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### Transcript of EXPERIMENT B8: EFFECT OF TEMPERATURE ON EQUILIBRIUM .equilibrium is established ... A saturated...

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EXPERIMENTB8:EFFECTOFTEMPERATUREONEQUILIBRIUMCONSTANT

LearningOutcomesUponcompletionofthislab,thestudentwillbeableto:

1) Predicttheeffectoftemperatureontheequilibriumconstant.2) Designanexperimenttomeasurethethermodynamicparametersforthe

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ConsidertheexothermictransformationofAtoB.Sinceheatisgeneratedinthisreaction,theequationcanbewrittenas:

A

C+heat

D

Ca(OH)2(s)

Ca2+(aq)+2OH(aq)

Theequilibriumconstantforthisprocessisgivenby:

K=[Ca2+][OH]2

NotetheabsenceofCa(OH)2(s)intheexpressionfortheequilibriumconstant.Sincetheamountofasolidisgenerallythoughttobeunchanged,itisexcludedfromtheequilibriumconstantexpression.Thefollowingequilibriumtableshowshowtheequilibriumconstantiscalculatedforsuchaprocess.Inthetable,Iisassumedtobesomeinitialamountofthesolid

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calciumhydroxide,andxistheamountofthesolidthatdissolvesinwatertoformtheaqueousions.

Ca(OH)2(s)

Ca2+(aq) + 2OH(aq)Initialconcentrations I 0 ~0Amountthatdissolves -x +x +2xEquilibriumamount Ix x 2xTherefore:

K = [Ca2+][OH ]2

K = x (2x)2 = 4x 3

2x=[OH]

Therefore:

x = [OH]

2

Theconcentrationofhydroxide,[OH],isrelatedtothehydrogenionconcentration,[H+],andthereforetothepHofthesolution,bytheionicproductofwater.

KW=1.010-14=[H+][OH]

pH=-log[H+]

Insummary,ifthepHofthesaturatedsolutionisdetermined,the[H+]canbeascertained.Usingtheionicproductofwater,onecanthendeterminethe[OH].Knowingthe[OH]onecancalculatexandthereforeK.

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DeterminationofthermodynamicparameterssuchasH,S,andGAsdiscussedearlier,chemicalequilibriumisimpactedbychangesintemperature.Therelationshipbetweentheabsolutetemperature,T,andtheequilibriumconstant,K,forareactionisgiveninEquation1below: G=-RTlnK Equation1InEquation1,GisthestandardfreeenergychangeandRistheuniversalgasconstant(8.314J/mol-K).Bydefinition,thestandardfreeenergychange,Gisalsorelatedtothestandardenthalpychange,Handthestandardentropychange,SaccordingtoEquation2. G=H-TS Equation2Inthermodynamics,thevaluesofHandSmaygenerallybeconsideredtobeinvariablewhenthetemperatureischangedandthevalueofGontheotherhandchangeswithtemperature.Therefore,Equations1and2maybecombinedasfollows: RTlnK=HTS Equation3Equation3canfurtherbemanipulatedtoisolatethedependentvariablesandtheconstants.Dividingbothsidesoftheequationby-RTresultsinEquation4.

lnK = H!

R1T

+

S !

R Equation4

InEquation4,H,S,andRareconstantsandKandTarevariables.AcomparisonofEquation4withtheequationofastraightline,y=mx+b,indicatesthefollowing:

lnKisanalogoustoy

1Tisanalogoustox

H !

Risanalogoustom

S !

Risanalogoustob

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ThereforealinearregressionofaplotoflnKvs.

1Tshouldresultinastraightline

whoseslopewillbe

H !

Randwhosey-interceptwillbe

S !

R.SinceRisthe

universalgasconstant,thevaluesofHandScanbedeterminedfromtheslopeandtheintercept,respectivelyofthebestfitline.

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2. ObtainapHmeter,thermometer,andahotplatefromthestockroom.3. TheinstructorwilldemonstratetheproperuseandcalibrationofthepHmeter.

4. Setupahotwaterbath.Inordertodothis,halffillalargebeakerwithtapwater

smallamountofsolidCa(OH)2tothetesttubetoensurethatthesolutionisindeedsaturated.

6. Obtainaringstandandaclampandsuspendthetesttubecontainingthe

Ca(OH)2intothewaterbathensuringthatthesolutioniscompletelysubmergedinsidethewater.

7. MeasureandrecordthetemperatureoftheCa(OH)2solution.Aimtostart

instructor.

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DataTable

Temperature,C

pH

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DataAnalysis1. Calculatethe[H+]fromthepHmeasuredateachtemperature.

[H +] =10pH 2. Calculatethe[OH]

[OH ] = 1.0 1014

[H +]

3. Calculatex.

x = [OH]

2

4. CalculateK.

K = 4x 3

5. CalculatelnK(naturallogarithmofK)

6. ConvertalltemperaturestotheKelvinscaleandcalculate

1T

7. PlotagraphoflnK(y-axis)vs.

1T(x-axis).

8. Useregressionanalysestofindtheequationofthebestfitlinearequationforthe

dataandobtaintheslope(m)andthey-intercept(b).9. Usingtheslope,calculatethevalueofH.

slope(m) = H!

RH ! = slope R( )

10. Usingthey-intercept,calculatethevalueofS.

y intercept = S!

RS ! = R (y intercept)

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1. Enterthedata(temperatureinCandpH)incolumnsAandB.UseRow1for

2. EnterformulasinRow2foreachcalculation,asshowninthetableabove.3. Ineachcolumn,pointthecursortothebottomrightcornerofacell(sayC2)and

dragdown(tillRow15)theplussigntocopytheformulatotheothercells.RepeatthisforallthecolumnsDthroughI.

4. Todrawagraph,selectthexandydata,whichwouldbedatainfieldsH:2-15

andI:2-15.5. ClickInsertandthenChart.ChooseXYscatterandselectMarkedScatter6. Whenthegraphisdisplayed,clickonanydatapointonthechartandfromthe

toolbar,selectChartandthenInsertTrendline.7. Fromthepop-upbox,selecttheOptionstabandchecktheboxes:1)Display

equationand2)DisplayR-squaredvalueandclickOK.

A B C D E F G H I1 Temperature,T

C pH [H+] [OH] x K T,Kelvin 1/T ln(K)2 70 pH1 =10^(-B2) =(1.0E-14)/C2 =D2/2 =4*((E2)^3) =A2+273.15 =1/G2 =LN(F2)3 65 pH2 4 60 pH3 5 55 pH4 6 50 pH5 7 45 pH6 8 40 pH7 9 35 pH8 10 30 pH9 11 25 pH10 12 20 pH11 13 15 pH12 14 10 pH13 15 5 pH14

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