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Unit2:ChemicalEquilibriumWhatthisunitisabout?Whentworeactingspeciesarereactedwitheachother,somethingwillhappen,butwhathappenswhenthereactionstops?Isitbecausethereactionranoutofreactantsorhasthereactionreacheditsequilibrium?Whatcontrolstheamountofproducts?DynamicEquilibriumManychemicalreactionsarereversible,meaningthereactantscanreacttomakeproductsortheproductscanreacttomakereactants.

Reversiblereactionsaredenotedby suchas

H2(g)+I2(g) 2HI(g)

ThisreactionisstatingthatwhenHydrogengasandIodinegasisreactedwitheachother,itwillproducehydrogeniodideBUThydrogeniodidegascanbedecomposedbackintothereactants.ThisiscalledDynamicEquilibrium.Whatisdynamicequilibrium?Theconditionwheretherateofforwardandbackwardreactionsareidentical.NoticethatitistheRATEandNOTtheCONCENTRATION!Atequilibrium,concentrationsofreactantsandproductsdonothavetobeidentical!

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Itdoesn’tmatterifwestartwithallproductsorallreactants,eventuallythesamerateofeachwouldresultafterreactionatequilibrium.Dynamicequilibriumcanonlybeachievedinaclosedsystem.

Inthediagramabove,atequilibrium,therateofproductsandreactantsarethesameeventhoughiftheconcentrationsaredifferent.

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TheequilibriumConstant,KTheequilibriumconstant,K,isusedtotoseeifthereactionfavorstheamountofproductsproducedorthereactants.K,iscalculatedbycomparingtherelativeamountsofproductstoreactants.Generalreactionequation:

aA+bB cC+dDThecoefficients,a,b,c,dareusedasexponentsintheK(LawofMassActionExpression)whileA,B,CandDareconcentrationsofthereactingcompoundsinmol/L.

KeyMathematicalConceptsthatcanbelearnedfromthisexpression:1. Asyoucansee,K,ismerelyaratiobetweentheamountofproductstotheamountofreactants.

2. WhenKisalargenumber(ex.1.5x1019),thatmeansthereareALOTmoreproductsproducedthanreactantsatequilibrium.

3. WhenKisasmallnumber(ex.2.5x10-4),thatmeansthereareMOREreactantsthanproductsatequilibrium.

4. Kcannotbenegative

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WritingLawofMassAction(Kconstantexpression)• LiquidsandsolidsarenotincludedintheLawofMassActionexpression,onlyaqueous(aq)andgas(g)areincluded!

• CoefficientsbecomeexponentsExample:

CO(g)+3H2(g) CH4(g)+H2O(g)

(Noticethe3becameanexponent)

Example2:

Example3:

Example4:

Determinethevalueof whentheequilibriumconcentrationsare:

Arethereactantsorproductsfavored?Whyorwhynot?

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Example5:

Atagiventemperature,thereaction producesthefollowingconcentrationsatequilibrium:.Calculatetheequilibriumconstant,K,if[H2]=0.320,[CO2]=0.420,[CO]=0.200and[H2O]=0.500.

Example6:

Commentonthefavorabilityofproductformationineachofthereactions.a.H2(g)+F2(g)↔2HF(g) KC=1.0×1013b.SO2(g)+NO2(g)↔NO(g)+SO3(g) KC=1.0×102c.2H2O(g)↔2H2(g)+O2(g) KC=6.0×10–28

Example7:Chemistshavedeterminedtheequilibriumconstantsforseveralreactions.Inwhichofthesereactionsaretheproductsfavouredoverthereactants?

a. KC=1.0×102b. KC=3.5c. KC=0.003d. KC=6.0×10–4

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CalculatingKfromequilibriumconcentrationsRecallthat:

1. EquilibriumconstantiscalculatedfromconcentrationsofreactantsandproductsATequilibriumnotinitialconcentrations(beforereaction).

2. Inordertocalculatetheequilibriumconstantwithinitialconcentrations,stoichiometrymustbeused.

3. The“I-C-E”tableisusedtocalculateKwheninitialconcentrationsareprovided.I–Initial,C–Change,andE–equilibrium

Example:

2CH4(g) C2H2(g)+3H2(g)

IfwestartwithonlyCH4(g)withaninitialconcentrationof0.300Mandatequilibriumthe[C2H2(g)is0.045M,calculatetheequilibriumconstant.(Note:thequestionhasINITIALandEQUILIBRIUMconcentrations!Recognizethisquestiontype)

1. SetupICEtable&placetheproperconcentrationattheappropriatebox.

2. MakesureyouhavethecorrectSigns(+/-).Use+signsforproductssinceyouaregaining,and–signsforreactantssinceyouareusingthemupifyouareprovidedwiththeequilibriumconcentration.

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3. Usestoichiometrytocalculatetheamountchangeforotherreactingspecies.

4. CalculateallconcentrationsatequilibriumandsolveforK.

5. ReasonifyourKindicatesproductorreactantfavouring.

Example2:(QuestiongivesyoutheK,andinitialvalues).N2(g)+O2(g)2NO(g)

Theequilibriumconstantis6.76.If6.0molesofnitrogenandoxygengasesareplacedina1.0Lcontainer,whataretheconcentrationsofallreactantsandproductsatequilibrium?

1. SetupICEtable.2. Inthisquestion,sinceyouarenotprovidedwithANYconcentrationsAT

equilibrium,youwillneedtouseAlgebratosubstituteintothe“Change”rowofyourICEtable.

3. Makesureyour“x”isconsistentwiththecorrect+/-signandalsothestoichiometryratio.

4. SolveforxonceyouhaveestablishedyourequilibriumconcentrationsandsubstituteyourvalueofxbackintoyourICEtable.

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CalculatingtheReactionQuotient,QUptothispoint,welookedatifareactionatequilibriumisfavoringtheproductsorreactantsATequilibrium.Butcanwepredictthedirectionofareactionatanytime?Atanyonetimeofareaction,isthereactionproducingmoreproducts(movingforward)ormorereactants(movingbackwards)?Thisiswherethereactionquotient,Qwillcomeintoplay.Thereactionquotient,Q,istheinstantaneousratioofproductstoreactants.WethencomparetheKandtheQtoseethedirectionofthereaction.SettingupReactionQuotient,QThesetupisexactlyidenticaltoK.

aA+bB cC+dD

ButwhatisimportantisknowingwhatQmeansifitislessthanorgreaterthanK.3scenariostoQtoK.IfQ=K.IfQisequaltoK,thatmeanstheratesofreactantsandproductsgeneratedareidenticalbetweenQandK.Thismeansthereactionisatequilibrium.IfQ<K.IfQislessthanK,thatmeansthatatthatparticularmomentoftime,thereactionwouldhavemorereactantsthanproductssinceyouwouldhavemoreof

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[A]and[B]thanof[C]and[D]whencomparedtoK.Inotherwords,inorderforQtobesmallerthanK,thedenominatorofQmustbegreater,meaningmorereactants.Thereactionisnotatequilibriumandismovingforwardinconvertingthereactantsintoproducts.IfQ>K.IfQisgreaterthanK,thatmeansthatatthatparticularmomentoftime,thereactionwouldhavemoreproductsthanreactants.Thereactionisnotatequilibriumandismovingbackwardsinconvertingthereactantsintoreactants.

Noticefromthegraphabove,thehorizontallinerepresentsK,andthecurveistheprogressionofthereactionthroughtime.Atthebeginningofthereaction,theconcentrationofreactantsislarge(QissmallerthanK–rememberlargerdenominator)andasthereactionprogresses,Qincreases,causingthereactantstodecrease.WhenQbecomesgreaterthanK,it

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meanstherearemoreproductsproducedandmustberevertedbytothereactantsfavoringabackwardsreaction.

ReactionQuotientQuestions

1. Forthereactionbelow,K=25.0:

A(s)+2B(g)↔2C(g)

If[A]=12.0mol/L[B]=2.0mol/L[C]=30.0mol/L

a) Isthesystemisatequilibrium.b)Whichreactionisfaster(favoured),forwardorreverse?c)Whichconcentrationsareincreasingordecreasing?

2. Thereexistsanequilibriumif5.0molesofCO2,5.0molesofCOand0.20molesofO2areina2.0Lcontainerat562°C.FindKCforthereaction

2CO(g)+O2(g)↔2CO2(g)

Wouldthesystembeatequilibriumif[CO2]=15.8mol/L,[CO]=10.0mol/Land[O2]=0.25mol/L?Ifnot,whichreactionisfavoured?

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3. Forthereactionbelow,K=16.0:

2SO2(g)+O2(g)↔2SO3(g)

Initially,[SO2]=5.0mol/L,[O2]=10.0mol/Land[SO3]=0.Aftertwohours[O2]=7.9mol/L.Isthesystematequilibrium?Ifnot,whichsubstancesareincreasingandwhicharedecreasing?

4. Thereaction

4NH3(g)+5O2(g)↔4NO2(g)+6H2O(g)

isatequilibriumwhen[H2O]=0.100mol/L,[O2]=2.00mol/L,[NO]=0.200mol/Land[NH3]=0.500mol/L.If0.75molesofH2O,12.0molesofNO,30.0molesofO2and0.30molesofNH3areina3.0Lcontaineratthesametemperature,isequilibriumachieved?Ifnot,whichreactionisfavoured?

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Review Questions

1. Why are solids and liquids not included in the equilibrium constant expression?

2. What does the value of mean in terms of the amount of reactants and products?

3. What is the correct equilibrium constant expression for the following reaction: ?

a.

b.

c.

d. 4. What is the correct equilibrium constant expression for the following

reaction: ?

a.

b.

c. d.

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5. Consider the following equilibrium system: . At a certain temperature, the equilibrium concentrations are as follows: , and . What is the equilibrium constant for this reaction?

a. b. c. d.

6. For the reaction , the equilibrium constant was found to be 3.86 at a certain temperature. If is placed in a container, what is the concentration of at equilibrium?

a. b. c. d. not enough information is available

7. Write the equilibrium constant expressions for each of the following equations:

a. b. c. d.

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CalculatingEquilibriumConstants

8. Amixtureatequilibriumat827°Ccontains0.552molesofCO2,0.552molesH2,0.448molesCO,and0.448molesofH2Oina1.00Lcontainer.Whatisthevalueoftheequilibriumconstant,Keq?

CO2(g)+H2(g) CO(g)+H2O(g)

9. Theequilibriumconstantforthereaction

4H2(g)+CS2(g) CH4(g)+2H2S(g)at755°Cis0.256.WhatistheequilibriumconcentrationofH2Sifatequilibrium[CH4]=0.00108mol/L,[H2]=0.316mol/L,[CS2]=0.0898mol/L?

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10. FindthevalueofKeqfortheequilibriumsystem

ZnO(s)+CO(g) Zn(s)+CO2(g)ifatequilibriumthereare3.0molesofCO,4.0molesofZnand4.0molesofCO2ina500.0mLcontainer.USEICETABLESforthefollowing

11.Thedecompositionofhydrogeniodidetohydrogenandiodineoccursbythereaction

2HI(g)H2(g)+I2(g)Hydrogeniodideisplacedinacontainerat450°Canequilibriummixturecontains0.50molesofhydrogeniodide.Theequilibriumconstantis0.020forthereaction.Howmanymolesofiodineandhydrogeniodidearepresentintheequilibriummixture?

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12. H2(g)+Cl2(g) 2HCl(g)Astudentplaces2.00molH2and2.00molCl2intoa0.500Lcontainerandthereactionisallowedtogotoequilibriumat516°C.IfKCis76.0,whataretheequilibriumconcentrationsofH2,Cl2andHCl?

13. IfKeq=78.0forthereactionA(s)+2B(g) 2C(g)andinitiallythereare5.00molesofAand4.84molesofBina2.00Lcontainer,howmanymolesofBareleftatequilibrium?

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LeChâtelier’sPrinciple

Inthelate1800s,achemistbythenameofHenry-LouisLeChâtelierwasstudyingstressesthatwereappliedtochemicalequilibria.Heformulatedaprinciple,LeChâtelier’sPrinciple,whichstatesthatwhenastressisappliedtoasystematequilibrium,theequilibriumwillshiftinadirectiontopartiallycounteractthestressandonceagainreachequilibrium.

Forinstance,ifastressisappliedbyincreasingtheconcentrationofareactant,theequilibriumpositionwillshifttowardtherightandremovethatstressbyusingupsomeofthereactants.Thereverseisalsotrue.Ifastressisappliedbyloweringareactantconcentration,theequilibriumpositionwillshifttowardtheleft,thistimeproducingmorereactantstopartiallycounteractingthatstress.Thesamereasoningcanbeappliedwhensomeoftheproductsisincreasedordecreased.

LeChâtelier'sprincipledoesnotprovideanexplanationofwhathappensonthemolecularleveltocausetheequilibriumshift.Instead,itissimplyaquickwaytodeterminewhichwaythereactionwillruninresponsetoastressappliedtothesystematequilibrium.

Thereare4conditions:

b) ConcentrationChangec) TheadditionofaCatalystd) Pressurechangee) Temperaturechange

1. Effect of Concentration Changes

Let's use Le Châtelier's principle to explain the effect of concentration changes on an equilibrium system.

N2O4(g) 2NO2(g)

If there is a sudden increase of NO2, then the is an added stress to the products side. Le Chatelier’s principle states that reactions will alleviate the stress by shifting the equilibrium, which is to change the products to reactants. Result: An increase of [N2O4(g)]

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The reaction can be viewed as a seesaw

N2O4(g) 2NO2(g)

Ifthereisanincreaseofthereactants,thestressisonthereactants,sothereactionwillthenshifttotherightbyproducingmoreproducts.Tippedontheleftsidesotheproductsideastoincrease.Likewise,ifthereisanincreaseofproducts,thestressisontheproducts,sothereactionwilltheshifttotheleftbyproducingmorereactants.IfthereisanDECREASEofreactants,thenthestressisonthereactantsside,sotheequilibriumshiftstowardsthereactants.Elevatedontheleftsidesotheproductsideastoincrease.IfthereisanDECREASEofproducts,thenthestressisontheproductsside,sotheequilibriumshiftstowardstheproducts.

Example:

For the reaction , what would be the effect on the equilibrium system if:

1. increases 2. increases 3. increases

Solution:

1. The equilibrium would shift to the right. would increase, more would be produced (but

that does not increase its concentration since its a solid), and would decrease.

2. The equilibrium would shift to the right. would decrease, more would be produced

(but again no change in concentration), and would increase.

3. The equilibrium would shift left. and ] would increase, and would be used up but not change its concentration.

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Example:

Forthereaction ,whichwaywilltheequilibriumshiftif:

1. decreases2. decreases3. decreases

Solution:

1. left 2. left 3. right

Example:

Here'sareactionatequilibrium.Notethephasesofeachreactantandproduct.

1. Whichwaywilltheequilibriumshiftifyouaddsome tothesystemwithoutchanginganythingelse?

2. After hasbeenaddedandanewequilibriumisreached,howwillthenewconcentrationof comparetotheoriginalconcentrationof ?

3. After hasbeenaddedandanewequilibriumhasbeenestablished,howwillthenewconcentrationof comparetotheoriginalconcentrationof ?

4. After hasbeenaddedandanewequilibriumhasbeenestablished,howwillthenewconcentrationof comparetotheoriginalconcentrationof ?

5. Whichwaywilltheequilibriumshiftifyouaddsome tothesystemwithoutchanginganythingelse?

6. After hasbeenaddedandanewequilibriumhasbeenestablished,howwillthenewconcentrationof compareitsoriginalconcentration?

7. After hasbeenaddedandanewequilibriumhasbeenestablished,howwillthenewconcentrationof compareitsoriginalconcentration?

8. Whichwaywilltheequilibriumshiftifyouaddsome tothesystemwithoutchanginganythingelse?

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Solution:

1. The equilibrium will shift toward the products. 2. The new concentration of will be higher than the original. 3. The new concentration of will be higher than the original, but lower than the concentration right after was added. 4. Since is a solid, its concentration will be the same as the original. There will be less of it since some was used in the

equilibrium shift, but the concentration will be the same. 5. The equilibrium will shift toward the reactants. 6. The new concentration of will be lower than the original. 7. The new concentration of will be higher than the original. 8. Since is a solid, adding will not change its concentration and therefore has no effect on the equilibrium. It is possible

that adding some will increase the surface area of and therefore increase the forward reaction rate, but it will also increase the reverse reaction rate by approximately the same amount, hence no shift in equilibrium.

2. Addition of a catalyst

Adding a catalyst to a system at equilibrium will NOT affect the equilibrium position. However, if a catalyst is added to a system which is not at equilibrium, the system will reach equilibrium much quicker since forward and reverse reaction rates are increased.

3. Pressure Change / Volume change

The pressure and volume change in a system have a inversely proportional effect since the increase in volume will decrease the pressure while increase in pressure will decrease in volume. If you can understand 1, then the other is the opposite.

In Haber’s Ammonia producing process

N2(g) + 3H2(g) 2NH3(g)

Pressurechangeeffects:

Ifthepressureholdingthenitrogenandhydrogenisincreased,thenumberofcollisionsbetweenthereactantswillbemorefrequentsincetherearemoreofthem,thus,moreproductswillform.Thereactionwillhavetodecreasethestressonthereactantsideduetotheincreasedinpressureandproducemoreproductstominimizethecollisions.

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Ifthepressureisdecreasedhowever,weneedtoshiftthereactiontothedirectionwheretherewillbemorecollisionstocompensatethedecreasedinpressure.

Summary:

Toseewhichdirectionareactionwillshift,thenumberofmoles(coefficients)mustbetakenintoaccount.Pressure=collisions,themorepressure,themorecollisions,themorestress,thustheshiftmustbetowardstheoppositetoalleviatethestress.

3a. The effects of Volume change

In order to see the effect of a volume change, understanding the effects of pressure change will make it easier.

Going back to Haber’s Ammonia producing process

N2(g) + 3H2(g) 2NH3(g)

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If the volume is decreased, where is the stress and which direction will the reaction shift?

If the volume is decreased, then the pressure is increased. That means the stress is where there are more collisions (reactant side with more moles). In order to alleviate the stress, the reaction shifts to the right to make less moles.

However, if the volume is increased, then the pressure is decreased. The equilibrium will now have to shift to the side where there are MORE moles, in this case, reactants.

If both sides contain the same number of reactant and product moles, increasing or decreasing volume will have no effect.

4. The effects Temperature change

Some reactions will be exothermic (produces heat)

A + B C + D + Heat

While some reactions are endothermic (takes in heat)

A + B + heat C + D

A simple way of looking at it is that the effects of temperature change is similar to concentration change that an increase in temperature will add stress to the temperature side. The result will be shifting the reaction to the opposite side.

Example:

N2(g) + 3H2(g) 2NH3(g) + heat

If reaction is heated up, the reaction will shift to make more reactants, to absorb the excess heat.

If the reaction is cooled down, then the reaction will shift to make products, to release more heat.

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Example:

N2O4(g) (Colorless) 2NO2(g) (Brown)

Is the reaction an endothermic or exothermic reaction if the reaction turns brown when heated?

It must be an endothermic reaction

N2O4(g) (Colorless) + heat 2NO2(g) (Brown)

Since if we were to heat it up, the stress will be on the reactant side, and to alleviate the stress, it has to shift to the right to absorb the heat.

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InterpretingGraphsduringreactionstress

� Thesegraphswillshowwhatishappeningwhenastressisapplied� Theconcentrationvs.timegraphsshowwhatishappeningtoeachreacting

specieswhenthestressisapplied� Onanygraphanequilibriumisshownbyaflathorizontalline� Afterthestresshasbeenapplied,thelinecurvesupordowndependingonthe

situation,andthenitlevelsoffatanewequilibriumlevel� Thestressisindicatedbyasharppeak(upforincreasesanddownfordecreases)� TherestofthelinesgentlycurveupordowndependingonLeChatelier� Temperaturechangescauseallreactingspeciestohavegentlecurvesinthe

appropriatedirections

Example: N2(g) + 3H2(g) 2NH3(g) + heat

Att2,thereisasharppeakatNH3(g),thatmeanstherewasasuddenincreaseofammoniagas.Asaresult,thestressisontheproductside,andthereactionshiftstothelefttoachieveequilibrium,whichisseenatt3.

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2NO2(g)ßàN2O4(g)

SuddendropofN2O4concentrationshiftsthereactiontotherightbycreatingMOREoftheN2O4compounduntilequilibrium.

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LeChatelier’sPrinciple

1. Forthereaction

PCl3(g)+Cl2(g)ßàPCl5(g)ΔH=–92.5kJ

predicttheeffectonthepositionoftheequilibriumthatresultsfrom

a)increasingthetotalpressurebydecreasingvolume.b)injectingmoreCl2gaswithoutchangingthevolume.c)increasingthetemperature.d)increasingthevolumeofthecontainer.e)addingacatalyst.

2. Forthereaction

CH4(g)+H2O(g)+49.3kJßàCO(g)+3H2(g)

predicttheeffectonthepositionoftheequilibriumthatresultsfrom

a)increasingtemperature.b)decreasingtemperature.c)decreasingthepressure.d)decreasingthevolumeofthecontainer.e)addingasoliddryingagentsuchasCaCl2whichreactswithH2O(g).

3. Forthereaction

9.4kJ+2HI(g)ßàH2(g)+I2(g)

a)Whatistheeffecton[HI]ifasmallamountofH2isadded?b)Whatistheeffecton[HI]ifthepressureofthesystemisincreased?c)Whatistheeffecton[HI]ifthetemperatureisincreased?d)Whatistheeffecton[HI]ifacatalystisadded?

4. ForthereactionCO(g)+2H2(g)ßàCH3OH(g)+energypredicttheeffectofthefollowingchangesontheequilibriumconcentrationofCH3OH(g)

a)adecreaseintemperature.b)anincreaseinpressure.c)additionofH2(g).d)additionofacatalyst.

5. Intheequilibriumreaction

2NO(g)+O2(g)ßà2NO2(g)+114.6kJ

Whatwillbethechangeintheequilibrium[NO2]undereachofthefollowingconditions?

a)O2isadded.b)NOisremoved.c)energyisadded.

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6. ForthefollowingreactionΔH=+58.9kJ

N2O4(g)ßà2NO2(g)

howwilltheequilibrium[NO2]beaffectedbythefollowing?

a) anincreaseinpressure.b)anincreaseintemperature.c)theadditionofacatalyst.

AdditionalLeChatelier’sQuestions

1. Whatistheeffectontheequilibriumpositionifthepressureisincreased?2. Whatistheeffectontheequilibriumpositionifthepressureisdecreased?3. UseLeChâtelier’sPrincipletopredictwhatwillhappentothefollowingreactionatequilibriumifthepressureis

increased: .Markallthatapply.a. equilibriumpositionshiftsrightb. equilibriumpositionshiftsleft

c. willdecrease

d. willincrease4. UseLeChâtelier’sprincipletopredictwhatwillhappentothefollowingreactionatequilibriumifthepressureis

decreased: .Markallthatapply.a. equilibriumpositionwillnotshiftb. equilibriumpositionshiftsleft

c. willincrease

d. willincrease5. UseLeChâtelier’sprincipletopredictwhatwillhappentothefollowingreactionatequilibriumifthevolumeis

decreased: .Markallthatapply.a. equilibriumpositionshiftsrightb. equilibriumpositionshiftsleft

c. willincrease

d. willdecrease

6. Forthereaction ,whatwouldbetheeffectontheequilibriumsystemifthepressureincreases(orthevolumedecreases)?

7. Forthereaction ,whatwouldbetheeffectontheequilibriumsystemifthepressuredecreases(orthevolumeincreases)?

8. Forthereaction ,whatwouldbetheeffectontheequilibriumsystemif

a. thepressureincreases?b. thevolumedecreases?

9. Forthereaction ,whatwouldbetheeffectontheequilibriumsystemifa. thepressureincreases?b. thevolumedecreases