2022_H1_Chemical_Equilibria_Tutorial(tutor).doc

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 forth