentplacestogiveavarietyofproductswhichcanalsobeseparatedby
differentplacestogiveavarietyofproductswhichcanalsobeseparatedbyfractionaldistillation.Forexample,C14H30moleculescouldcrackintooctaneandpropene
orethene,propeneandhydrogen.
Examiner’stipStudentswillnotbeaskedtopredictwhatthedefiniteproductsofcrackingarewithoutbeingprovidedwithfurtherinformation.
Sampleexam-stylequestionWriteanequationforthecrackingofhexaneintoanalkaneandanalkene,bothhavingthesamenumberofcarbonatoms.
AnswerBoth
and
areacceptableanswers,becausebothproduceanalkaneandanalkenewiththesamenumberofcarbonatoms.Neitheranswerismorecorrectthantheother.Ifthequestionhadspecifieda1:1moleratiooftheproducts,onlyC6H14→
C3H8+C3H6wouldbecorrect.
Exam-stylequestions1Drawthestructures,showingalltheatomsandallthebonds,oftwodifferentunbranchedalkeneswiththemolecularformulaC5H10.Youarenotexpectedtonamethealkenes.
[2marks]2Ethanereactswithchlorineinasubstitutionreaction.aUnderwhatconditiondoesthereactiontakeplace?
[1mark]bNametheorganicproductformedwhenethaneandchlorinereactina1:1
moleratio.[1mark]
cIfanexcessofchlorineisused,givethemolecularformulaofoneotherorganicproductthatcouldform.
[1mark][Total:3marks]
3Propenereactswith
iaqueousbromineiihydrogeniiisteam.
aWhatnameisgiventothetypeofreactionthatoccursinallthreecases?[1mark]
bStatetheobservationyouwouldexpecttoseeinreaction(i)ifexcesspropeneisused.
[2marks]cWritedownthemolecularformulaeoftheproductsthatforminreactions
(i),(ii)and(iii).[3marks]
dWhatisthenameofthecatalystusedin(ii)?[1mark]
[Total:7marks]4Alkanesareconvertedintoalkenesbycracking.aWhatisthemolecularformulaofthealkanethatcontainsninecarbon
atoms?[1mark]
bDrawthestructureandgivethenameofanalkenewithfourcarbonatoms.[2marks]
cWriteanequationforthecrackingofoctane,C8H18,into
ianalkaneandanalkeneformedinthemoleratio1:2
iihydrogenandtwootherproducts.[2marks]
[Total:5marks]
15Organicchemistry2
KeyobjectivesBytheendofthissection,youshouldbeableto
•describethemanufactureofethanolbyfermentationandbythecatalyticadditionofsteamtoethene
•outlinetheadvantagesanddisadvantagesofthesetwomethodsofmanufacturingethanol
•describethepropertiesofethanolintermsofburning•nametheusesofethanolasasolventandasafuel
•nameanddrawthestructuresofunbranchedalcoholsandcarboxylicacidscontaininguptofourcarbonatomspermolecule•nameanddrawthestructuresoftheesterswhichcanbemadefromunbranchedalcoholsandcarboxylicacids,eachcontaininguptofourcarbonatoms
•describethepropertiesofaqueousethanoicacid
•describetheformationofethanoicacidbytheoxidationofethanolwithacidifiedpotassiummanganate(VII)•describeethanoicacidasatypicalweakacid•describethereactionofacarboxylicacidwithanalcoholinthepresenceofacatalysttogiveanester
•definepolymersaslargemoleculesbuiltupfromsmallmoleculescalledmonomers
•explainthedifferencesbetweenadditionandcondensationpolymerisationandunderstandthatdifferentpolymershavedifferentunitsand/ordifferentlinkages
•describetheformationofpolyetheneasanexampleofaddition
polymerisation•namesomeusesofplasticsandofman-madefibressuchasnylonandTerylene•describethepollutionproblemscausedbynon-biodegradableplastics
•deducethestructureoftheadditionpolymerfromagivenalkeneandviceversa•describetheformationofnylon(apolyamide)andTerylene(apolyester)bycondensationpolymerisation
•nameproteinsandcarbohydratesasconstituentsoffood
•describethestructureofproteinsaspossessingthesameamidelinkagesasnylon,butwithdifferentunits•describethehydrolysisofproteinstoaminoacids•describecomplexcarbohydratesintermsofalargenumberofsugarunitsjoinedtogetherbycondensationpolymerisation•describehydrolysisofcomplexcarbohydrates(e.g.starch)byacidsorenzymestogivesimplesugars•describeinoutlinetheusefulnessofchromatographyinseparatingandidentifyingtheproductsofhydrolysisofcarbohydratesandofproteins.
KeytermsEthanol Analcoholusedasafuel(e.g.inspiritcampingstovesandin
petrol)Polymers Largemoleculesmadeupbythereactionsofsmallmolecules
calledmonomersPolymerisationTheformationofpolymersfrommonomers
AlcoholsManufactureofethanolEthanolismanufacturedonalargescalebyfermentationofcarbohydratesandhydrationofethene.
FermentationofcarbohydratesCarbohydrates,suchassugar,arebrokendownbyenzymesinyeasttoproduceglucose,C6H12O6.Theenzymesinyeastalsocatalysethebreakdownofglucosetoformethanolandcarbondioxide.Thereactionoccursatatemperatureof37°Candiscarriedoutunderanaerobicconditions(intheabsenceofoxygen).
Whentheconcentrationofethanolreaches14%,itkillstheyeast.Theyeastcellsareremovedbyfiltrationandtheethanolispurifiedbyfractionaldistillation.
HydrationofetheneEtheneisproducedfrompetroleumbyfractionaldistillationfollowedbycrackingoflong-chainalkanes.Ethenereactswithsteamusingacatalystofphosphoricacid,H3PO4,at300
°Cand60atmospherespressure.
TheadvantagesanddisadvantagesofthetwoprocessesareshowninTable15.1.
Table15.1Advantagesanddisadvantagesoffermentationandhydrationinthemanufactureofethanol
Fermentation HydrationAdvantages Usescarbohydratesfromplants
whicharearenewableresourceThereisonlyoneproductinthereactionwhichmeans
whicharearenewableresource thereactionwhichmeansthereisnowaste
Requiresatemperatureof37°Cwhichmeansenergycostsarelow
Acontinuousflowprocessisusedwhichisefficient
Disadvantages Abatchprocessisusedwhichisinefficient
Usesethenefrompetroleumwhichisanon-renewableresource
Landwhichcouldbeusedtogrowplantsforfoodisusedforethanolproduction
Requiresatemperatureof300°Cwhichmeansenergycostsarehigh
PropertiesofethanolEthanolisusedasafuelinspiritcampingstovesanditisalsoaddedtopetrolandusedinethanolfuelcells.Itundergoescompletecombustiontoproducecarbondioxideandwater.
Ethanolisalsousedonalargescaleasasolvent.
ConversionofalcoholstocarboxylicacidsCarboxylicacidsareformedinthelaboratorybyoxidationofalcoholsusingaqueouspotassiummanganate(VII)whichactsasanoxidisingagentwhenacidifiedwithsulfuricacid.Ethanolisoxidisedtoethanoicacidbythismethod.Asimplifiedversionoftheequation,whichrepresentsoxygenfromthe
oxidisingagentas[O],is
Formulaeandnamesofalcohols•AlcoholsaremembersofahomologousserieswiththegeneralformulaCnH2n+1OH.
•Thenamesofalcoholsallendin-ol.•Alcoholscontainthefunctionalgroupwhichisthe-O–Hgroup.•AlcoholswithmorethantwocarbonatomshaveunbranchedstructuralisomersinwhichtheO–Hgroupcanbeindifferentpositionsonthecarbonchain.AnumberisusedtoindicatethepositionoftheO–Hgroup(Table15.2).
UnbranchedalcoholswithuptofourcarbonatomsareshowninTable15.2.
Carboxylicacids•Carboxylicacidsaremembersofahomologousseries.•Thenamesofcarboxylicacidsallendin-oicacid.•Thefunctionalgroupincarboxylicacidsis–COOHwhichcanalsobewrittenas–CO2H.
•ThisisdisplayedasshowninFigure15.1
UnbranchedcarboxylicacidswithuptofourcarbonatomsareshowninTable15.3.
PropertiesofaqueousethanoicacidEthanoicacid,CH3COOH,isatypicalweakacid.Itreactswithmetals,basesandcarbonatestoproducesalts(seeChapter8).Thesaltsarecalledethanoatesandcontaintheethanoateion,CH3COO−.
Esters•Estersaresweet-smellingliquids.•Thenamesofestersallendin-oate.•EstershaveageneralformulaofCnH2nO2.•Thefunctionalgroupinestersis–COORwhichcanalsobewrittenas–CO2R.Rrepresentsagroupcontainingcarbonandhydrogenatoms.
•ThefunctionalgroupinestersisdisplayedasshowninFigure15.2.•Estersaremadebythereactionbetweenacarboxylicacidandanalcohol.•Thetypeofreactioniscalledesterification.Thealcoholandcarboxylicacidareheatedwithacatalystofconcentratedsulfuricacid.
Thegeneralequationinwordsis:
Themoleculescanberepresentedas
Thereactionoccursas
Anexampleis
Namingestersisunlikenaminganyotherorganicmoleculeswehavemetsofar.TheformulaisdividedintotwoandeachpartisnamedaccordingtothenumberofcarbonatomsitcontainsasshowninFigure15.4.ThenamesandformulaeofunbranchedestersareshownTable15.4.
ThenamesandformulaeofunbranchedestersareshowninTable15.4.Allnamesshouldbetwocombinedwords.Thefirstofthesewordsismethyl,ethylorpropyl.
Examiner’stipsRememberthatmostformulaeoforganiccompoundsbeginwithaCatom.However,methanoicacidiswrittenHCOOHandbecauseitbeginswithanH,itsstructuralformulaisconstantlywrittenincorrectlybystudents.Thesamethingappliestomethanoateesters,suchasmethylmethanoatewhichshouldbewrittenHCOOCH3.
•Therearenoesterswithonecarbonatom.•Therearetwoisomericesterswiththreecarbonatoms.•Therearethreeisomericunbranchedesterswithfourcarbonatoms.
PolymersPolymersarelargemolecules(ofnodefinitesize)madeupbythereactionsofsmallmoleculescalledmonomers.Theformationofpolymersfrommonomersiscalledpolymerisation.Therearetwotypesofpolymerisationknownasadditionpolymerisation
andcondensationpolymerisation.
AdditionpolymerisationIfalkenessuchasethenearetreatedtoconditionsofhightemperatureandhighpressureinthepresenceofasuitablecatalyst,thedoublebondsbecomesinglebonds,makingmoreelectronsavailableforthecarbonatomstojointogetherandformlongchains.Thishappenstothousandsofethenemoleculeswhichjointogethertoform
onelong-chainmolecule.Thechemicalnameoftheproductispolyethene.Itscommercialnameispolythene.Thisreactionisknownasadditionpolymerisation,becausethemonomers
jointogetherwithouttheremovalofanyatoms.Thereisonlyoneproductjustasinotheradditionreactionsofalkenes(seeChapter14).TheequationforpolymerisationofetheneisshowninFigure15.5wheren
representsanumberlargerthan10000.
OtherexamplesofadditionpolymerisationTheoreticallyanymoleculewithacarbon–carbondoublebondcanformanadditionpolymer.Thechemicalnameofthepolymerisalwaysthesameasthatofthe
Thechemicalnameofthepolymerisalwaysthesameasthatofthemonomerwithpoly-infront.Propene,CH3CH==CH2,undergoesadditionpolymerisationtoform
polypropene.ThestructureofthepolymercanonlyberepresentedbydrawingthemonomerasinFigure15.6,changingthedoublebondtoasinglebondanddrawingtwoextensionbondsoneithersidewhichshowthatthepolymerextendsinbothdirections.
UsesofadditionpolymersAdditionpolymershaveseveraluseswhicharedependentonstrength,hardness,poorelectricalandthermalconductivity.ExamplesarePolytheneisusedforcarrierbags,bucketsandbowls.PVCisusedforwindowframes,gutteringandinsulatingelectricalwires.Polystyreneisusedforinsulationandpackaging.
Commonerror•Whenaskedtowriteanequationforthepolymerisationofpropene,acommonresponseis
Thisisincorrect.ThecarbonatomintheCH3grouphasfivebondsandthecarbonatomintheCH2grouphasthreebonds.Themonomermustbedrawnasadisplayed(orpartiallydisplayed)structure.
ThecorrectanswerisshowninFigure15.7.
CondensationpolymerisationAcondensationreactionisareactioninwhichtwomoleculesjointogetherandasimplemolecule,suchaswater,isremovedatthesametime.Esterificationisanexampleofacondensationreaction.Condensationpolymersareformedfrommonomerswithtwofunctional
groupseach.Asimplemolecule,suchaswater,iseliminatedasthemonomersjointogether.Examplesofsuchfunctionalgroupsare—OH,—COOHand—NH2.Polyestersandpolyamidesareexamplesofcondensationpolymers.
PolyestersPolyesterscanbemadefromdicarboxylicacids(moleculeswithtwo—COOH
groups)anddiols(moleculeswithtwo—OHgroups).Thesemonomerscanbe
representedas
Thepolymerisationoccursbytheremovalofamoleculeofwaterwhena—COOHgroupandan—OHgroupreact.Themonomersjointogetheras
Becausethereare—COOHgroupsand—OHgroupsatbothendsofthemonomers,morelinkagescanformandthepolymerchaincangrowinbothdirections.
UseofTeryleneTeryleneisanexampleofpolyestermadefromadicarboxylicacidandadiol.
Itisasyntheticfibreusedinclothingmanufacture.
Polyesterscanalsobemadeusingonemonomerwithbothan—OHanda—COOHgroup,forexample,
Inthiscase,therepeatunitshowstheresidueofthesinglemonomermolecule.
PolyamidesPolyamidescanbemadefromdicarboxylicacids(moleculeswithtwo—COOHgroups)anddiamines(moleculeswithtwo—NH2groups).Thesemonomers
canberepresentedas
Thepolymerisationoccursbytheremovalofamoleculeofwaterwhena—COOHgroupanda—NH2groupreact.Themonomersjointogetheras
Becausethereare—COOHgroupsand—NH2groupsatbothendsofthemonomers,morelinkagescanformandthepolymerchaincangrowinbothdirections.
UseofnylonNylonisanexampleofapolyamidemadefromadicarboxylicacidanda
diamine.Nylonisasyntheticfibreusedinclothingmanufacture.Itisalsousedin
ropes,parachutesandstringsfortennisrackets.
Polyamidescanalsobemadeusingonemonomerwithbothan—NH2anda—COOHgroup.
Table15.5Additionandcondensationpolymers
Additionpolymers CondensationpolymersMonomers ContainaC=Cdoublebond Containtworeactivefunctional
groups,e.g.—NH2,—COOH,—OH
PolymerisationOccurswithoutanylossofatomsproducingonlyoneproduct(thepolymer)
Occurswithremovalofsimplemolecule,e.g.water,producingtwoproducts
Polymers Havesameempiricalformulaasthemonomer
Havedifferentempiricalformulafromthemonomers
DisposalofpolymersHouseholdwastecontainslargequantitiesofpolymers(plasticobjects).Thesecanbedisposedofbyburyinginlandfillsitesincineration(burning).
Bothmethodsofdisposalcontributesignificantlytoenvironmentalpollution.
•Buryinginlandfillsitesmeansthatplasticswillremainintheenvironmentandtakeuplargeamountsofspace,especiallyiftheyarenon-biodegradable(whichadditionpolymersare).
•Incinerationcanleadtoproductionoftoxicgases,suchascarbonmonoxide,andacidicgases,suchashydrogenchloride,whichcontributetoacidrain.
Attemptstoovercometheseproblemsinclude•developmentofbiodegradableplastics(thosethatbreakdownintheenvironmentasaresultofbacterialactivity)•developmentofphotodegradableplastics(breakdowninsunlight)•sortingandrecyclingschemes.
NaturalpolymersFoodscontainproteinsandcomplexcarbohydrates(polysaccharides,suchasstarch).
ProteinsProteinsarenaturalpolyamideswhicharemadefromaminoacidmonomers.Thereare20differentaminoacids.Allhavean—NH2(amine)anda—COOH(carboxylicacid)functionalgroup.Thesegroupsreacttogetherbycondensationpolymerisationtoproduceproteinswhichhaveaminoacidresiduesinasequencewhichisspecifictoeachindividualprotein.
Proteinscontainthesameamidelinkageasthatpresentinsyntheticpolyamides,suchasnylon,althoughbiologistsusuallyrefertoitasapeptidelinkagewhenitexistsinproteins.
ComplexcarbohydratesComplexcarbohydrates,suchasstarch,arenaturallyoccurringcondensationpolymersmadewhenglucosemonomers
jointogetherwiththeremovalofwatermoleculestoformstarch.
HydrolysisofnaturalpolymersProteinsTheconstituentaminoacidsthatarepresentindifferentproteinscanbeidentifiedbyheatingfor24hourswith6.0moldm−3(concentrated)hydrochloricacid.Theproteinundergoesacidcatalysedhydrolysis(breakdownbyreactionwithwater)andisconvertedtoitsconstituentaminoacids.(Thisisthereverseofcondensationpolymerisation.)•separatingtheaminoacidsbypaperchromatography(seeChapter2)•sprayingthechromatogram(chromatographypaper)withalocatingagent(ninhydrinisused),sothatthecolourlessaminoacidsproducevisiblebluespots•determiningRfvaluesandcomparingwithRfvaluesofknownaminoacidsinadatabook(seeChapter2).
ComplexcarbohydratesComplexcarbohydratescanalsobehydrolysedbyhydrochloricacid.Thisbreaksthecomplexcarbohydratedowntosimplesugars.Thecomplexcarbohydratestarchwouldbebrokendownintoglucosebyhydrolysis.Hydrolysiscanalsobecatalysedbyenzymes.Starchisbrokendownbythe
enzymeamylaseinsalivatoproducemaltose(adisaccharidemadefromtwoglucoseunits).
Exam-stylequestions1Threecompounds,A,BandC,allhavethemolecularformulaC3H8O.Areactswithethanoicacidtoproduceacompoundwiththestructureshownbelow.
BreactswithethanoicacidtoproduceacompoundwiththemolecularformulaC5H10O2.
Cdoesnotreactwithethanoicacid,butitundergoescompletecombustionwhenburnedinexcessoxygen.
aWhatismeantbytheterm‘molecularformula’?[1mark]
bWhatisthetermusedtodescribecompoundswiththesamemolecularformulabutdifferentstructuralformulae?
[1mark]cWhatistheempiricalformulaofthecompoundwiththemolecular
formulaC5H10O2?
[1mark]dWhattypeofreactionoccursbetweenAandethanoicacid?
[1mark]eWhataretheconditionsthatarerequiredforAtoreactwithethanoicacid?
[2marks]fCompletethechemicalequationforthereactionoccurringwhenC
undergoescompletecombustioninexcessoxygen.Statesymbolsarenotrequired.
[2marks]
gGivethestructuresofmoleculesA,BandC.Showalltheatomsandallthebonds.
[3marks][Total:11marks]
2Thediagramshowspartofapolymer.
aWhattypeofapolymerisshown?[1mark]
bDrawacirclearoundonerepeatunitofthepolymer.[1mark]
ciDrawthestructureofonemoleculeofthemonomer.Showalltheatomsandallthebonds.
iiNamethemonomer.[2marks]
[Total:4marks]3Thediagramshowspartofapolymerwhichisformedbycondensationpolymerisation.
aWhatismeantbycondensationpolymerisation?[2marks]
bWhattypeofcondensationpolymerisshown?[1mark]
cDrawacirclearoundonerepeatunitofthepolymer.Labeltherepeatunit.[1mark]
dDrawacirclearoundthelinkageinthepolymer.Labelthelinkage.[1mark]
eWhattypeofbiologicalmoleculecontainsthesamelinkageasthepolymershown?
[1mark]fDrawthestructuresofthetwomonomers,showingalltheatomsand
bondsinthefunctionalgroups.(YoumayleaveC6H4asitiswritten.)[2marks]
[Total:8marks]4Thediagramshowspartofacondensationpolymermadefromonemonomer.
aWhattypeofacondensationpolymerisdrawn?[1mark]
bDrawacirclearoundtherepeatunitofthepolymer.[1mark]
cDrawthestructureofthemonomershowingallatomsandbonds.[1mark]
dWhatarethenamesofthetwofunctionalgroupsinthemonomer?[2marks]
[Total:5marks]
16Experimentalchemistry
KeyobjectivesBytheendofthissection,youshouldbeabletodescribeteststoidentify
•aqueouscations:aluminium,ammonium,calcium,chromium(III),copper(II),iron(II),iron(III),andzinc•cations:useofflameteststoidentifylithium,sodium,potassium,copper(II)•anions:chloride,bromide,iodide,carbonate,sulfite,sulfateandnitrate•gases:ammonia,carbondioxide,oxygen,hydrogen,chlorineandsulfurdioxide.
Testingforcations(positiveions)Cations(positiveions)canbeidentifiedusingaqueoussodiumhydroxide•aqueousammonia•flametests.
Usingaqueoussodiumhydroxide
Usingaqueousammonia
FlametestsFlametestscanbecarriedoutonsolidsoronaqueoussolutions.
•Startingwithasolid,afewdropsofconcentratedhydrochloricacidshouldbeaddedtoasampleofthesolidonawatchglass.Dilutehydrochloricacidcanbeusedtoavoidsafetyissues.
•Asmallamountofthemixtureshouldthenbeplacedonanichromewire.•ThenichromewirecontainingsomeofthemixtureisthenplacedinthehotpartofaBunsenflame.
•Thecolouroftheflameidentifiesthecation(positiveion)(Table16.1).
Table16.1
Cation(positiveion) FlamecolourLithium,Li+ Red
Sodium,Na+ Yellow
Potassium,K+ Lilac
Copper(II),Cu2+ Blue-green
Testingforanions(negativeions)TestsforanionsandtheirresultsareshowninTable16.2.
Table16.2
Test Result Anion
Adddilutenitricacidfollowedbyaqueoussilvernitrate
Whiteprecipitate Chloride,CI−
Creamprecipitate Bromide,Br−
Yellowprecipitate Iodide,I−
Addanydiluteacid
BubblesGasgivenoffturnslimewatermilky(gasisCO2)
Carbonate,
GasgivenoffwhenwarmedGasturnsacidifiedaqueouspotassiummanganate(VII)colourless(gasisSO2)
Sulfite,
Adddilutenitricacidfollowedbyaqueousbariumnitrate
Whiteprecipitate Sulfate,
Addaqueoussodiumhydroxidefollowedbyaluminium.Warmgently
Gasgivenoffturnsdampredlitmuspaperblue(gasisNH3)
Nitrate,
TestingforgasesTestsforgasesandtheirresultsareshowninTable16.3.
Table16.3
Test Result GasDampredlitmus Turnsblue Ammonia,NH3Limewater Turnsmilky Carbondioxide,
CO2Glowingsplint Lights Oxygen,O2Burningsplint Pops Hydrogen,H2Damplitmuspaper Bleached Chlorine,Cl2Acidifiedaqueouspotassiummanganate(VII)
Changesfrompurpletocolourless
Sulfurdioxide,SO2
Answerstoexam-stylequestions
Chapter1
1Solid[1]Bothmeltingpointandboilingpointareabove50°C.[1]Althoughthemeltingpointbeingabove50°Cisreallyenoughofanexplanation,itisadvisabletorefertobothmeltingpointandboilingpoint.
2aC[1]Solidshaveparticlesthatareveryclosetogether,orderedandvibrate
aboutafixedposition.bB[1]Liquidshaveparticlesthatarefairlyclosetogether,irregularlyarranged
andmoveslowly.cD[1]Gaseshaveparticlesveryfarapart,arrangedrandomlyandmovingathigh
speeds.dA[1]Particlesthatareveryfarapartmustbeinagaswhichmeanstheycannot
beorderedorvibrateaboutfixedpositions.Thequestiondoesnotaskforexplanations,soitisunnecessarytogiveany.
Chapter21aDissolving(sugarinwater)[1]Filtration(toremovesand)[1]Crystallisation[1]bDistillation[1](Simple)distillation,ratherthanfractionaldistillationisusedtoseparatea
pureliquidfromasolution.cFractionaldistillation[1]Fractionaldistillationisusedtoseparateamixtureoftwoormoreliquids
withdifferentboilingpoints.(Simple)distillationisincorrect.dFiltration[1]Washingwithdistilledwater[1]Dryingonawarmwindowsillorinalowoven[1]Aprecipitateisaninsoluble/undissolvedsolid.Centrifugationisanother
alternative.Neitheroftheseprocesseswillproduceapuresolid,becausethesolidwillbecontaminatedwithasmallamountofthesolutionitwasseparatedfrom.Therefore,washingwithdistilledwateranddryingarebothrequiredinaddition.
2Heatuntilcrystalsformonaglassrodplacedinthesolutionandwithdrawn.[1]
Leavethehotsaturatedsolutiontocoolslowly.[1]Crystalsshouldthenform.Removecrystals(byfiltrationifthereisanyliquidleft).[1]Washwithasmallamountofcolddistilledwaterandthendryinalowovenoronawarmwindowsill.[1]
Commonmistakesinclude•Notstatingwhentheheatingshouldbestopped.•Evaporatingtodryness.Inthiscasewaterofcrystallisationwouldbe
removed,leavingtheanhydroussalt.•Notspecifyingthatasmallamountofcolddistilledwaterbeusedfor
washing.Thereisadangerofdissolvingthecrystalsaswellasremovingimpuritiesiftoomuchisusedorifthewaterisnotcold.
•Ifanovenisusedfordryingitshouldbelow.Toomuchheatwouldcause
thecrystalstodecompose(particularlythosethatcontainwaterofcrystallisation).
3Carryoutpaperchromatographyusingasuitablesolvent.[1]Allowsolventtoreachthetopofthechromatographypaper.[1]Removechromatographypaperandallowtodry.[1]Spraywithlocatingagent.[1]MeasureRfvaluesandcomparewithdatabookvaluestoidentifyaminoacids.[1]
Asimplediagramofhowpaperchromatographyiscarriedoutisthebestwaytodescribehowtheapparatusissetupevenifthequestiondoesnotaskforadiagram.
Itisacommonerrortodrawthesolventlevelabovethestartinglineonthechromatographypaper.
Chapter31
2a
b
c
d
e
[Total:5marks]Studentsareencouragedtousethetypeofdiagramsshown.Inmanycasesitisverydifficulttodecidewhichelectronsaresharedifdiagramsarebadlydrawn.
‘Outershellelectrons’onlyarerequested.3aMg(OH)2bCaCl2c(NH4)3PO4dLi2S
ePb(NO3)2fCaCO3gAl(NO3)3hK2SO3iZnSO4j(NH4)2SO4
[1markeach][Total:10marks]
Pleasenote:(d)asksforasulfideand(h)asksforasulfite,whereasboth(i)and(j)askforasulfate.
4aB,C,DandESolidshavemeltingpointsandboilingpointsabove25°C.bAAhasameltingpointbelow25°Candaboilingpointabove25°Csoitis
aliquidat25°C.cFFhasbothmeltingpointandboilingpointbelow25°Csoitisagasat25
°C.dDDistheonlyonetoconductelectricitywhensolid;thereforeDcouldhave
agiantmetallicstructure.(Itcouldalsobegraphiteorgraphene.)eCChasahighmeltingpointandahighboilingpointandconductselectricity
whenmoltenbutnotwhensolid.ThereforeChasagiantionicstructure.fEEhasahighmeltingpointandahighboilingpointwhichmeansithasa
giantstructure.Eisanon-conductorbothwhensolid(soitisnotagiantmetallicstructure)
andwhenmolten(soitisnotagiantionicstructure).ThereforeEhasagiantmolecularstructure.
[1markeach][Total:6marks]
Chapter41MrofH2=(1×2)=2MolesofAl=8.1÷27=0.3moles[1]Moleratiofromtheequation2moleAl:3moleH20.30molesAl:0.30×3/2=0.45molesH2[1]MassofH2=moles×massof1mole=0.45×2=0.90g[1]
[Total:3marks]2MrofKO2=39+(16×2)=71MolesofKO2=0.142÷71=0.002moles[1]Moleratiofromtheequation4moleKO2:3moleO20.002molesKO2:0.002×3/4=0.0015molesofO2[1]VolumeofO2(g)=moles×volumeofonemoleofgas=0.0015×24=0.036dm3[1]
[Total:3marks]3MrofCaC2=40+(12×2)=64MolesofC2H2(g)=120÷24000=0.005molesC2H2(g)[1](Thevolumesmustbothbeinthesameunits:seebelow.)Becausethevolumeinthequestionisgivenincm3,thevolumeof1moleofagasmustbeconvertedfrom24dm3to24000cm3.Alternatively,120cm3couldbeconvertedto0.120dm3.
Moleratiofromtheequation1moleC2H2:1moleCaC20.005molesC2H2:0.005molesCaC2[1]MassofCaC2=moles×massof1mole=0.005×64=0.32g[1]
4Molesof [1]
Moleratioinequation
1moleH2SO4:2molesKOH0.007molesH2SO4:2×0.007=0.014molesKOH[1]aconcentrationofKOH
=0.70moldm−3[1]bMrofKOH=39+16+1=56
Toconvertconcentrationinmoldm−3toconcentrationingdm−3Mass(grams)=moles×Mr=0.70×56=39.2gdm−3[1]
[Total:4marks]5aCalculatethenumberofmolesofatomsofeachelement.Carbon,C54.5÷12=4.54Hydrogen,H9.1÷1=9.1Oxygen,O36.4÷16=2.275DividealltheabovebythesmallestC4.54÷2.275=2H9.1÷2.275=4O2.275÷2.275=1Empiricalformula=C2H4ObIftheMrofthecompound=44TheMrofC2H4O=(12×2)+(1×4)+16=44n=Mrofthecompound÷Mrofempiricalformulan=44÷44=1Therefore,molecularformula=C2H4O×1=C2H4O6MrofTiCl4=48+(35.5×4)=190MolesofTiCl4=0.38÷190=0.002moles[1]Moleratio1moleofTiCl4:1moleofTiTherefore,0.002molesofTiCl4:0.002molesofTi[1]
MassofTi=0.002×48=0.096g[1]0.096gis100%.Buttheyieldisonly0.024g.Percentageyield=actualyield÷100%yield×100%Percentageyield=0.024÷0.096×100=25.0%[1]
[Total:4marks]Itisessentialtoshowallworkingoutinallcalculations.
Chapter51
Electrolyte Nameofproductatanode(+)
Nameofproductatcathode(−)
Moltenpotassiumbromide
Bromine[1] Potassium[1]
Aqueouspotassiumbromide
Bromine[1] Hydrogen[1]
Moltenleadiodide Iodine[1] Lead[1]Aqueouscopper(II)chloride
Chlorine[1] Copper[1]
Aqueoussodiumsulfate Oxygen[1] Hydrogen[1]
Moltenelectrolytescanonlyproducethetwoelementspresentinthemoltencompound.
Aqueouselectrolytesproduce•halogensoroxygenattheanode(exceptionsapplywhentheanodeismade
ofanunreactivemetalsuchascopper)•metalsbelowhydrogeninthereactivityseriesorhydrogenatthecathode.
Metalsabovehydrogeninthereactivityseriesareneverproducedbyelectrolysisofaqueoussolutions.
2aNickelistheanode.[1]bAsolutioncontainingnickelionsistheelectrolyte.[1]cTheobjecttobeplated,i.e.theknife,isthecathode.[1]Studentsmaynotbefamiliarwithnickelcompounds,butifaskedtonamea
solublecompounditissafetoassumethatnitratesarealwayssolubleinwater.Thereforeaqueousnickelnitratecouldbechosenastheelectrolyte.
3aIodine[1]Halogensarealwaysproducedattheanodefromhalides(inpreferenceto
oxygen).b [1]Potassiumisneverproducedbyelectrolysisofanaqueoussolution.
Itisunnecessarytogivestatesymbolsinequationsunlessrequested.cOxidation[1]Oxidationalwaysoccursattheanodebecauseelectronsarealwayslostby
ionsbeingdischargedattheanode.d(Aqueous)potassiumhydroxide[1]Hydrogenions(fromwater)aredischargedatthecathodeandiodideions
aredischargedatthecathode.Thisleavespotassiumions,K+,andhydroxideions,OH−(fromwater),inthesolution,whichbecomes(aqueous)potassiumhydroxide.
Thisissimilartotheelectrolysisofconcentratedaqueoussodiumchlorideinwhichhydrogenandchlorineareproducedattheelectrodesandtheelectrolytebecomesaqueoussodiumhydroxide.
eElectrons[1]Conductingwires,beingmetallic,conductelectricitybecausetheycontain
movingelectrons.‘Ions’isacommonincorrectanswer.f(K+andI−)ions[1]Electrolytesconductelectricitybecausetheycontainmovingions.‘Electrons’isacommonincorrectanswer.
Chapter61a
[2]b2C–C,8C–H,5O=O[2]Itisacommonerrortoinclude3C–CbondsbecausetheformulaisC3H8.c(2×347)+(8×435)+(5×497)=6659kJ[1]d6C=O,8O–H[2]e(6×803)+(8×464)=8530kJ[1]f8530−6659=1871kJ/mole.Thereactionisexothermic.[3]Becausetheamountofenergygivenoutwhennewbondsforminthe
products(8530)isbiggerthantheamountofenergytakenintobreakbondsinthereactants(6659),theoverallenergychangeisexothermic.
Chapter71aPhysical[1]Thisproducesaqueoussodiumchloride.Thesodiumchlorideandwater
arenotchangedintoanysubstancesthatarenotthereatthestart.Sinceanewchemicalsubstanceisnotproduceditisaphysicalchange.
bChemical[1]Thisproducesthreenewchemicalsubstances,i.e.chlorine,hydrogenand
sodiumhydroxide.Therefore,achemicalchangeoccurs.cChemical[1]Thesilverchlorideischemicallychangedintosilverandchlorine.dPhysical[1]Thisisamethodofseparationanddoesnotproduceanynewchemical
substances.ePhysical[1]Thisisamethodofseparationanddoesnotproduceanynewchemical
substances.2
Thevolumeofhydrogenproduceddependsonlyonthenumberofmolesofhydrochloricacidused(becausethecalciumcarbonateisinexcess).
Experiment2istheonlyexperimentinwhichmoremolesofhydrochloricacidareproduced.Thenumberofmolesofhydrochloricacidisdoubledandsothevolumeofhydrogenisdoubled.Becausetheconcentrationofhydrochloricacidisgreater,therateofreactionisfasterandthegraphissteeper(GraphD).
Increasingthesurfacearea(Experiment3)andincreasingthetemperature(Experiment5)bothincreasetherateofreactionwithoutchangingthevolumeofhydrogen.Thusthegraphissteeperinbothcasesandlevelsoffatthesamevolume(GraphB).
InExperiment4,decreasingthetemperaturedecreasestherateandthegraphislesssteepbutlevelsoffatthesamevolumebecausethevolumeofhydrogenisunchanged(GraphE).
3aTherearethesamenumbersofgasmolecules(2)onbothsidesofthesign;thereforeincreasingthepressuredoesnotfavoureitherreaction.[1]
bTherearetwomoleculesofgasontherightandthreemoleculesofgasontheleftofthe sign;thereforeincreasingthepressurecausesequilibriumtoshiftinthedirectionoffewergasmolecules,whichmeanstotheright.[1]
cTherearetwomoleculesofgasontherightandonemoleculeofgasontheleftofthe sign;thereforeincreasingthepressurecausesequilibriumtoshiftinthedirectionoffewergasmolecules,whichmeanstotheleft.[1]
4aTheforwardreactionisexothermic,whichmeansthatthereversereactionisendothermic.Whentemperatureisdecreased,equilibriumalwaysshiftsintheexothermicdirection.Inthiscase,equilibriumshiftstotheright.[1]
bTheforwardreactionisendothermic,whichmeansthatthereversereactionisexothermic.Whentemperatureisdecreasedequilibriumalwaysshiftsintheexothermicdirection.Inthiscase,equilibriumshiftstotheleft.[1]
5aMg(s)+Cu2+(aq)→Mg2+(aq)+Cu(s)[1]
Thesulfateions,So42+,arespectatorions.Statesymbolsneednotbegiveninequationsunlessaskedfor.bOxidation:Mg(s)→Mg2+(aq)+2e−[1]
Reduction:Cu2+(aq)+2e−→Cu(s)[1]Thetotalnumberofchargesonbothsidesofanyequationmustbeequal.cCu2+[1]isanelectronacceptor/gainselectrons.[1]Oxidisingagentsarealwaysreduced.
Oxidisingagentsarealwaysonthesamesideastheelectronsinanionichalf-equation.
dMg[1]isanelectrondonor/loseselectrons.[1]Reducingagentsarealwaysoxidised.Reducingagentsarealwaysontheoppositesidetotheelectronsinanionic
half-equation.
Chapter81aiMethod1ii(Dilute)hydrochloricacid[1]Hydrochloricacidproduceschlorides.iiiCoCO3+2HCl→CoCl2+CO2+H2O[1formulae,1balancing]Cobaltcompoundsarenotallthatwellknown,butthe(II)incobalt(ii)
carbonateandcobalt(II)chloridemeanthatacobaltionisCo2+,thusmakingitstraightforwardtoderivetheformulaeofthecompounds.
biMethod3ii(Aqueous)sodiumiodide[1]Todecideonachoiceofanaqueoussolutioncontainingiodideions,you
shouldrememberthatallsodium(orpotassium)saltsaresolubleinwater.
iii2NaI+Pb(NO3)2→PbI2+2NaNO3[1formulae,1balancing]ciMethod2[1]ii(Dilute)nitricacid[1]Nitricacidproducesnitrates.iiiKOH+HNO3→KNO3+H2O[1formulae,1balancing]2•Pourdilutesulfuricacid[1]intoabeaker.•Addmagnesiumcarbonate.[1]•Stirorwarm.[1]•Stopaddingmagnesiumcarbonatewhensomeremainsundissolved/nomore
bubblesofgasevolved.[1]•Filteroffexcessmagnesiumcarbonate.[1]•Heatthefiltrateuntilcrystalsformonaglassrodplacedinthesolutionand
withdrawn.[1]•Leavethehotsaturatedsolutiontocoolslowly.[1]Crystalsshouldthen
form.•Removecrystals(byfiltrationifthereisanyliquidleft).[1]•Washwithasmallamountofcolddistilledwaterandthendryinalowoven
oronawarmwindowsill.[1]•MgCO3+H2SO4→MgSO4+CO2+H2O[1]3•Addaqueoussodiumhydroxideorpotassiumhydroxide.[1]•Stir/warmtodissolvescandiumoxide.[1]•Filteroffcopper(II)oxide.[1]•Washwithdistilledwater.[1]•Drythecopper(II)oxideonawarmwindowsill/inalowoven.[1]
Chapter91aB[1]GroupIelementsbecomemorereactivedownthegroup.bD[1]GroupVIIelementsbecomemorereactiveupthegroup.cEitherForG[1]dA[1]Periodsarethehorizontalrows.eA[1]AtomsofGroupIVelementshavefourelectronsintheiroutershells.All
atomshavethesamenumberofouterelectronsastheirgroupnumbers.2aBubbles,orlithiumfloats,orlithiumdisappears,orlithiummelts,orlithium
movesaroundAnythree[3]Itisacommonerrortosaylithiumburstsintoflame.Onlytheelements
frompotassiumdownwardsburstintoflame.b2Li(s)+2H2O(l)→2LiOH(aq)+H2(g)[1formulae,1balancing,1state
symbols]Thisequationisverycommonlyaskedfor(withanyGroupIelement)in
exams.TheformulaeandbalancingnumbersarethesameforallGroupIelements;onlythesymboloftheelementchanges.
cYellow[1]Thecoloursofmethylorangeandlitmusinacidicandalkalinesolutions
shouldbememorised.3aCl2(g)+2KI(aq)→2KCl(aq)+I2(aq)[1formulae,1balancing,1statesymbols]Cl2(g)+2I−(aq)→2Cl−(aq)+I2[1]bBr2(l)+2KI(aq)→2KBr(aq)+I2(aq)[1formulae,1balancing,1state
symbols]Br2(l)+2I−(aq)→2Br−(aq)+I2(aq)[1]I2(s)isacceptableinbothcasesbecausesomeofitmayformasa
precipitate.
Thehalogenelementsareoftendissolvedinwatertocarryoutthesereactions,inwhichcase(aq)wouldbeusedasthestatesymbol.
Potassiumionsarespectatorionsinbothreactions.Ifaqueoussodiumiodidewasusedineitherorbothcases,thesodiumionswouldbespectatorionsandtheionicequationswouldbeexactlythesame.
4aF2+2KCl→2KF+Cl2[1formulae,1balancing]bNoreaction[1]cBr2+2KAt→2KBr+At2[1formulae,1balancing]dNoreaction[1]HalogensnearerthetopofGroupVIIdisplacethoselowerdownbutnot
viceversa.Reactionsinvolvingfluorineonlyoccurintheory,becauseinpractice
fluorinereactsviolentlywithwatersocouldnotbeused.Astatineisradioactive.5aCu2O[1]bCu(NO3)2[1]cFeCl2[1]dFe2(SO4)3[1]Theromannumeralsusedtorepresentoxidationstatesarethesameasthe
numberofpositivechargesonthecations:copper(I)referstoCu+,iron(II)referstoFe2+andiron(III)referstoFe3+.
Chapter101aB,C,A,D[1]bB+A(NO3)2→A+B(NO3)2[1]
cC+D2+→D+C2+[1]BecausethenitrateionisNO3−,itcanbededucedthatallthechargeson
themetalionsare2+because1metalioniscombinedwith2nitrateionsineachcompoundascanbeseenfromtheformulae.
dB[1]Themostreactivemetalisalwaysthenegativeterminalinacellcontaining
twodissimilarmetalsinanelectrolyte,becausethemostreactivemetalhasthegreatesttendencytoreleaseelectrons(whichmakesitnegative)asitformspositiveions.
eB+DO→BO+D[1]BecauseBandDhaveionswiththeformulaB2+andD2+,andbecausethe
oxideionisO2−,theformulaeofthemetaloxidesareBOandDO.2•Addadiluteacidinexcess(preferablyhydrochloricorsulfuricbutnot
nitric).[1]•Stirorwarmorboth.•Zincreactsanddissolvesbutcopperdoesneither;bubblingisalsoseenwith
zincbutnotwithcopper.[1]•Filteroffcopper.[1]•Washwithdistilledwateranddryinalowoven.[1]
[Total:4marks]Studentsshouldmemorisethepositioninthereactivityseriesofthose
elementsreferredtoonthesyllabus.Zincisabovehydrogenbutcopperisbelowhydrogen.Thuszincreactswithdilutehydrochloricandsulfuricacidstoproduceanaqueoussaltsolution.Hydrogengasisevolved.
3
[Total:4marks]Becausebothelectrodescontainnickel,theanodemustbeclearlylabelledasimpurenickelandthecathodeaspurenickel.Studentsshouldrememberthatallnitratesaresolubleinwater;thereforeaqueousnickelnitrate(ornickelnitratesolution)isasuitableelectrolyte.
4aB,copper(II)nitrate[1]C,copper(II)oxide[1]D,nitrogendioxide[1]E,oxygen[1]Studentsareaskedtonamethesubstances.Itisacommonerrortowrite
formulae.biCuO+2HNO3→Cu(NO3)2+H2O[1formulae,1balancing]ii2Cu(NO3)2→2CuO+4NO2+O2[1formulae,1balancing]cCopper(II)hydroxide[1]Copper(II)carbonate[1]Bases(copper(II)hydroxide)andcarbonates(copper(II)carbonate)are
usedtoreactwithdiluteacidstoformaqueoussolutionsofsolublesalts.Coppermetalisnotsuitablebecausecopperisbelowhydrogeninthe
reactivityseriesandthereforeitdoesnotreactwithdiluteacids.
Chapter111aBrown[1]toblack[1]Acolourchangeisrequested;thereforeitisessentialtogivetheoriginal
andfinalcolours.b2Cu+O2→2CuO[1formulae,1balancing]cOxygengasistakenout/removedfromtheair.[1]d‘Excess’inthiscasemeansthatthereismorethanenoughcoppertoreact
withalltheoxygen.Somecopperwillbeleftoverwhenalltheoxygenhasreacted.[1]
Studentscommonlyanswerthatexcessmeans‘morethanenough’butitisimportanttoaddmorethanenoughtoreact.Thepointisemphasisedbysayingsomewillbeleftoverafterthereaction.
eNitrogen[1]2aAcatalystincreasestherateofachemicalreaction[1]andischemically
unchangedattheendofthereaction.[1]b
Yieldofammonia
Rateofproductionofammonia
Highertemperaturethan450°C Decrease[1] Increase[1]Higherpressurethan200atmospheres
Increase[1] Increase[1]
Notusingacatalyst Nochange[1] Decrease[1]
Thetablemakesdistinctionsbetweenyield(equilibriumposition)andrateofreaction.Studentsshouldmakesurethattheydonotconfuseonewiththeother(seeChapter7).Wheretherearechangestheanswersmustbecomparative,e.g.loweryieldandnotlowyield.
ciSulfuricacid[1]ii2NH3+H2SO4→(NH4)2SO4[1formulae,1balancing]d3NH3+H3PO4→(NH4)3PO4[1formulaof(NH4)3PO4,1balancing]
Chapter121aVanadium(V)oxideorvanadiumpentoxide[1]Theoxidationstateofvanadiumisanessentialpartofthename
vanadium(V)oxide.Iron(thecatalystintheHaberprocess)isacommonerror.bBelow450°C,therateofreactionwouldbeslower.[1]cTherateofreactionwouldincreaseatahigherpressure[1]becausethegas
moleculeswouldbeclosertogetherandthereforetherewouldbemorecollisionsinanygiventime.[1]
Statingthatgasmoleculeswouldbeclosertogetherthereforetherewouldbemorecollisionsisnotenoughforthesecondmark.Referencetotimeisessential.
dTheyieldofsulfurtrioxideandtherateofreactionwouldbehighenoughat1–2atmospheres.
orTherewouldberisksofexplosions/leakagesathigherpressure.orItwouldbetooexpensivetobuildanindustrialplanttowithstandhigher
pressure.Anytwo[2]eSulfurtrioxideisdissolvedin98%concentratedsulfuricacidtoproduce
oleum.SO3+H2SO4→H2S2O7[1]Theoleumisthenaddedtothecorrectamountofwatertoproducesulfuric
acidoftherequiredconcentration.H2S2O7+H2O→2H2SO4[1]2aH2SO4+CuCO3→CuSO4+CO2+H2O[1formulaeofCuCO3andCuSO4,1restofequationcompletelycorrect]bH2SO4+2NaOH→Na2SO4+2H2O[1formulae,1balancing]cH2SO4+KOH→KHSO4+H2O[1]dH2SO4+Zn→ZnSO4+H2[1]
Chapter131PlantsgrowbetterinsoilsatspecificpHvalues.[1]Calciumoxideorcalciumhydroxide[1]Lime,quicklimeorslakedlimearecommonnames(asalsoarelimestoneandlimewater)andnotchemicalnames.
2ai2C8H18+25O2→16CO2+18H2O[1formulae,1balancing]Fractions/multiplesareacceptableinequationsunlessstatedotherwise.iiCaCO3→CaO+CO2[1]iiiFe2O3+3CO→2Fe+3CO2[1formulae,1balancing]ivCaCO3+2HCl→CaCl2+CO2+H2O[1formulae,1balancing]biWater[1]iiGlucose[1],oxygen[1]iiiUVlight/sunlight[1],chlorophyll[1]
Chapter141
[2]Whendrawingmoleculesitisessentialthatallcarbonatomshavefourbondsonly.
Itisalsoessentialtodrawtwodifferentmolecules,andnotdrawthesamemoleculetwiceasinthefollowingexamples.
Theseshowthesamemoleculedrawnseveraldifferentways.Inalltheexamples,thedoublebondisbetweenthefirsttwocarbonatomsandallfivecarbonatomsareina‘straightchain’eveniftheyaredrawnatanangleinsomecases.
2aUltravioletlight[1]bChloroethane[1]cAnyoneofthefollowing:C2H4Cl2/C2H3Cl3/C2H2Cl4/C2HCl5/
C2Cl6[1]
Someorallofthehydrogenatomsinethanecanbesubstitutedbychlorineatoms.Thetotalnumberofatomsbondedtothetwocarbonatomsmustbe
sixineachcase.3aAddition[1]bThebrominechangescolourfromyellow/brown[1]tocolourless.[1]Whenthebrominetestforunsaturationisdescribed,itisessentialtogive
theinitialandfinalcolours.Ifexcesspropeneisnotusedtheremaybesomebromineleftandsothe
colourofthebrominemaystillbevisible.ciC3H6Br2[1]Twobromineatomsareaddedtothepropene.iiC3H8[1]Twohydrogenatomsareaddedtothepropene.iiiC3H8O[1]Awatermoleculeisaddedtothepropene,i.e.twohydrogenatomsand
anoxygenatomareadded.dNickel[1]4aC9H20[1]Thecorrectanswerisobtainedusingn=9inthegeneralformulafor
alkaneswhichisCnH2n+2.bAnyonefromthefollowing:
[formula1,name1]Allowstructuralformulaeordisplayedformulaebecause‘showingallthe
atomsandbonds’isnotrequested.StructuralformulaemustshowtheC=Cdoublebond.
ciC8H18→C4H10+2C2H4[1]iiC8H18→C5H10+C3H6+H2orC8H18→C2H4+C6H12+H2orC8H18→C2H4+2C3H6+H2[1]ThereareacceptableanswerswithanalkyneCnH2n−2andanalkaneas
theproductsaswellashydrogen.
Chapter151aTheformulathatshowsthenumberofatomsofeachelementinone
moleculeofanelementorcompound.[1]bStructuralisomers[1]Theword‘structural’isoftenomitted.cC5H10O2[1]Thisformulacannotbesimplifiedanyfurther.Thereforethemolecular
formulaandtheempiricalformulaarethesame.dEsterification[1]Condensationwouldalsobeacceptable.eHeat[1]andacatalystofconcentratedsulfuricacid[1]f2C3H8O+9O2→6CO2+8H2O[1formulae,1balancing]Fractions/multiplesareacceptedinequations.g
[3]ThediagrambelowshowspartsoftheestersformedfromAandB.
TheoxygenatomintheesterformedfromAisjoinedtocarbonnumber2.ThereforetheO–HgroupinalcoholAmusthavebeenoncarbonnumber2.
TheoxygenatomintheesterformedfromBisjoinedtocarbonnumber1.ThereforetheO–HgroupinalcoholBmusthavebeenoncarbonnumber1.
CdoesnotreactwithethanoicacidwhichmeansthatCisnotanalcoholanddoesnothaveanO–Hgroup.TheonlymoleculethatcanbedrawnwiththemolecularformulaC3H8Othatisnotanalcoholandobeystherulesthat
•carbonatomshaveonlyfourbonds•oxygenatomshaveonlytwobonds,
and•hydrogenatomshaveonlyonebondis2aAdditionpolymer[1]b
[1]Thecircleshouldbedrawnaroundtwoconsecutivecarbonatomsinthe
mainchainandalltheatomsandgroupsofatomsjoinedtothem.Oneexampleisgiven.
ci
[1]iiBut-1-ene[1]3aCondensationpolymerisationistheformationofalong-chainmolecule(the
polymer)fromsmallmolecules(monomers)[1].Asimplemoleculesuchaswateriseliminatedasthemonomersjointogether.[1]
bApolyamide[1]c,d
[1each]eAprotein[1]f
[2]Incarboxylicacidsandalcohols,itisacommonerrorforstudentsnotto
drawthebondbetweenOandHatoms,i.e.theydraw–OHinsteadof–O–H.
4aApolyester[1]b
[1]c
[1]dCarboxylicacid[1]Alcohol[1]
Index
Aacidicoxides60acids57–9activationenergy51additionpolymerisation103additionreactions93air79–80airpollution80alcohols98–100alkalimetals64alkalis58,59alkanes92–3alkenes93–7allotropes91alloys69aluminium(Al)71,73–4,76aluminiumchloride20ammonia(NH3)21,82,111ammoniumsalts59,61amphotericoxides60anions,testingfor111anode37,39aqueousammonia111aqueouspotassiumiodide55aqueouspotassiummanganate55aqueoussodiumhydroxide110argon(Ar)67atomicnumber(protonnumber)14atomicstructure14–15atoms,electronarrangement15–16
Avogadroconstant26
Bbases57,58,59basicoxides60benzene33boiling1,2,3boilingpoint1,2,69bondenergies43,45bromide(Br−),testingfor111bromine(Br)65–6,95Brownianmotion1,4butane33,91,92
Ccalcium(Ca),reactionwithwater70carbohydrates,fermentation98carbonates(CO32−)58,61,72,88,111carboncycle88carbondioxide(CO2)21,87,111carbon,inorganicchemistry87–9carbonmonoxide(CO)80carboxylicacids98,100–1catalysts47catalyticconverters78,80catalyticcracking96cathode37,39cations,testingfor110–11cells(batteries)41ceramics,usesfor41changesofstate2chemicalenergetics43–6chemicalequilibrium52–3chemicalpropertiesofasubstance48chemicalreactions47–56chlorides(Cl−)61,111
chlorination78,79chlorine(Cl2)21,65–6,93,111chromatography8–10cobaltchloride78collisiontheory49–51combustion,alkanes93completecombustion93complexcarbohydrates107compounds7–13condensation1,2condensationpolymerisation103conductors37coolingcurves3copper(Cu)75–6copper(II)sulphate40,78covalentbonding21cracking96crystallisation10–11
Ddensity,metals69diamond23–4diffusion1,4displacementreactions,metals70displayedformula91dissolving10–11distillation11–12drinkingwater79ductility,metals69
Eelectricalconductivity,metals69electricity37–42electrodes37electrolysis37,38–40,41electrolytes37–8electrons14,15–16
electronshell15electroplating40–1elements7–13empiricalformula26,32–3,90,91endothermicreaction43,44energyleveldiagrams43enzymes47equations,Molecalculation30equilibriumsystems52–4esters101–3,102ethane92ethanol98–9ethene(C2H4)21,98evaporation1,2exothermicreaction43experimentalchemistry110–11extractionofmetals73–6
Ffertilisers78,82–3filtration10–11,78,79flametests111formulae18–19,19–20,91fractionaldistillation11–12,78,79freezing1,2freezingpoint1fuelcells44fuels44,96functionalgroups90
Ggalvanising81gases1–2,4–5,111gasvolume,Molecalculation29giantcovalentstructure(macromolecularstructure)22giantionicstructure22,23giantmetallicstructure22,23
giantmolecularstructure23glucose33graphite23–4greenhouseeffect87greenhousegases87Group0elements66–7GroupIelements65GroupVIIelements65–6groups,PeriodicTable64,65–7
HHaberprocess53–4,82halogens64,65–6heatingcurves3helium(He)67homologousseries90hydrocarbons92hydrogen(H2)21,44,95,111hydrogenperoxide33hydrolysis,naturalpolymers107hydroxides,thermaldecomposition72
Iincompletecombustion93indicators60inorganiccarbonchemistry87–9insolublebases58iodide(I−),testingfor111iodine(I)65–6ionicbonding16–20ioniccompounds16–20ionicequations28iron(Fe)70,74,75,76iron(III)sulfate20isomers91isotopes14,91
Kkinetictheory3
Llattices14leadsalts61,80limestone88limitingreactants33liquids1–2,4litmus60
Mmacromolecularstructure(giantcovalentstructure)22magnesiumfluoride,ionicbonding18magnesiumhydroxide20magnesium(Mg),reactionwithwater70malleability,metals69masses,Molecalculation29massnumber(nucleonnumber)14matter,particulatenatureof1–6melting1,2,3meltingpoint1,69metallicbonds22metalliccompounds,thermaldecomposition72metals7,69–77extractionof41,73–6reactionswithacids57–8usesof41,76
methane(CH4)21,33,87,92methanol(CH3OH)21methylorange60mixtures7,8–12Mole26,29molecularformulae26,33–4,90,91
N
naturalpolymers106–7neutraloxides60neutron14nitrates61nitrates(NO3–)72,111nitrogen(N2)21,82–3noblegases64,66–7non-metals7nucleonnumber(massnumber)14nucleus15nylon106
Oorganicchemistry90–109oxidation47oxides60oxidisingagents47,54,55oxygen(O2),testingfor111
Ppercentagepurity35percentageyield34PeriodicTable16,19,64–8periods,PeriodicTable64petroleumindustry96photochemicalreactions51pHscale60physicalpropertiesofasubstance48plastics,usesfor41pollution,air80polyamides105polyesters104polyethene103polymerisation98,103polymers98,103–7potassiumsalts61
propane92proteins106–7proton14protonnumber(atomicnumber)14
Qquicklime88
Rrateofreaction48reactivityseries,metals70–2redoxreactions54–5reducingagents47,55reduction47,54relativeatomicmass(Ar)8,26relativemolecularmass(Mr)8,26,28reversiblereactions52rust78,81
Ssalts57,61–2separationofmixtures8–12silicon(IV)oxide(silicondioxide)24simpledistillation11simplemolecularstructures23slakedlime88sodiumchloride,ionicbonding17sodiumhydroxide(NaOH)110sodiumsalts61solids1–2,22–3solutes7solutions7,29solvents7stainlesssteel,usesof76starch107state,changesof2steam,reactionwithalkenes95
steel75,76stoichiometry26–7strongacids57,58strongbases57structuralformula90,91structuralisomerism90,91–2,94sublimation1substance7substitutionreactions93sulfate(SO42–)61,111sulfite(SO32–),testingfor111sulfurdioxide(SO2)80,84–6,85,111sulfuricacid84–5symbolequations26
Tterylene105thermalconductivity,metals69thermalcracking96thermaldecomposition,metalliccompounds72transitionelements64,67
Uunbranchedesters102universalindicators60unsaturationtesting,alkenes94
Wwater(H2O)21,65,70,78–9weakacids57,58wordequations26
Zzinc(Zn)76,81