topic 9.3 2009 group vii prelim soln

7/27/2019 Topic 9.3 2009 Group VII Prelim Soln

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GroupVIChemistry-SuggestedSolutions

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Topic9.3:GroupVIChemistry

ACJC/2009/P2/Q2d,e1

ACJC/2009/P3/Q2f2

AJC/2009/P3/Q5a,b3


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CJC/2009/P3/Q2b4(i) Passchlorinegasintohotaq.sodiumhydroxide

ORHeatNaClO. 3Cl2+6NaOH5NaCl+NaClO3+3H2O 3ClO-2Cl-+ClO3

- (ii) ClO3

-+H2OClO4-+2H+2e-

Overall:4ClO3-3ClO4

-+Cl- (iii) pHindependent. H+orOH-isnotinvolvedintheoverallequation. HCI/2009/P2/Q5a-b

5 (a) (i)

dehydrationofalcoholtoformalkene: OResterification:

nitrationofbenzene:

OResterification:ORhydrationofalkene:

H2C CH2 + H2OCH3CH2OHconc.H2SO4

RCO2H+R'OH RCO2R'+H2Oconc.H2SO4

+conc.HNO3conc.H2SO4

NO2

+H2O

RCO2H+R'OH RCO2R'+H2Oconc.H2SO4

CH2=CH2+H2O CH3CH2OHconc.H2SO4


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(ii) H2SO4(l)+NaBr(s)HBr(g)+NaHSO4(aq)

H2SO4(l)+2HBr(g)Br2(g)+SO2(g)+2H2O(l)

(b) (i) Tetrachloromethane/hexane(oranyothernon-polarorganicsolvent)

(ii) Thehalideisiodide.2I(aq)+Cl2(g)2Cl

(aq)+I2(aq)

HCI/2009/P3/Q1d6 (i) BothCl2andI2existasdiatomicmoleculesandhavedispersionforcesexisting

betweenmolecules.I,beinglowerdowninthegroupthanC l,hasabiggerelectroncloudsizeandhence,strongerdispersionforces.ThereforeI 2islessvolatilethanC l2.

(ii) 4Cl2(aq)+S2O32(aq)+5H2O(l)8Cl

(aq)+2SO4

2(aq)+10H+(aq)

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

2(aq)

E,/VCl2+2e=2Cl

+1.36I2+2e=2I

+0.54

E,Cl2/ClismorepositivethanE

,I2/I,indicatingthat

Cl2isastrongeroxidizingagentthanI 2.Hence,Cl2canoxidizeS2O3

2toSO42whileI2canonlyoxidize

S2O32toS4O6

2.

JJC/2009/P3/Q3b,c

7 (b) AwhiteprecipitateofAgC lisobtainedwhensodiumchlorideismixedwith silver nitrate. Precipitatedissolves in aqueous ammoniato give acolourlesssolutionofAg(NH 3)2

+. [1]

Ag+(aq)+Cl(aq)AgCl(s)

Ag+(aq)+2NH3(aq)Ag(NH3)2+(aq)

WhenNH3isadded,theformationofthecomplexiondecreasestheconcentrationofAg +,causingequilibriumpositionoftheequilibriumAgC lAg++Cltoshifttotherightsuchthationicproduct


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(c) NaAt(s)+H2SO4(l)HAt(g)+NaHSO4(s) [1]

2HAt(g)+H2SO4(l)At2(g)+SO2(g)+2H2O(l)

8HAt(g)+H2SO4(l)4At2(g)+H2S(g)+4H2O(l)

MI/2009/P3/Q5d

8 2ClCl2+2e E(Cl/Cl2)=1.36V []

2II2+2e E(I/I2)=0.54V []

IisastrongerreducingagentthanCl,i.e.morelikelytobeoxidisedbyconc.

H2SO4,toformI2instead,thanCl.

I+H2SO4HI+HSO4 ;

8HI+H2SO44I2+H2S+4H2O ;

Pungentsmelldetectedwiththeformationofablacksolid. ;

MJC/2009/P3/Q29a(i) Gasliberated-Chlorinegas ClO-(aq)+Cl-(aq)+H2O(l)Cl2(g)+2OH

-(aq)(ii) Chlorine is only slightly/ moderately soluble in water as the VDW of attraction is not

compatibletothehydrogenbondingbetweenthewatermolecules

(iii) ConcentratedH2SO4isonlyabletooxidizeHItoI 2orBr-toBr2

NaCl+H2SO4HCl+NaHSO4

NaBr+H2SO4HBr+NaHSO4

2HBr+H2SO4Br2+SO2+2H2O

orNaI+H2SO4HI+NaHSO4

8HI+H2SO44I2+H2S+4H2O(iv) Thermal stabilitydecreases from HBr to HAt as the covalent bond strengthdecreases

downthegroupresultinginthedecreaseofthebonddissociationenergy

BEofHI(+299kJmol1)


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RI/2009/P3/Q4a,b10 (a) Iodine oxidises thiosulfate to tetrathionate resulting in a change of average

oxidationnumberfrom+2to+2.5:

I2+2S2O322I+S4O6

2

Chlorine oxidises thiosulfate to sulfate(VI) resulting in a change of averageoxidationnumberfrom+2to+6:

4Cl2+S2O32+5H2O8Cl

+2SO42+10H+

SincechlorineisabletoincreasetheoxidationstateofSinthiosulfatemoregreatlythaniodine,chlorinehashigheroxidisingpowerthaniodine.

(b)(i)NaCl(s)+H2SO4(l)NaHSO4(s)+HCl(g) (ii) Hydrogen iodide formed is oxidised by conc H2SO4 to iodine since HI is easily

oxidised.

(iii) Concentratedphosphoric(V)acid. SAJC/2009/P2/Q7a,b11

(a) Fromdatabooklet,atomicsize:F0.072nm;

Cl:0.099nm;Br:0.114nm;I:0.133nm Assizeofhalogenatomincreasesdownthegroup,thebondlengthofHX

increases/effectivenessofoverlapoforbitalsdecreases.BondenergyofHX

decreasesandeaseofdecompositionofHXincreases. Or

Fromdatabooklet:Bondenergyvalues:HF:562

kJmol-1;HCl:431kJmol-1;Br:366kJmol -1andHI:299kJmol-1AsthebondenergyofHXdecreasesdownthegroup,theeaseofdecomposition

ofHXincreases.Thebondenergydecreasesdownthegroupastheatomicsizeof

halogenatomsincreasesandHXbondlengthincreases.

(b) (i) 2HBr+H2SO4

SO2+Br2+2H2O (ii) HIisastrongerreducingagentthanHBr

ItisabletoreduceSfromoxidationstate +6(inH2SO4)to-2(inH2S).

HBrisonlyabletoreduceSto+4oxidationstate(inSO 2)


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TPJC/2009/P3/Q2a142(a) Reducingpowerofhalides(oreaseofoxidationofhalides)increasesdownthegroup

andthisisevidentfromtheirdecreasingE values

OR E

valuesbecomeslesspositiveasshown: Cl2(g)+2e2Cl(aq) E=+1.36V

Br2(g)+2e2Br(aq) E=+1.07V

I2(g)+2e2I(aq) E=+0.54V

Sodiumchloride*Sodiumchloridereactswithconc.H 2SO4toproducewhitefumesofhydrogenchloride.

2NaCl(s)+H2SO4(l) 2HCl(g)+Na2SO4(aq)OR NaCl(s)+H2SO4(l) NaHSO4(aq)+HCl(g)

Sodiumbromide*Sodiumbromidereactswithconc.H 2SO4toproducewhitefumesofhydrogenbromide.SomeHBrgascanbefurtheroxidisedbyconcentratedH 2SO4toformreddishbrownBr 2gas.Conc.H2SO4isreducedtoformpungentSO2.

2NaBr(g)+H2SO4(l) 2HBr(g)+Na2SO4(aq)OR NaBr(s)+H2SO4(l) NaHSO4(aq)+HBr(g)2HBr(g)+H

2SO

4(l) Br

2(g)+SO

2(g)+2H

2O(l)

Sodiumiodide*Sodiumiodidereactswithconc.H 2SO4toproducewhitefumesofhydrogeniodide.MostoftheHIcanbefurtheroxidisedbyconcentratedH 2SO4toformpurplefumesofI 2gasConc.H2SO4isreducedtoformpungentH 2S.

2NaI(g)+H2SO4(l) 2HI(g)+Na2SO4(aq)OR NaI(s)+H2SO4(l) NaHSO4(aq)+HI(g)8HI(g)+H2SO4(l) 4I2(s)+H2S(g)+4H2O(l)


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YJC/2009/P2/Q1b15

YJC/2009/P3/Q4a16

topic 9.3 2009 group vii prelim soln

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