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Analytical Chemistry

Unit 53 Qualitative analysis — detecting the presence of inorganic chemical species

Unit 54 Tests for functional groups, separation and purification of compounds

Unit 55 Quantitative methods of analysis

Unit 56 Instrumental analytical methods

Unit 57 Contribution of analytical chemistry to our society

Topic16

KeyC o ncepts

Quantitative methods of analysis• Gravimetric analysis• Precipitation titrations• Redox titrations involvingKMnO4

• Redox titrations involving I2

Qualitative analysis — detecting the presence of inorganic

chemical species• Cations• Common gases andvapours• Anions• Devising a separation scheme• Risks associatedwith tests

Contribution of analytical chemistry to our society

• Food anddrug analyses• Air quality control• Forensic chemistry• Clinical laboratory tests

Tests for functional groups, separation and purification

of compounds• Tests for organic functional groups• Distillation and fractionaldistillation• Liquid-liquid extraction• Re-crystallization• Chromatography• Checking thepurityof aproduct

Instrumental analytical methods• Colorimetry• Infrared spectroscopy• Mass spectrometry

Analytical Chemistry

Topic 16� Analytical Chemistry �Unit 53 Qualitative analysis — detecting the presence of inorganic chemical species

53.1 – 53.21

Summary

1 The following table summarizes results of some tests for cations.

Ion Test ActionofNaOH(aq) ActionofNH3(aq) Other test(s)

Aluminiumion,Al3+

whiteprecipitate, soluble inexcessNaOH(aq) to give acolourless solution

Al3+(aq) + 3OH–(aq)Al(OH)3(s)

Al(OH)3(s) +OH–(aq) [Al(OH)4]

–(aq)

whiteprecipitate

Al3+(aq) + 3OH–(aq)Al(OH)3(s)

—

Ammoniumion,NH4

+colourless gas (NH3) givenoff onwarming

NH4+(aq) +OH–(aq)

NH3(g) +H2O(l)

— —

Calciumion,Ca2+

whiteprecipitate

Ca2+(aq) + 2OH–(aq)Ca(OH)2(s)

— flame test—brick-red flame

actionofH2SO4(aq)—whiteprecipitate

Ca2+(aq) + SO42–(aq)

CaSO4(s)

Copper(II)ion,Cu2+

blueprecipitate

Cu2+(aq) + 2OH–(aq)Cu(OH)2(s)

blueprecipitate, soluble inexcessNH3(aq) to give adeepblue solution

Cu2+(aq) + 2OH–(aq)Cu(OH)2(s)

Cu(OH)2(s) + 4NH3(aq) [Cu(NH3)4]

2+(aq) +2OH–(aq)

flame test—bluishgreen flame

Iron(II)ion, Fe2+

greenprecipitate, turningbrownon standing

Fe2+(aq) + 2OH–(aq) Fe(OH)2(s)

greenprecipitate, turningbrownon standing


—

Iron(III)ion, Fe3+

reddishbrownprecipitate


reddishbrownprecipitate


—

Lead(II)ion, Pb2+


Pb2+(aq) + 2OH–(aq) Pb(OH)2(s)

Pb(OH)2(s) + 2OH–(aq) [Pb(OH)4]

2–(aq)

whiteprecipitate

Pb2+(aq) + 2OH–(aq) Pb(OH)2(s)

actionofHCl(aq)—whiteprecipitate,soluble inhotwater

Pb2+(aq) + 2Cl–(aq) PbCl2(s)

actionofH2SO4(aq)—whiteprecipitate

Pb2+(aq) + SO42–(aq)

PbSO4(s)

Magnesiumion,Mg2+

whiteprecipitate

Mg2+(aq) + 2OH–(aq)Mg(OH)2(s)

whiteprecipitate

Mg2+(aq) + 2OH–(aq)Mg(OH)2(s)

—

Potassiumion,K+

— — flame test— lilacflame

Sodiumion,Na+

— — flame test—goldenyellow flame

53.1 Introduction to analytical chemistry

53.2 Identifying the cation in a sample

53.3 Appearance

53.4 Solubility inwater

53.5 Flame test

53.6 Action ofheat

53.7 Action ofdilutehydrochloric acid

53.8 Action ofdilute sulphuric acid

53.9 Action ofdilute aqueous solutionof sodiumhydroxide

53.10 Action ofdilute aqueous ammonia

53.11 Identifying the cation in an unknown sample in thelaboratory

53.12 Devising a scheme to separate the cations in an aqueoussolution

53.13 Specific tests for identifying commongases andvapours

53.14 Identifying the anion in a sample

53.15 Action ofheat

53.16 Action ofdilutehydrochloric acid

53.17 Action of concentrated sulphuric acid

53.18 Action of aqueous solutionof silvernitrate

53.19 Devising a scheme to separate anions in an aqueous solution

53.20 Identifying the cation and anion in an unknown sample inthe laboratory

53.21 Risks associatedwith chemical tests

Qualitative analysis — detecting the presence of inorganic chemical speciesUnit 53


Ion

Test ActionofdiluteHCl(aq) orH2SO4(aq)

ActionofBaCl2(aq)Actionof

concentratedH2SO4(aq)

Actionofdilute

HNO3(aq),followedbyAgNO3(aq)

Otherreagent(s)

Iodideion, I–

— — purple vapour(I2, SO2 andH2S)givenoff

NaI(s) +H2SO4(l)NaHSO4(s)

+HI(g)

8HI(g) +H2SO4(l) 4I2(s) +

H2S(g) + 4H2O(l)

yellowprecipitate

Ag+(aq) +I–(aq)

AgI(s)

actionofCl2(aq) +organicsolvent—apurpleorganiclayerforms*

*Cl2(aq)oxidizesBr–(aq) / I–(aq) toBr2(aq) / I2(aq).

3 The following table summarizes specific tests for some common gases andvapours.

Gasor vapour Colour Test Observation

Ammonia Colourless

Insert a piece of moist red litmuspaper into the gas.

It turns moist red litmuspaperblue.

Dipaglassrodindilutehydrochloricacid and insert it into the gas.

Dense white fumes areformed.

Carbondioxide Colourless Bubble the gas through limewater. The limewater turnsmilky.

Chlorine Greenishyellow

Insert a piece of moist blue litmuspaper into the gas.

Chlorine turns the moistblue litmus paper red andthenbleaches it.

Hydrogen Colourless Insert a burning splint into the gas. A ‘pop’ sound isheard.

Oxygen Colourless Insert a glowing splint into the gas. Theglowing splint relights.

Sulphurdioxide Colourless

Insert a piece of filter paper soakedwithacidifiedpotassiumdichromatesolution into the gas.

Thepaperturnsfromorangeto green.

Water vapour Colourless Insert a piece of blue cobalt(II)chloridepaper into thevapour.

The blue cobalt(II) chlorideturnspink.

Ion Test Action ofNaOH(aq) ActionofNH3(aq) Other test(s)

Zinc ion,Zn2+


Zn2+(aq) + 2OH–(aq)Zn(OH)2(s)

Zn(OH)2(s) + 2OH–(aq) [Zn(OH)4]

2–(aq)

whiteprecipitate, solublein excessNH3(aq) to give acolourless solution

Zn2+(aq) + 2OH–(aq)Zn(OH)2(s)

Zn(OH)2(s) + 4NH3(aq) [Zn(NH3)4]

2+(aq) +2OH–(aq)

—

2 The following table summarizes results of some tests for anions.

Ion

Test ActionofdiluteHCl(aq)orH2SO4(aq)

Action of BaCl2(aq)Action of

concentratedH2SO4(aq)

Actionofdilute

HNO3(aq),followedbyAgNO3(aq)

Otherreagent(s)

Carbonateion,CO3

2–colourless gas(CO2) givenoff

CO32–(aq) +

2H+(aq)CO2(g) +

H2O(l)

whiteprecipitatesoluble indiluteHCl(aq)withcolourless gas (CO2)givenoff

Ba2+(aq) +CO32–(aq)

BaCO3(s)

vigorous reaction,colourless gas(CO2) givenoff

— —

Hypochloriteion,OCl–

greenishyellowgas (Cl2) givenoff

OCl–(aq) +2H+(aq) +Cl–(aq)

Cl2(g) +H2O(l)

— — — —

Sulphiteion, SO3

2–colourless gas(SO2) givenoff

SO32–(aq) +

2H+(aq) SO2(g) +

H2O(l)

whiteprecipitatesoluble indiluteHCl(aq)withcolourless gas (SO2)givenoff

Ba2+(aq) + SO32–(aq)

BaSO3(s)

vigorous reaction,colourless gas(SO2) givenoff

— —

Sulphateion, SO4

2–— whiteprecipitate

insoluble indiluteHCl(aq)

Ba2+(aq) + SO42–(aq)

BaSO4(s)

— — —

Chlorideion,Cl–

— — steamy fumes(HCl) givenoff

NaCl(s) +H2SO4(l)NaHSO4(s) +

HCl(g)

whiteprecipitatesoluble inNH3(aq)

Ag+(aq) +Cl–(aq)

AgCl(s)

—

Bromideion,Br–

— — reddishbrownvapour (HBr,Br2and SO2) givenoff

NaBr(s) +H2SO4(l)NaHSO4(s) +

HBr(g)

2HBr(g) +H2SO4(l) Br2(g) +

SO2(g) + 2H2O(l)

creamyprecipitatesoluble inconcentratedNH3(aq)

Ag+(aq) +Br–(aq)

AgBr(s)

actionofCl2(aq) +organicsolvent—anorangeorganiclayerforms*

ExamtipsExamtipsExamtipsExamtips ♦ Barium ions give a green flame in flame test.

♦ Ag2O(s),NOTAgOH(s), forms whenNaOH(aq) is added toAgNO3(aq). ✔ ✘

2Ag+(aq)+2OH–(aq) Ag2O(s) +H2O(l) brown

♦ Questions often ask about identifying a solution of sulphate ions byusing an aqueous barium chloride.

♦ Questions often ask about identifying a solution of chloride ions byusing an aqueous solution of silver nitrate.


♦ Chlorineandsulphurdioxideformanacidicsolutionwhendissolvedinwater.

♦ Thechemicaltestscanbeusedtodistinguishbetweendifferentinorganicchemical species.

e.g.

Ba2+(aq) and Pb2+(aq) can be distinguished by one of the followingmethods:

– adding HCl(aq); only Pb2+(aq) gives awhite precipitate.

– adding NaOH(aq); only Pb2+(aq) gives a white precipitate which issoluble in excessNaOH(aq).

– adding KI(aq); only Pb2+(aq) gives a yellowprecipitate (PbI2(s)).

Cl–(aq) and Br–(aq) can be distinguished by one of the followingmethods:

– adding Cl2(aq); only Br–(aq) gives a brown solution.

– adding acidified AgNO3(aq); Cl–(aq) gives a white precipitate whileBr–(aq) gives a creamyprecipitate.

Mg(NO3)2(aq)andAgNO3(aq)canbedistinguishedbyoneofthefollowingmethods:

– adding zinc strip; only AgNO3(aq) forms a deposit on zinc.

– adding KCl(aq); only AgNO3(aq) gives awhite precipitate.

Na2CO3(aq), KCl(aq) and HNO3(aq) can be distinguished by addingacidified AgNO3(aq). Only Na2CO3(aq) shows effervescence while onlyKCl(aq) gives awhite precipitate.

♦ Questionsoftenaskaboutidentifyingsolutionsofbromideions/iodideions by adding aqueous chlorine.

– Chlorine oxidizes bromide ions to bromine.

Cl2(aq)+2Br–(aq) 2Cl–(aq)+Br2(aq)

Thebrominedissolvesinanorganicsolventtoformanorangeorganiclayer.

– Chlorine oxidizes iodide ions to iodine.

Cl2(aq)+2I–(aq) 2Cl–(aq)+ I2(aq)

The iodine dissolves in an organic solvent to form a purple organiclayer.

♦ Aqueoussodiumcarbonatesolutioncanprecipitateinsolublecarbonatesfrom solutions containing some cations.

♦ Questions often ask about the action of aqueous solution of lead(II)nitrate onother aqueous solutions.

unknownsamplesolution

Al3+(aq),Ca2+(aq), Mg2+(aq),

Pb2+(aq) or Zn2+(aq)

add a few drops of Na2CO3(aq)

whiteprecipitate

forms

precipitateturns brown upon

heating

blueprecipitate

colouredprecipitate

forms

Cu2+(aq)Ag+(aq)*

greenprecipitate

Fe2+(aq) or Ni2+(aq)

reddish brown precipitate

Fe3+(aq)

* Ag2CO3 precipitate decomposes to Ag2O upon heating.

Adding Pb(NO3)2(aq) to

solution containingObservation Ionic equation(s)

Bromide ions, Br–

Chloride ions,Cl–white precipitate forms,soluble in hotwater togive a colourless solution

Pb2+(aq)+2Br–(aq) PbBr2(s)Pb2+(aq)+2Cl–(aq) PbCl2(s)

Iodide ions, I–yellowprecipitate forms,soluble in hotwater togive a colourless solution

Pb2+(aq)+2I–(aq) PbI2(s)

Sulphate ions, SO42– white precipitate forms Pb2+(aq)+ SO4

2–(aq) PbSO4(s)

Carbonate ions,CO3

2–

Sulphite ions, SO32–

white precipitate forms,soluble in dilute nitricacid

Pb2+(aq)+CO32–(aq) PbCO3(s)

Pb2+(aq)+ SO32–(aq) PbSO3(s)

Example

State the expected observation in each of the following experiments, and account fortheobservationwith the aidof chemical equation(s).

a) AddingNaOH(aq)dropwise toAl(NO3)3(aq)until in excess. (4marks)

b)Adding excess H2SO4(aq) to K2CrO4(aq), and then excess FeSO4(aq) to the resultingmixture. (4marks)

c) Warming amixtureof (NH4)2SO4(aq) andCuSO4(aq)with excessNaOH(aq). (4marks)

Answer

a) Awhiteprecipitate forms. (1)

Theprecipitatedissolves in excessNaOH(aq) to give a colourless solution. (1)

Al3+(aq) + 3OH–(aq) Al(OH)3(s) (1)

Al(OH)3(s) +OH–(aq) [Al(OH)4]–(aq) (1)

Topic 1610 Analytical Chemistry 11Unit 54 Tests for functional groups, separation and purification of compounds

b)YellowK2CrO4(aq) turnsorangeupon the additionofH2SO4(aq). (1)

2CrO42–(aq) + 2H+(aq) Cr2O7

2–(aq) +H2O(l) (1) yellow orange

A green solution resultswhenFeSO4(aq) is added to theorange solution. (1)

6Fe2+(aq) +Cr2O72–(aq) + 14H+(aq) 6Fe3+(aq) + 2Cr3+(aq) + 7H2O(l) (1)

orange green

c) A gas that turnsmoist red litmuspaperblue is evolved. (1)

NH4+(aq) +OH–(aq) NH3(g) +H2O(l) (1)

Ablueprecipitate forms. (1)

Cu2+(aq) + 2OH–(aq) Cu(OH)2(s) (1)

➤QuestionsoftenaskaboutthecolourchangewhenH2SO4(aq)orHCl(aq) isadded toK2CrO4(aq).

➤Questions often ask about the expected observation when an ammoniumsalt iswarmed withNaOH(aq).

RemarksRemarks*

54.1 Chemical tests for various functional groups in carboncompounds

54.2 Tests for the alkene functional group

54.3 Tests for the alcohol functional group

54.4 Tests for the aldehyde andketone functional groups

54.5 Tests for the carboxylic acid functional group

54.6 Identifying functionalgroup(s) inanunknownsample in thelaboratory

54.7 Separation andpurificationof compounds

54.8 Distillation

54.9 Fractional distillation

54.10 Liquid-liquid extraction

54.11 Re-crystallization

54.12 Chromatography

54.13 Paper chromatography

54.14 Column chromatography

54.15 Thin layer chromatography

54.16 Tests forpurityof aproduct

Tests for functional groups, separation and purification of compoundsUnit 54

Topic 1612 Analytical Chemistry 13Unit 54 Tests for functional groups, separation and purification of compounds

54.1 – 54.6

Summary

1 The following table summarizes results of chemical tests for the alkene functionalgroup.

Test Alkene functional group –C=C–

Aqueousbromine the yellow-brown aqueous bromine becomescolourless rapidly

Cold acidified dilute aqueous solution ofpotassiumpermanganate

the purple solution becomes colourlessrapidly

2 The following table summarizes results of chemical tests for functional groupscontainingoxygen.

* 1°, 2° and3° alcohols canbedistinguishedby treatingwith theLucas reagent.

Functionalgroup

Test

Alcohols

HO

WarmwithK2Cr2O7 /H3O+ 1° and 2° alcohols* — clear orange solution turns green almost

immediately

K2Cr2O7 / H3O+ K2Cr2O7 / H3O

+

RCH2OH CR H CR OH

O O

1° alcohol

2° alcohol

K2Cr2O7 / H3O+

CHOHR

R1

CR R1

O

3° alcohols*—noobservable change

Treatmentwith2,4-dinitrophenylhydrazine

—

WarmwithTollens’ reagent(an aqueous solutionofsilvernitrate in ammonia)

—

Iodoform test (warmwithiodine in an aqueoussolutionof sodiumhydroxide)

alcohols containing the CH3C

H

OH

group gives a bright yellowprecipitate

O–Na++ CHI3

iodoformC

OR

H

C CH3R

OH

I2 / NaOH

Treatmentwith aqueoussodiumhydrogencarbonatesolution

—

Warmwith ethanol in thepresenceof concentratedsulphuric acid, followedbypouring themixture intoan aqueous solutionofsodiumcarbonate

—

Functionalgroup

Test

Aldehydes

HC

O

Ketones

C

O

Carboxylicacids

OHC

O

WarmwithK2Cr2O7 /H3O

+

clear orange solution turns green

K2Cr2O7 / H3O+

CR H CR OH

O O— —

Treatmentwith2,4-dinitro-phenyl-hydrazine

ayellow to redprecipitate forms

(H)R’

RC

H2N

aldehyde or ketone H

NO2

H

NO2 + H2O

NO2

NO2

(H)R’

RC NO +

N N

—

WarmwithTollens’reagent (anaqueoussolutionof silvernitrate inammonia)

silvermirror formson thewall of reactionvessel

R C H + 2[Ag(NH3)2]+ + 3OH–

aldehyde

O

warmR C O–

O

+ 2Ag + 4NH3 + 2H2O

— —

Iodoformtest (warmwith iodinein anaqueoussolutionof sodiumhydroxide)

aldehydesandketoneswiththe CH3C

O

groupgivesabrightyellowprecipitate

R C

O

CH3 + 3I2 + 3OH– R C

O

CI3 + 3I– + 3H2O

R C

O

CI3 + OH– R C

O

O– + CHI3

—

Treatmentwithaqueoussodiumhydrogen-carbonatesolution

— — a colourless gas(CO2) givenoff

RCOOH+NaHCO3

RCOO–Na++CO2+H2O

Warmwithethanolin thepresenceofconcentratedsulphuricacid,followedbypouring themixture intoan aqueoussolutionof sodiumcarbonate

— — a sweet, fruity-smelling estercanbedetected

RCOOH+CH3CH2OH

H+

RCOOCH2CH3

+H2O

Topic 161� Analytical Chemistry 1�Unit 54 Tests for functional groups, separation and purification of compounds

Example

a) At room temperature, acyclic carbon compound X is a liquid. It has the followingcompositionbymass:

C 64.9% H 13.5% O 21.6%

Calculate the empirical formulaof compoundX.

(Relative atomicmasses:H=1.0,C=12.0,O=16.0) (2marks)

b)0.185g of X, upon complete vaporization, occupies 60.0cm3 at room temperatureandpressure.

Deduce themolecular formulaof compoundX.

(Molar volumeof gas at room temperature andpressure=24.0dm3mol–1) (2marks)

c) Compound X gives a yellow precipitate when warmed with iodine in an aqueoussolutionof sodiumhydroxide. It showspositive resultswhen treatedwithacidifiedK2Cr2O7(aq).

i) What is the chemical formulaof the yellowprecipitate. (1mark)

ii) What does its reaction with iodine indicate about the structure of compoundX? (2marks)

iii) Suggest apossible structureof compoundX. Explainyour answer. (2marks)

ExamtipsExamtipsExamtipsExamtips ♦ Students should be able to give the correct structure of 2,4-dinitrophenylhydrazine.

♦ The chemical tests canbeused to distinguish between compounds.

e.g.

The following five colourless liquids

CH3CH2OH CH3CHO (CH3)3COH CH3CH2COOH CH3(CH2)7OH

canbedistinguished in the following way:

– OnlyCH3(CH2)7OH is immiscible withwater.

– OnlyCH3CHOreactswith2,4-dinitrophenylhydrazinetogiveayellowtoredprecipitate /onlyCH3CHOgivesasilvermirrorwhenwarmedwith the Tollens’ reagent.

– Only CH3CH2COOH reacts with NaHCO3(aq) to give a colourlessgas.

– When warmed with K2Cr2O7 / H3O+, CH3CH2OH turns the orange

solution greenwhile (CH3)3COHdoes not.

Answer

a) Suppose we have 100g of compound X, so there are 64.9g of carbon, 13.5g ofhydrogen and21.6gof oxygen.

Carbon Hydrogen Oxygen

Massofelement in thecompound

64.9g 13.5g 21.6g

Numberofmolesof atomsthat combine

64.9g12.0gmol–1 =5.41mol

13.5g1.0gmol–1 =13.5mol

21.6g16.0gmol–1 =1.35mol (1)

Simplest ratioof atoms

5.41mol1.35mol

=4.0113.5mol1.35mol

=10.01.35mol1.35mol

=1.00 (1)

∴ the empirical formulaof compoundX isC4H10O.

b)Let (C4H10O)n be themolecular formulaof compoundX.

Numberofmolesof compoundXvaporized= 60.0cm3

24000cm3mol–1

= 0.00250mol

Molarmassof compoundX = 0.185g0.00250mol

= 74.0gmol–1 (1)

Molarmassof compoundX = n(4 x 12.0+10 x 1.0 +16.0) = 74 n = 1 (1)

∴ the molecular formulaof compoundX isC4H10O.

c) i) CHI3 (1)

ii) CompoundX contains a CH3

O

C (1) /

OH

CH3C

H

(1) group.

iii)As compoundX showspositive resultswhen treatedwith acidifiedK2Cr2O7(aq), it is not a ketone. Thus, it is probably a secondary alcohol containing a

OH

CH3C

H

group. (1)

Apossible structureof compoundX:

(1)

OH

CH3CH3CH2 C

H

➤The chemical formula of iodoform is CHI3,NOTCH3I. ✔ ✘

RemarksRemarks*


54.7 – 54.16

Summary

1 The following table summarizes common separation and purification methods forliquid and solidproducts.

Typeofproduct Separationandpurificationmethod to employ

Liquidproduct

• simpledistillation

• fractionaldistillation

• liquid-liquid extraction

Solidproduct• re-crystallization

• chromatography

2 During liquid-liquid extraction, place the aqueous solution containing the liquidproductinaseparatingfunnel.Thenaddanorganicsolventthatisimmisciblewithwater, such as ethoxyethane.

ring with pieces of rubbertubing to cushion funnel

ethoxyethane layer

separating funnel

aqueous layer containing the product

3 A re-crystallizationprocess involves threemain stages:

Stage1 Choose a suitable solvent.

Stage2 Use the minimum amount of hot solvent to dissolve the crude solidproduct. Filter to remove the insoluble impurities.

Stage3 Allowthefiltratetocoolslowlyforcrystalstore-form.Collectthecrystalsby filtrationunder reducedpressure anddry them.

4 The following table summarizes the working principles of three chromatographicmethods.

Chromatographicmethod

Stationaryphase

Mobilephase Working principle

Paper water in thepaper fibres

liquidsolvent

Eachcomponentinamixturedistributes itselfbetween thewater in thepaper fibresand theliquid solvent.

Compoundsthataremoresolubleintheliquidsolvent travel up more quickly. Thus, thecomponents separate.

Columnadsorbent invertical glasscolumn

liquidsolvent

Each component in a mixture has its ownequilibrium between adsorption onto thesurface of the adsorbent and solubility in thesolvent.

Compounds that are more soluble in theliquid solvent move more quickly. Thus, thecomponents separate.

Thin layer

fine layer ofa l u m i n a o rsilicagelcoatedonto a glassplate

liquidsolvent

The component in amixture canbe identified from its retention ratio Rf.

Rf = distance travelledby the componentdistance travelledby the solvent

5 The purity of a solid product may be checked by its melting point and that of aliquidproductby its boilingpoint.

ExamtipsExamtipsExamtipsExamtips ♦ Remember the importance of releasing the pressure in the separatingfunnel regularly when shaking a liquid mixture in a separating funnel.

♦ Ethoxyethane is frequently used in liquid-liquid extraction due to thefollowing reasons:

– itischemicallyinertandunlikelytoreactwiththecarboncompounddissolved in it;

– it dissolves most carbon compounds;

– it is immiscible withwater;

– itcanbeeasilyremovedafterextractionbysimpledistillation,owingto its great volatility (boiling point 37°C).

♦ Paper chromatography can be used to separate Cu2+(aq) ions froma mixture containing Cu2+(aq) ions and Cr3+(aq) ions. This is becauseCu2+(aq) ions and Cr3+(aq) ions havedifferent colours.


Example

Benzoincanbeobtainedbyheatingbenzaldehydewithpotassiumcyanideinasolvent-free condition.

OH

C

O

C

H

KCN

benzoin (m.p. 137 °C)

CHO2

a) What is the functionofpotassiumcyanide in this conversion? (1mark)

b)Give TWO reasons why this reaction can be considered as an example of greenchemistry. (2marks)

c) Describehow thepotassiumcyanide canbe removed from the crudeproduct. (2marks)

d)Thebenzoinobtained canbepurifiedby re-crystallization.

i) SuggestTHREE criteria for an appropriate solvent for the re-crystallization. (3marks)

ii) Outline theprocedure in the re-crystallizationprocess. (3marks)

iii) Suggestamethodtoverifywhetherornot there-crystallizedsampleofbenzoinis pure. (1mark)

Answer

a) As a catalyst (1)

b)Any twoof the following:

• The reactiondoesnot involve theuseof solvents. (1)

• The reactionhas ahigh atomeconomy. (1)

• A catalyst (KCN) is used. (1)

c) Extract benzoinwith an appropriateorganic solvent (e.g. ethoxyethane). (1)

Wash theorganic layerwithwater to remove thepotassiumcyanide. (1)

d)i) Any threeof the following:

• Benzoin should have a high solubility in the solvent while the impuritiesshouldnot. (1)

• The solubility of benzoin in the solvent should be high at high temperaturesbut lowat room temperature. (1)

• The solvent shouldbe volatile. (1)

• The solvent shouldnot reactwithbenzoin. (1)

ii) Dissolve the crudebenzoin in theminimumamountofhot solvent. (1)

Filter themixturewhilehot. (1)

Allow the filtrate to cool and collect the crystals by filtration. (1)

Wash the crystals anddry. (1)

➤Questionsoftenaskstudentstodescribethere-crystallizationprocedureforthepurification of a crude solid product.

– Use activated charcoal to remove coloured impurities.

– Use a short-stem funnel when filtering thehot solution.

➤Studentsshouldbeabletodrawadiagramoftheexperimentalset-upusedfor filtration under reduced pressure.

DrawaBuchner funnel (NOT a filter funnel). ✔ ✘

to aspirator

water trap Buchnerflask

Buchner funnelfilter paper

RemarksRemarks*

iii)Determinethemeltingpointoftheproductandcomparetheresultwithliteraturedata. (1)

Topic 1620 Analytical Chemistry 21Unit 55 Quantitative methods of analysis

55.1 Quantitative analysis

55.2 Steps of theprecipitationmethod in gravimetric analysis

55.3 Possible major sourcesof error

55.4 Gravimetricdeterminationofphosphoruscontentinasampleof fertilizer

55.5 Gravimetric determination of calcium content in a samplesolution

55.6 Precipitation titrations in volumetric analysis

55.7 Redox titrations in volumetric analysis

55.8 Redox titrations involving an aqueous solution of potassiumpermanganate

55.9 Determining the iron content in commercial iron tablets

55.10 Analyzingthequalityofwaterbydeterminingitspermanganateindex

55.11 DeterminingthevitaminCcontentinasampleusingaredoxtitration involving iodine

55.12 Determining the concentration of sodium hypochlorite in ahouseholdbleachusing an iodine / thiosulphate titration

Quantitative methods of analysisUnit 55

ExamtipsExamtipsExamtipsExamtips ♦ Todeterminethepercentagebymassof iron inasample,H2SO4(aq) isusedtoconvert the irontoFe2+(aq) ions.Thenthe resultingsolution istitrated against KMnO4(aq).

HCl(aq) is NOT used in this case because MnO4–(aq) ions may oxidize

theCl–(aq) ions.

55.1 – 55.10

Summary

1 Examplesof gravimetric analysis:

a) determiningthephosphoruscontentinasampleoffertilizerbyprecipitatingthephosphorus asmagnesiumammoniumphosphate (MgNH4PO4•6H2O);

b)determiningthecalciumcontentinasamplesolutionbyprecipitatingthecalciumas calciumoxalate (CaC2O4•H2O).

2 The concentration of chloride ions in a sample solution can be determined byprecipitation titrationwith a standard aqueous solutionof silvernitrate.

Ag+(aq) + Cl–(aq) AgCl(s)

Useachromateindicatorinthetitration.Whenallthechlorideionsareprecipitated,thefirstexcesssilvernitratesolutiongivesareddishbrownsilverchromateprecipitatewith the chromate indicator. This signals the endpointof the titration.

2Ag+(aq) + CrO42–(aq) Ag2CrO4(s)

reddishbrown

3 Redox titration involving an aqueous solution of potassium permanganate can beused todetermine the iron content in commercial iron tablets.

MnO4–(aq) + 8H+(aq) + 5Fe2+(aq) Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)

Example

Awater-soluble solid sampleXcontains tin(II) ions.Thepercentagebymassof tin inX canbedetermined from the experimental data listedbelow:

Step 1 25.0cm3of0.0326moldm–3Na2C2O4(aq),afteracidification, requires26.5cm3of a certainKMnO4(aq) for complete reaction.

Step 2 An aqueous solution of 1.05g of X, after acidified, requires 32.2cm3 of thesameKMnO4(aq) for complete reaction.

a) i) Write an equation for the reaction thatoccurs in Step 1. (1mark)

ii) Describehow the titration endpoint canbedetected in Step 1. (1mark)

iii)Calculate the concentrationof theKMnO4(aq)used. (2marks)

b)The equation for the reaction involved in Step 2 is as follows:

5Sn2+(aq) + 2MnO4–(aq) + 16H+(aq) 5Sn4+(aq) + 2Mn2+(aq) + 8H2O(l)

Calculate thepercentagebymassof tin in solid sampleX.

(Relative atomicmass: Sn=118.7) (3marks)

Topic 1622 Analytical Chemistry 23Unit 55 Quantitative methods of analysis

c) StateONEassumption for the experiment. (1mark)

Answer

a) i) 2MnO4–(aq) + 5C2O4

2–(aq) + 16H+(aq) 2Mn2+(aq) + 10CO2(g) + 8H2O (1)

ii) KMnO4(aq)servesasitsownindicator.AddKMnO4(aq)untilthefirstappearanceof apersistentpalepink colour. (1)

iii)2MnO4–(aq) + 5C2O4

2–(aq) + 16H+(aq) 2Mn2+(aq) + 10CO2(g) + 8H2O(l)

?moldm–3 0.0326moldm–3

26.5cm3 25.0cm3

NumberofmolesofC2O42– ions in25.0cm3 solution

= 0.0326moldm–3 x 25.01000

dm3

= 8.15 x 10–4mol (1)

According to the equation, 2molesofMnO4– ions reactwith5molesofC2O4

2–ions.

i.e. numberofmolesofMnO4– ions in26.5cm3 solution

= 25

x 8.15 x 10–4mol

= 3.26 x 10–4mol

ConcentrationofKMnO4(aq)=3.26 x 10–4mol

26.51000

dm3

= 0.0123moldm–3 (1)

b)5Sn2+(aq) + 2MnO4–(aq) + 16H+(aq) 5Sn4+(aq) + 2Mn2+(aq) + 8H2O(l)

1.05g 0.0123moldm–3

32.2cm3

NumberofmolesofMnO4– ions in32.2cm3 solution

=0.0123moldm–3 x 32.21000

dm3

= 3.96 x 10–4mol (1)

According to the equation, 5molesof Sn2+ ions reactwith2molesofMnO4– ions.

i.e. numberofmolesof Sn2+ ions = 52

x 3.96 x 10–4mol

= 9.90 x 10–4mol

Massof Sn in solid sample= 9.90 x 10–4mol x 118.7gmol–1

= 0.118g (1)

Percentagebymassof Sn in solid sample= 0.118g1.05g

x 100%

= 11.2% (1)

c) Xdoesnot contain anyother species that can reactwithKMnO4(aq). (1)

➤It is necessary to standardize KMnO4(aq) prior to its use for titrimetricanalysis.

➤StudentsshouldbeabletodescribehowtheendpointinatitrationinvolvingKMnO4(aq) canbedetected.

RemarksRemarks*

ExamtipsExamtipsExamtipsExamtips ♦ Questions often ask about titrations involving iodine.

♦ The reaction below can be used to determine the concentration of asodium thiosulphate solution:

IO3–(aq)+5I–(aq)+6H+(aq) 3I2(aq)+3H2O(l)

– FirstpreparestandardIO3–(aq)bydissolvingaknownmassofKIO3(s)

in distilled water and diluting the solution to a known volume.

– AddexcessKI(aq)andH2SO4(aq)toaknownvolumeofthestandardIO3

–(aq) to obtain standard I2(aq).

– Titrate the I2(aq) against theNa2S2O3(aq).

Studentsshouldknowthatstandardiodinesolutioncanbepreparedbyadding excess KI(aq) and H2SO4(aq) into a known volume of standardIO3

–(aq) (refer to the example in Unit 57).

55.11 – 55.12

Summary

1 The concentration of a reducing agent in a sample solution can be determined bymeasuring the amount of iodine required to undergo a redox reaction with thereducing agent in a knownvolumeof the sample.

reducing agent+ I2 I– +otherproducts

Asaqueousiodinesolutionisunstable,iodinecanbegeneratedin situbythereactionof iodate ions (IO3

–)with iodide ions in an acidic solution:

IO3–(aq) + 5I–(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)

2 To determine the concentration of an oxidizing agent in a sample solution, allowtheoxidizingagent inaknownvolumeof thesamplesolutionto reactwithexcessiodide ions to liberate iodine. Then titrate the iodine liberated against standardNa2S2O3(aq).

oxidizing agent Stage 1 being analyzed + excess I– I2 + otherproducts in sample solution

Stage 2 I2 + 2S2O32– 2I– + S4O6

2–

knownvolume and concentration

Topic 162� Analytical Chemistry 2�Unit 55 Quantitative methods of analysis

Example

Inanexperimenttodeterminethepercentagebymassofcopperinasampleofcopperore, 4.20g of the sample were warmed with excess HNO3(aq) to convert all copper-containing substances to Cu2+(aq) ions. The resulting solution was boiled to removeallnitrogenoxides formed.

Uponcooling,thesolutionwasdilutedto250.0cm3.25.0cm3ofthedilutedsolutionwerewithdrawnandtreatedwithexcessKI(aq)toliberateCuI(s)andI2(aq).Thetitrationof the I2(aq) liberated required27.7cm3of 0.112moldm–3Na2S2O3(aq).

a) Write equations for (2marks)

i) the reactionofCu2+(aq)with I–(aq); and

ii)the reactionof I2(aq)withNa2S2O3(aq).

b)SuggesthowtheendpointofthetitrationbetweentheI2(aq)liberatedandNa2S2O3(aq)canbedetected. (2marks)

c) Calculate thepercentagebymassof copper in the sampleof copperore.

(Relative atomicmass:Cu=63.5) (4marks)

Answer

a) i) 2Cu2+(aq) + 4I–(aq) 2CuI(s) + I2(aq) (1)

ii)I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O6

2–(aq) (1)

b)TitratetheiodineliberatedagainstNa2S2O3(aq)untilthecolourofthemixturechangestopale yellow. (1)

Atthispoint,addafewdropsofstarchsolution,producingadarkbluecolour.AddNa2S2O3(aq)dropwiseuntil themixturebecomes colourless. (1)

c) I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O6

2–(aq)

?mol 0.112moldm–3

27.7cm3

Numberofmolesof S2O32– ions reactedwith iodine liberated

=0.112moldm–3 x 27.71000

dm3

=3.10 x 10–3mol (1)

According to the equation, 1moleof I2 reactswith2molesof S2O32– ions.

i.e. numberofmolesof I2 reactedwith S2O32– ions

= 3.10 x 10–3

2mol

= 1.55 x 10–3mol

=numberofmolesofI2liberatedinthereactionbetweenCu2+ionsin25.0cm3ofdiluted solution andKI(aq)

2Cu2+(aq) + 4I–(aq) 2CuI(s) + I2(aq)

4.20g 1.55 x 10–3mol 250.0cm3

(used) 25.0cm3

➤The titrationbetween the I2(aq) liberatedandNa2S2O3(aq) is carriedoutbyaddingNa2S2O3(aq) to I2(aq),NOT the reverse order.

➤Questionsoften askhow the endpointof the titrationbetween the I2(aq)liberated and Na2S2O3(aq) can be detected (refer to Fig.55.23 of thetextbook).

RemarksRemarks*

According to the equation, 2molesofCu2+ ions react to give1moleof I2.

i.e. numberofmolesofCu2+ ions in25.0cm3diluted solution = 2 x 1.55 x 10–3mol = 3.10 x 10–3mol (1)

NumberofmolesofCu2+ ions in250.0cm3dilute solution =10 x 3.10 x 10–3mol =3.10 x 10–2mol (1) =numberofmolesofCu in the sampleof copperore

MassofCu in the sampleof copperore= 3.10 x 10–2mol x 63.5gmol–1

= 1.97g

PercentagebymassofCu in the sampleof copperore = 1.97g4.20g

x 100%

= 46.9% (1)

Topic 162� Analytical Chemistry 2�Unit 56 Instrumental analytical methods

56.1 The useof instruments in analytical chemistry

56.2 Electromagnetic radiation

56.3 The interactionof radiationwithmatter

56.4 Colorimetry

56.5 Infrared spectroscopy

56.6 Basic featuresof an infrared spectrum

56.7 Characteristicabsorptionwavenumberrangesfordifferentbondsin carbon compounds

56.8 Using infrared spectrum in the identification of bonds (orfunctional groups) in carbon compounds

56.9 Interpreting infrared spectra

56.10 Mass spectrometry

56.11 Finding relative atomic masses and relative molecular massesfrommass spectra

56.12 Using fragmentation patterns to determine the structures ofmolecules of carbon compounds

56.13 Fragmentation patternsof alkanes

56.14 Fragmentation patternsof aromatic compounds

56.15 Fragmentation patternsof aldehydes andketones

56.16 Differences inmass/charge ratiobetweenpeaks

56.17 Interpreting mass spectra

Instrumental analytical methodsUnit 56 56.1 – 56.17

Summary

1 a) Colorimetry determines the concentration of a substance based on its ability toabsorb certainparts of radiationof the visible spectrum.

b)The concentration of a species being analyzed in a sample solution can bedetermined froma calibration curve.

2 a) Infrared (IR) spectroscopy is theabsorptionmeasurementof infraredradiationofdifferentwavenumbersby a sample.

b)The main goal of infrared spectroscopy is to determine the functional groupspresentincarboncompoundsasdifferentcovalentbondsabsorbinfraredradiationof characteristicwavenumbers.

c) The following table* shows the characteristic infrared absorption wavenumberranges for some covalentbonds (stretchingmodeof vibration).

Bond Compound type Wavenumber range(cm–1)

Intensity(s = strong,m=medium)

C–Cl — 700 – 800 s

C–O alcohols, esters 1000 – 1300 s

C=C alkenes 1610 – 1680 m

C=O aldehydes, ketones, acids,esters 1680 – 1750 s

C C alkynes 2070 – 2250 m

C N nitriles 2200 – 2280 m

O–H acids (hydrogen-bonded) 2500 – 3300 m (broad)

C–H alkanes, alkenes 2840 – 3095 m– s

O–H alcohols (hydrogen-bonded) 3230 – 3670 s (broad)

N–H amines 3350 – 3500 m

* Youneednotrememberthesecharacteristicwavenumberranges.Thistablewillbeprovidedin examinations.

3 a) Massspectrometryisaformofanalysisthatseparatesandidentifiessubstancesonthe basis of mass-to-charge ratios of positive ions formed from substances whenthey arebombardedbyhigh energy electrons in a vacuum.

b)Massspectracanbeusedtomeasurerelativeatomicmassesandrelativemolecularmasses.

c) Structural information of a compound can be determined from fragmentationpatterns of the molecular ion. The following table shows typical fragment ionsproduced in amass spectrometer.

Ion Mass/charge ratio (m/e) Ion Mass/charge ratio (m/e)

CH3+ 15 C4H9

+ 57

C2H5+ 29 CH3CH2CO+ 57

CH3CO+ 43 C6H5+ 77

C3H7+ 43 C6H5CH2

+ 91

Topic 162� Analytical Chemistry 2�Unit 56 Instrumental analytical methods

Example

Compound X contains carbon, hydrogen and oxygen only. It shows negative resultswhentreatedwithacidifiedK2Cr2O7(aq).Theinfraredspectrumandmassspectrumareshownbelow.Deduceonepossible structureofX. (7marks)

Tran

smit

tan

ce (

%)

wavenumber (cm–1)

100

50

04 000 3 000 2 000 1 500 1 000 500

Rel

ativ

e in

ten

sity

(%

)

Mass/charge ratio (m/e)

10 20 30 40 50 60 70 80 90

0

50

100

ExamtipsExamtipsExamtipsExamtips ♦ Infrared spectroscopy can be used to follow the progress of areaction.

e.g.

Addition polymerization is involved when Superglueworks.

CN n

COOCH2CH3

CC

H

H

CN

COOCH2CH3

CCn

H

H

additionpolymerization

Theprogressof thepolymerizationcanbemonitoredbyobservingthechangeofIRabsorptionofthepeakat1614cm–1duetothestretchingof theC=Cbond.

As the polymerization proceeds, the concentration of the monomerdecreases. Thus, the intensity of theC=Cabsorption reduces.

50

60

70

80

90

100

1 614 cm–1

t = 600 st = 400 st = 200 s

t = 0 s

Tran

smitt

ance

(%)

♦ Questionsoftengivethemassspectraofaromaticcompoundsandaskabout possible structures of the compounds. Pay special attention tothe following peaks:

– m/e =77due toC6H5+ ion; and

– m/e =91due toC6H5CH2+ ion.

Topic 1630 Analytical Chemistry 31Unit 57 Contribution of analytical chemistry to our society

➤Questions may give the IR spectra of carbon compounds and ask aboutpossiblestructuresofthecompounds.Payspecialattentiontothefollowingcharacteristic infrared absorptions that often appear.

RemarksRemarks*

Bond Compound typeWavelength range

(cm–1)Intensity

C=O aldehydes, ketones, acids, esters 1680 – 1750 strong

O–H acids (hydrogen-bonded) 2500 – 3300 medium (broad)

O–H alcohols (hydrogen-bonded) 3230 – 3670 strong (broad)

57.1 Introduction

57.2 Food analysis

57.3 Gas chromatography

57.4 Carbon monoxide in the air

57.5 Dioxins in the air

57.6 Formaldehyde —a common indoor air pollutant

57.7 Forensic chemistry

57.8 Drink driving

57.9 Fingerprint analysis

57.10 The roleof analytical chemistry in clinical diagnoses

Contribution of analytical chemistry to our societyUnit 57

Answer

The infrared absorption at 1680 – 1750cm–1 indicates the presence of a C=O bondinX.

HenceXmaybe an aldehyde, a ketone, an acidor an ester. (1)

Theabsenceofstronginfraredabsorptionat1000–1300cm–1and2500–3300cm–1suggests thatXdoesnot containC–OandO–Hbonds.

HenceXmaybe an aldehydeor a ketone. (1)

AsXshowsnegativeresultswhentreatedwithacidifiedK2Cr2O7(aq),thusXisprobablya ketone. (1)

The peak at m/e = 86 in the mass spectrum suggests that the relative molecular massofX is 86. (1)

Molecular ions of a ketone (RCOR’) tend to undergo fragmentation to give the RCO+ion andR’CO+ ion.Thepeak atm/e=57 is likely tobedue to theC2H5CO+ ion. (1)

Thepeakatm/e=29islikelytobeduetotheC2H5+ionwhichformedwhenC2H5CO+

ion loses aCO fragment. (1)

X is likely tobeC2H5COC2H5. (1)

Topic 1632 Analytical Chemistry 33Unit 57 Contribution of analytical chemistry to our society

57.1 – 57.10

Summary

1 Volumetricanalysisorgaschromatographycanbeemployedtodeterminetheethanolcontentof a spirit.

2 Ingas-liquidchromatography,themobilephase isagasandthestationaryphase isanon-volatile liquid.

3 Carbonmonoxide level in the airmaybedeterminedby infrared spectrometry.

4 The level of dioxins in the air may be determined by gas chromatograph-massspectrometry (GC-MS).

5 Formaldehyde is a common indoor pollutant. The level of formaldehyde in indoorairmaybedeterminedbyhighperformance liquid chromatography (HPLC).

6 The working of an alcohol breathalyzer is based on the redox reaction involvingpotassiumdichromate.

7 Latent fingerprints canbedevelopedwith iodine fumes.

ExamtipsExamtipsExamtipsExamtips ♦ When orange dichromate ions are brought into contact with ethanol,the following redox reactionoccurs.

2Cr2O72–(aq)+3CH3CH2OH(aq)+16H+(aq)

4Cr3+(aq)+3CH3COOH(aq)+11H2O(l)

Green chromium(III) ions form.

Example

Anexperiment,consistingof the twostages,wascarriedout todeterminethesulphurdioxide content in awhitewine.

Stage 1 Excess KI(aq) and H2SO4(aq) were added to 25.0cm3 of 0.00360moldm–3KIO3(aq). The mixture was then diluted to 250.0cm3 using distilled water togive a standard aqueous iodine solution.

Stage 2 25.0cm3portionsof thewinewere titratedwith the standard aqueous iodinesolution,using starch solution as an indicator. Themean titrewas11.4cm3.

a) The reaction in Stage 1 canbe representedby the following equation:

IO3–(aq) + 5I–(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)

Calculate themolarityof the standard aqueous iodine solutionprepared.(2marks)

b)State the colour change at the titration endpoint in Stage 2. (1mark)

c) i) Write a chemical equation for the reaction involved in the titration. (1 mark)

ii)Calculate the concentration, inmoldm–3, of sulphurdioxide in thewhitewine. (2marks)

Answer

a) IO3–(aq) + 5I–(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)

0.00360moldm–3 ?moldm–3

25.0cm3 250.0cm3

NumberofmolesofKIO3 in25.0cm3 solution

=0.00360moldm–3 x 25.01000

dm3

=9.00 x 10–5mol (1)

According to theequation,1moleof IO3– ions reactswith I– andH+ ions togive3

molesof I2.

i.e. numberofmolesof I2 formed =3 x 9.00 x 10–5mol = 2.70 x 10–4mol

Concentrationof I2(aq) =2.70 x 10–4mol

250.01000

dm3

= 1.08 x 10–3moldm–3 (1)

b)Fromcolourless toblue (1)

c) i) I2(aq) + SO2(aq) + 2H2O(l) 2I–(aq) + SO42–(aq) + 4H+(aq) (1)

ii) I2(aq) + SO2(aq) + 2H2O(l) 2I–(aq) + SO42–(aq) + 4H+(aq)

1.08 x 10–3moldm–3 ?moldm–3

11.4cm3 25.0cm3

Numberofmolesof I2 in11.4cm3 solution

=1.08 x 10–3moldm–3 x 11.41000

dm3

=1.23 x 10–5mol (1)

According to the equation, 1moleof I2 reactswith1moleof SO2.

i.e. numberofmolesof SO2 in25.0cm3whitewine=1.23 x 10–5mol

Concentrationof SO2 inwhitewine =1.23 x 10–5mol

25.01000

dm3

= 4.92 x 10–4moldm–3 (1)

➤Whenstandard I2(aq) isused todetermine theconcentrationofa reducingagentinasample(e.g.sulphurdioxidecontentinwine)usingstarchsolutionas an indicator, the colour change at the end point is from colourless toblue.

Distinguishthisfromthecasewhenusingstarchsolutionasanindicator inthetitrationbetween I2(aq) liberatedandNa2S2O3(aq) (refer totheexampleinUnit 55).

RemarksRemarks*

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