chemical reactions 3eso...a law in chemistry, the law of conservation of mass, states, “in an...

_________________________________________________________________________________________________________JoséManuelSánchezHernándezAntonioJoséLechugaNavarro

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Unit:CHEMICALREACTIONS


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1.INTRODUCTION

Asyouknow,matteriscomposedofparticles(atomsandmolecules).Infact,

thereisahugeamountofdifferentatomsandplentyofcombinationsofatomsto

formmoleculesorcompounds.

Probably,youhavealwayslistenedthat"matterisnotcreatedordestroyed

onlytransformed".Thelawimpliesthatmasscanneitherbecreatednordestroyed,

althoughitmayberearrangedinspace,sotheamountofmatterinourUniverseis

alwaysthesame.ImagineacubewithmanylegobrickspiecesCouldyoubuildonly

onestructureormorethanone?

ü ActivityA1.Rememberwhatyouhavelearnedintheunitandgivealittledefinitionforatomandmolecule.Furthermore,drawanexampleofeachone.

ü ActivityA2.Theimagesbelowareatomsormolecules?


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2.PHYSICALANDCHEMICALPHENOMENA

Physical phenomena: they are the processes in which substances do notbecomedifferentones. This typeofphenomena is studied inPhysics. Forexample,themovementofacarordissolutionofsugarinwater.

Chemical phenomena: they are the processes in which substances becomedifferentones.ThistypeofphenomenaisstudiedinChemistry.Forexample,petrolcombustionorironoxidation.

ü Physicalchangesareusuallyconnectedto

physicalstatesofmatter.Physicalchangerearrangesmoleculesbutdoesn'taffecttheirinternalstructuresneithersubstancenorcomposition.

ü Chemical changes happen on amolecularlevel when you have two or moremolecules interacting among them. Chemical changes happen whenatomicbondsare broken or created during chemicalreactions. A chemical changeis anychangethatproducestheformationofnewchemicalsubstances.


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PHYSICALVS.CHEMICALCHANGES

PHYSICALCHANGES CHEMICALCHANGES

Physicalchangesaresimplychangeofstatewithoutchangesofmatter.

Chemicalchangesoccurwhennewmaterialsareformedbyachangeinthewayatomsarebondedtogether.

_ Colourchange,gascreated,smellchange,newmattercreated,hotorlightcreated…areevidencesorcluestoidentifyachemicalchange.

_ reactivitychangeswiththeformationofnewsubstances.

_ heat,lightorelectricalenergyisoftenemittedorabsorbed.


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3.WHATISACHEMICALREACTION?

Achemicalreactionisaprocessthatleadstothetransformationofonesetofchemicalsubstancestoanother.

Reactants:Thesubstance(orsubstances)initiallyinvolvedinachemicalreaction.Products:Thesubstance(orsubstances)obtainedinachemicalreaction.

ü ActivityA3.Whichofthefollowingphenomenaarephysicalandwhicharechemical?Why?(thechemicalcomposition

a)Fooddigestion.b)Theturnofthewheelofacar.c)Thetransformationofgrapejuiceintowine.d)Thechangeofwaterfromliquidintogas.e)Thereflectionoflightonamirror.f)Theattractionofironfilingsbyamagnet.g)Theoxidationofcopperstatues.


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4.CHEMICALEQUATION

Chemical reactions are described with chemical equations which graphicallypresentthestartingmaterials,endproductsandreactionconditions.

REACTANTS→PRODUCTS

Reactantsandproductsarerepresentedbyformulas.Formulasareexpressionsmadeup by chemical symbols and subscripts which inform us of the composition orstructureofsubstances.Molecularformula:Expressesthenumberofatomsofeachelementinthemolecule.Examples:HCl,NaCl,H2O…HowmanyatomsofeachelementarethereinAl(OH)3?1atomofaluminum,3atomsofoxygenand3atomsofhydrogen.

ü ActivityA4.Howmanyatomsofeachelementaretherein:a)Mg(OH)2b)Al2(SO4)3c)C6H12O6d)H2CO3e)NH3


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5.BALANCEOFCHEMICALREACTIONS.LAWOFCONSERVATIONOFMASS.

When you write an equation for a chemical reaction, the two sides of theequation should balance (you need the same number of each kind of element onbothsides.) Ifyoucarryoutachemicalreactionandcarefullysumupthemassesofall the reactants, and then compare the sum to the sum of themasses of all theproducts,youseethatthey’rethesame.

A law in chemistry, the Lawof Conservation ofMass, states, “In an ordinary

chemical reaction, matter is neither created nor destroyed.” This means that youhaveneithergainednor lostanyatomsduringthereaction.Theymaybecombineddifferently,butthey’restillthere.


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6.STOICHIOMETRY

Stoichiometryisthecalculationofreactantsandproductsinchemicalreactions.

Thebranchofchemistrywhichiscalledstoichiometrydealswiththecalculationof various quantities of reactants or products of a chemical reaction. The word“stoichiometry”itselfisderivedfromtwoGreekwords“stoichion”meaningelementand“metry”meaningmeasure.

Stoichiometry is foundedon thelawof conservationofmasswhere the totalmassof the reactantsequals the totalmassof theproducts.Thismeans that if theamountsoftheseparatereactantsareknown,thentheamountoftheproductcanbecalculated. Conversely, if some product of reaction is in known quantity and thechemicalequationofthereactioncanbedetermined,thentheamountoftheotherreactantscanalsobecalculated.


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ü ActivityA5.Balancethefollowingchemicalreactions:


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7.MOLECULARMASS

The molecular mass of a substance is the mass of one molecule of thatsubstance expressed in unified atomic mass units (u). The molecular mass can becalculatedasthesumoftheindividualmasses(asfoundintheperiodictable)ofalltheatomsinanymolecule.Example:calculatethemolecularmassofCO2MCO2=1·C+2·O=(1·12)+(2·16)=12+32=44u

8.MOLARMASS

InChemistryinsteadofworkingwithmasseswehavetoworkintermsofmoles.But,whatistherelationshipbetweenmolesandmass?Molarmass.

Molarmassisthemassofonemoleanditisthesamenumberasthemolecular

massbutwithdifferentunits.Molarmassisexpressedingrammes.Therefore,ifwecancalculatemolecularmasses,weknowthevalueofthemolarmass(expressingitingrammes),whichisofgreatuseinalaboratory.

Asanexample,themolarmassofwater:

MH2O=2·H+1·O=(2·1)+(1·16)=2+16=18g/mol

ü ActivityA6.Calculatethemolecularmassthesechemicacompounds:a) H2CO3b) Mg(OH)2c)Al2(SO4)3d)Cl2O5e)O2f)CuSO3

Forknowingthedataofatomicmassyoucanuseaperiodictable

SOLUTIONS:a)62ub)58uc)342ud)151ue)32uf)143,5u


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Wewillbeabletomakechemicalcalculationsbyusingasimplechemicalformula:

where…

§ naremoles§ mismass(g)§ Mismolarmass(g/mol)

Asanexample,themolarmassofwater:

MH2O=2·H+1·O=(2·1)+(1·16)=2+16=18g/mol

Examples:

§ In a laboratory glass bottle there are 300 g of CuSO4. Work out how manymolesofthatsubstancearecontainedintheglass.

ü WeworkoutthemolarmassMoftheCuSO4:

M(CuSO4)=1·63,5+1·32+4·16=159,5g

𝑛 = !!= !""

!"#,!= 1,88 𝑚𝑜𝑙𝑒𝑠 𝐶𝑢𝑆𝑂!

ü ActivityA7.Howmanymolesarein10gofNaOH?

(Na=23u;O=16u;H=1u)

Sol:0.25molesNaOH

ü ActivityA8.Howmanygramsarein2molesofH3PO4?

(H=1u;P=31u;O=16u)

Sol:196gofH3PO4

ü


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9.AVOGADRO’SNUMBER

Avogadro’snumber is the totalnumberofparticlespresent inonemoleof asubstance.Itisthenumberofatomspresentinexactly12gofC-12.

Avogadronumberisvaluedas6.022·1023particles/mol

This number can be the number of particles (atoms,molecules,electrons…)inonemolofsubstance.


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10.MolarVolume

The molar volume, Vm of a substance is the volume occupied by a mole ofatomsormoleculesofthatsubstance.Ifweworkwithgases(verycommon),insteadofusingmasseswearegoingtousevolume.

In order to work out the molar volume of gases, 1 mole of any gas at STP

(standardconditionsof temperaturaandpressure,whichare273Kand1atm) it isequal to22.4L.Amoleofanygas instandardconditions (273Kand1atm)alwaysoccupies22.4L,nomatterwhatthegasis.

Thismeansthatifweareworkingat0oC=273Kand1atmofpressure,1mole

ofanygas…- O2(M=32g/mol)- SO2(M=64g/mol)- CH4(M=16g/mol)

…occupythesamevolume.Example:Howmanymolesofoxygenarethereinacontainerof45LatSTP?Solution:2.04moles

ü ActivityA9.Whatisthemassof150Lofnitrogen(N2)atSTPconditions?Data:MN=14uSol.:187,5g

ü ActivityA10.Whatisthevolumeof80gSO2gasatSTPconditions?

Data:MS=32uMO=16u

Sol.:28L


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11.StoichiometriccalculationsStoichiometryisthestudyofquantitative

relationshipsinvolvedinchemicalreactions.Tomakestoichiometriccalculationsyouhavetofollow4steps:

1.-Balancetheequation.2.-Convertunitsofagivensubstancetomoles.3.-Usingthemoleratio,calculatethemolesofsubstanceyieldedbythereaction.4.-Convertmolesofwantedsubstancetodesiredunits.Example:

§ Whatwill be themass of iodine needed to react completelywith 7 grams ofaluminum?

Al+I2→AlI3Data:mAl=27g/molmI=127g/mol

ü ActivityA11.WhatwillbethemassofKClobtainedwhenwehave5gofKClO3?KClO3→KCl+O2

Data:mK=39umCl=35.5umO=16u


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12.COLLISIONTHEORYTherateofa reaction is thespeedatwhicha reactionhappens. Ifa reaction

hasalowrate,thatmeansthemoleculescombineataslowerspeedthanareactionwith a high rate. Some reactions take hundreds, maybe even thousands, of yearswhileotherscanhappeninlessthanonesecond.Therateofreactiondependsonthetypeofmolecules thatare combining. If youwant to thinkofa very slow reaction,thinkabouthowlong it tookdinosaurbonestobecomefossils throughbreakdown.Youcanthankchemicalprocessesinbacteriaformostofthosedinosaurbonesinthemuseum.

There is another big idea for rates of reaction called collision theory. Thecollision theory says that asmore collisions in a system occur, there will bemorecombinationsofmoleculesbouncingintoeachother.Ifthereareahighernumberofcollisionsinasystem,morecombinationsofmoleculescanoccur.Thereactionwillgofasterandtherateofthatreactionwillbehigher.Eventhoughtheyarebothliquids,think about how slowly molecules move in honey when compared to your soda.Therearealowernumberofcollisionsinthehoney.

ü ActivityA13.HydrochloricacidHClreactswithcalciumcarbonate,CaCO3,andproduces calcium chloride, CaCl2, carbon dioxide CO2 andwater.Whatmass ofwaterisobtainedwhen18gofcalciumcarbonatereactfully?

Data:mCa=40umC=12umCl=35.5umO=16u

ü ActivityA12.The followingreaction isoneof themethods forobtainingnitricacid:NO2+H2O→HNO3+NOCalculatehowmanygrammesofnitrogendioxidemustreactinordertoproduce8.8gofnitricacid.

Data:mN=14umO=16umH=1u


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Reactions happen - no matter what. Chemicals are always combining orbreaking down. The reactions happen over and over, but not always at the samespeed. A few things affect the overall speed of the reaction and the number ofcollisionsthatcanoccur.

13.FACTORSTHATAFFECTREACTIONRATES

§ Concentration: If there ismoreofasubstance inasystem, there isagreaterchance that molecules will collide and speed up the rate of the reaction. Ifthereislessofsomething,therewillbefewercollisions and the reaction will probablyhappenat a slower speed. Sometimeswhenyouare in a chemistry lab, youwill addonesolutiontoanother.Whenyouwanttherateof reaction tobeslower,youwilladdonlyafewdropsatatimeinsteadoftheentirebeaker.


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§ Temperature: When you raise the temperature of a system, the moleculesbouncearoundalotmorebecausetheyhavemoreenergy.Whentheybouncearoundmore, they aremore likely to collide. That factmeans they are alsomore likely to combine.When you lowerthe temperature, the molecules areslowerandcollide less.That temperaturedroplowerstherateofthereaction.Backto the chemistry lab! Sometimes youwillmix solutions in ice so that thetemperatureofthesystemstayscoldandtherateofreactionisslower.

§ Pressure: Pressure affects the rate of reaction, especially when you look at

gases.Whenyouincreasethepressure,themoleculeshavelessspaceinwhichthey can move. That greater density ofmolecules increases the number ofcollisions. When you decrease thepressure, molecules don't hit each otheras often. The lower pressure decreasestherateofreaction.

§ Surface area of reactants: Increasing the

surfaceareaofsolidreactantsicreasesthenumberofparticlestharareexposedandavailable to react, and as conseqyuencethis increases the frequency of particlecollisions,increasingrate.


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§ Catalyst:Catalysisistheincreaseintherateofachemicalreactionduetothe

participation of a substance called a catalyst. Unlike other reagents in thechemical reaction, a catalyst is not consumed. A catalyst may participate inmultiplechemicaltransformations.Theoppositeofacatalyst,asubstancethatreduces the rate of a reaction, is aninhibitor. Surface area .This means thatthe more finely divided a solid or liquidreactant the greater its surface area perunit volume and the more contact itmakes with the other reactant, thus thefasterthereaction.

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ü PROJECT,workingingroups:§ you do a little presentation in English for your classmates and try to

answerthisquestionexpressingyourownopinion:

Timeaproximate5minutes.Youcanuseimagesashelpsource.


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VOCABULARY Inthetablebelowwritedownthekeywordslearnedfromtheunitwiththeirmeaning.

key word meaning key word meaning

chemical reactions 3eso...a law in chemistry, the law of conservation of mass, states, “in an...

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