2022 H1 Chemical Equilibria Tutorial(tutor)

H1PhysicalChemistryTutorial ChemicalEquilibrium St.Andrew’sJuniorCollegeH1Chemistry2022PhysicalChemistryTutorialChemicalEquilibrium DISCUSSIONQUESTIONS Equilibriumconstant Kc calculation1. N2 (g)+O2 (g) 2NO(g) Δ H=+180kJmol -1 Theequilibriumconstantforthereactionaboveis6.2x10 -4 at2000°C.(a) Writedownanexpressionfor KC fortheaboveequilibrium,statingitsunits. Units: nounits(b) A25dm 3 flaskcontains0.27molesofN2 ,0.001moleofNOand xmolesofO2 atequilibrium.Calculatethevalueof x.[Ans:0.00597mol] Concentrationof N2 atequilibrium= =0.0108moldm -3 Concentrationof O2 atequilibrium= moldm -3 Concentrationof NOatequilibrium= =4.0x10 -5 moldm -3 1

H1PhysicalChemistryTutorial ChemicalEquilibrium 2.N2010P3Q2b(iv) Theadditionofglucono-delta-lactone,GDL,tosoymilkproducesasoftformofdofuduetoagradualacidificationofthemixture.Inaqueoussolutionthefollowingequilibriumisslowlysetup. When1.00gofGDL(Mr=178)wasdissolvedin50.0cm 3 waterandthesolutionallowedtoreachequilibrium,theconcentrationofgluconicacidwasfoundtobe0.0670moldm -3 . (i) Writeanexpressionfor Kc for theequilibriumabove, andusethedatagiventocalculateitsvalue.Youcanassumethat[H2 O]=55.5moldm -3 throughout.[Ans:0.0266mol ‒1 dm 3 ] Initial[GDL]=(1.00/178)/0.050=0.1124moldm ‒3 GDL H2 O gluconicacid Initialconc/moldm -3 0.1124 55.5 0 Changeinconc/moldm -3 ‒0.0670 0 +0.0670 Equilibriumconc/moldm ‒3 0.0454 55.5 0.0670 Kc =[gluconicacid]/[GDL][H2 O]Kc =0.0266mol ‒1 dm 3 (3sf) 2

H1PhysicalChemistryTutorial ChemicalEquilibrium LeChatelier’sPrinciple&EquilibriumPosition3. [J95/I/2](a) StateLeChatelier’sPrinciple.(b) Inrelationtothefollowingequilibria,Equilibrium1 H2 O(g) +C(s) H2 (g) +CO(g) Δ H=+131kJmol -1 Equilibrium2 2CrO4 2- (aq) +2H + (aq) Cr2 O7 2- (aq) +H2 O(l)UseLeChatelier’sprincipletopredictandexplaintheeffectontheequilibriumpositionandKc onthefollowingchanges:(i) increasingpressureonEquilibrium1,(ii) increasingthetemperatureinEquilibrium1,(iii) increasing[H + (aq)]onEquilibrium2. (a)LeChatelier’sPrinciplestatesthatifasystematdynamicequilibriumissubjectedtoachange, thesystemrespondsinsuchawaysoastooppose/counteractthechange,andanewequilibriumisformed. (b)(i)WhenpressureisincreaseinEquilibrium1,theequilibriumpositionshiftsLEFTtodecreasethepressurebydecreasingtheamountofgaseousparticles.Kc hasnochangeasit’sonlyaffectedbytemperature. b(ii)WhentemperatureincreasedinEquilibrium1, theequilibriumpositionshiftsRIGHTtodecreasethetemperaturebyabsorbingtheadditionalheat,favouringtheendothermicreaction.SinceKc =kf /kb ,whentemperatureincreases,bothrateforwardandrate backward increases. However, since the forward reaction isfavoured,rateforwardincreasesmorethantheratebackward.Hence,Kc increases. b(iii) When[H + ]increases,theequilibriumpositionshiftsRIGHTtodecrease[H + ].Kc hasnochangeasitsonlyaffectedbytemperature. 3

H1PhysicalChemistryTutorial ChemicalEquilibrium INTEGRATEDQUESTION4. At200°C,Kc forthereaction,PCl5 (g) PCl3 (g) +Cl2 (g) Δ H=+124kJmol −1 hasanumericalvalueof8x10 −3 .(a) Writeanexpressionfor Kc forthisreaction.Statetheunitsfor Kc . units:moldm −3 (b) Whatisthevalueof Kc forthereversereactionat200°Candwhatareitsunits?[Ans:125] KC ofthereversereaction= =125mol -1 dm 3 (c) Howwouldtheequilibriumpositionandthevalueof KC changeif(i) morePCl5 isadded;ThepositionofequilibriumshiftsRIGHTtodecreasetheamountofPCl5 .Valueof KC remainsunchangedasitisonlyaffectedbytemperature. (ii) thepressureisincreased; ThepositionofequilibriumshiftsLEFTtodecreasetheamountofgasparticlessoastodecreasepressure.ValueofKC remainsunchangedasitisonlyaffectedbytemperature. (iii) thetemperatureisincreased?TheequilibriumpositionshiftsRIGHTtoabsorbtheexcessheatsoastodecreasethetemperaturebyfavouringtheendothermicreaction.SinceKc =kf /kb ,whentemperatureincreases,bothrateforwardandratebackwardincreases. However, sincetheforwardreactionisfavoured, rateforwardincreasesmorethantheratebackward.Hence,Kc increases. 4

H1PhysicalChemistryTutorial ChemicalEquilibrium (d) AsampleofpurePCl5 wasintroducedintoanevacuatedvesselat200°C.Whentheequilibriumwasobtained,theconcentrationofPCl5 was5.0x10 -2 moldm -3 .WhataretheconcentrationsofPCl3 andCl2 atequilibrium?[Ans:0.020moldm -3 ] PCl5 (g) PCl3 (g) +Cl2 (g)Initialconc/moldm -3 5.0x10 -2 +x 0 0Change/moldm -3 -x +x +xequilibriumconc/moldm −3 5.0x10 -2 x X 8x10 -3 =x=0.020[PCl3 ]=[Cl2 ]=0.020moldm -3 5

H1PhysicalChemistryTutorial ChemicalEquilibrium 5. 2018A’levelP2/7(c) Chlorine and bromine are formed when bromine monochloride, BrCl,decomposes. 2BrCl(g) Br2 (g)+Cl2 (g)Measuredamountsofeachofthegasesweremixedandsealedinacontainer.The mixture was allowed to reach equilibrium at 100°C. The equilibriumconcentrationofeachgaswasmeasuredandisshowninTable7.1. Table7.1Temperature/°C [BrCl]/moldm -3 [Br2 ]/moldm -3 [Cl2 ]/moldm -3 100 6.26x10 -2 1.62x10 -3 3.48x10 -2 (i) Writetheexpressionfor Kc andgiveitsunits. Kc =Nounits (ii) Calculatethevalueof Kc at100°C.[Ans:0.0144] Kc = =0.0144 (iii) Theexperiment wasrepeatedat 250°C. Thenumerical valueof Kc wasfoundtobe0.132.Deducewhetherthedecompositionreactionisexothermicorendothermic.Explainyouranswer.Whentemperatureincreases, Kc increases. Thismeant that forwardrateincreasesmorethanbackwardrate.Hence,theforwardreactionisfavouredandisendothermic. (iv) Thefirst experiment at 100°Cwasrepeatedbut thetotal pressurewasdoubled.Predicttheeffectthiswouldhaveonthevalueof Kc at100°C.Explainyouranswer.[Nocalculationsarerequired.]ThereisnochangeinKc . Kc isonlyaffectedbythechangeintemperature. 6

H1PhysicalChemistryTutorial ChemicalEquilibrium 6.Twogases,AandB,reactasfollows.A(g) +B(g) 2C(g)Amixturecontaining1moleeachofAandBwasheatedinaclosedvesselandthereactionwasallowedtoreachequilibriumat400K.Thegraphshowshowthenumberofmolesofeachgasvarieswithtime. (a) CalculatethevalueoftheequilibriumconstantKc forthisreactionat400K.[Ans:9] (b)Onreachingequilibrium,anadditional1moleeachofAandBwasinjectedintothevessel and the systemwas allowedtoreachthenewequilibriumat thesametemperatureof400K.CalculatetheamountofA, BandCatthenewequilibrium.[Ans:A&B=0.8mol,C=2.4mol]LetthechangeinnumberofmolesofAbex. A +B 2CInitialmole/mol 1.4 1.4 1.2Changeinmole/mol -x -x +2xFinalmole/mol (1.4–x) (1.4–x) (1.2+2x) NoofmolesofAandBatnewequilibrium=1.4–0.6=0.8mol 7

H1PhysicalChemistryTutorial ChemicalEquilibrium NoofmolesofCatnewequilibrium=1.2+2(0.6)=2.4mol (c)Extendthegraphandincludethesketchofhowthenoofmolesofthegaseschangewithtimewhen1moleeachofAandBwerelaterinjectedintothevessel. 7. 2015A’levelP2/5(d) *Note: Though this question takes reference fromthe industrial conditions of theproductionofSO3 ,itdidnotasktojustifywhythisconditionsareusedbuthowtheseindustrialconditionsaffectthepositionofequilibrium.Hence,theseindustrialconditionswill take reference fromroomtemperature and pressure and hence theindustrialconditionsareconsideredashightemperatureandhighpressure.● Ahighpressurewouldshiftthepositionoftheequilibriumtotherighttolower pressure by favoring the side with less amount of gasmoleculesorparticles.Atthesametime,highpressurewillleadtoanincreaseintherateof 8

H1PhysicalChemistryTutorial ChemicalEquilibrium reaction(astherearemoreparticlesperunitvolumeleadingtoanincreaseinfrequencyofeffectivecollision) ● Ahightemperaturewouldshiftthepositionoftheequilibriumtothelefttolower temperaturebyfavortheendothermicreactiontoabsorbtheadditionalheat.Atthesametime,hightemperaturewillleadtoanincreaseintherateofreactionaswell (asthekineticenergyoftheparticlesincreaseleadingtoanincreaseinfrequencyofeffectivecollision) ● Acatalyst doesnotaffectthepositionofequilibrium. However, acatalystdoes increase the rate of both the forward and backwardreaction(asitprovidesanalternativepathwaywithloweractivationleadingtoanincreaseinfrequencyofeffectivecollion) 9