Experiment6∙Calorimetry 6‐1

Name___________________________________________________________LabDay__________LabTime_________Experiment6∙CalorimetryPre‐labquestionsAnswerthesequestionsandhandthemtotheTFbeforebeginningwork.(1)Whatisthepurposeofthisexperiment?__________________________________________________________________________________________________________________________________________________________________________________________________________________(2)HowisheatqrelatedtoenthalpychangeΔHatconstantpressure?__________________________________________________________________________________________________________________________________________________________________________________________________________________(3)YouwilldeterminethecalorimeterconstantCcalofyourcalorimeter.WhatpropertyofacalorimeterdoesCcalmeasure?__________________________________________________________________________________________________________________________________________________________________________________________________________________(4) Youwillmeasure the enthalpy changeΔH at constant pressure of several processes.WilltheseΔHvaluesbepositiveornegative?Willheatbeabsorbedorreleased?__________________________________________________________________________________________________________________________________________________________________________________________________________________(5) Youwill calculate several temperature changesΔT using the formulaΔT =Tmix –Ti.HowwillyoudeterminethevalueofTmix?__________________________________________________________________________________________________________________________________________________________________________________________________________________

Experiment6∙Calorimetry 6‐2

Experiment6

Calorimetry

MathematicaldevelopmentThecalorimeterconstantCcalCalorimetry is thescienceofmeasuring thequantitiesofheatreleasedorabsorbedduringachemical reaction.Theamountof heat that flows in or out of a system depends on (1) thequantityofmatterthatconstitutesthesystem,(2)theidentityofthatmatter,and(3)temperaturechangeexperiencedbythesystemasitabsorbsorreleasesheat.Inequationform

q=mCΔTwhereqrepresentsheat,m isthemassofthesystem,C istheheatcapacityofthesystem,andΔTisthetemperaturechange.TheheatcapacityCisameasureofhowasubstancerespondstotheabsorptionorreleaseofheat;substancesthathavealowvalueofCsuchasiron(C=0.45J/(g·°C))tendtobegoodcon‐ductorsofheatwhereassubstancesthathaveahighvalueofCsuchaswater(C=4.18J/(g·°C))tendtobegoodinsulators. A vessel called a calorimeter is needed to hold the sub‐stancesunderstudy.Ideally,thecalorimeterisaperfectinsula‐tor,thatis,itshouldpreventthelossofheatfromthesystemtothe surroundings and it should not allow heat from the sur‐roundingstoenterthesystem.Inpractice, thisstateofaffairsis extremely difficult to achieve: heat is inevitably exchangedbetweenthesystemandthesurroundings.

Experiment6∙Calorimetry 6‐3

Consider what happens when a quantity of hot water ispouredintoaquantityofcoldwaterinsideacalorimeter.Thefollowingrelationshipaccountsfortheheatexchanged:heatlostby = heatgainedby + heatgainedbythehotwater thecoldwater thecalorimeterThisrelationshipisexpressedsymbolicallyas –qhw=qcw+qcal –mhwChwΔThw=mcwCcw∆Tcw+Ccal∆Tcw (Eqn.6‐1)wheremhwisthemassofthehotwater,mcwisthemassofthecoldwater,Chwistheheatcapacityofthehotwater,Ccwistheheat capacity of the cold water, ΔThw is the temperaturechangeexperiencedbythehotwaterasaresultofmixingwiththecoldwater,∆Tcwisthetemperaturechangeexperiencedbythecoldwaterasaresultofmixingwiththehotwater,andCcaldenotes the calorimeter constant, which is a measure of thecalorimeter’sabilitytoactasaninsulator. Wenowemploythesubstitutions mhw=ρhwVhw and mcw=ρcwVcwwhereρ representsdensityandV representsvolume.For thesakeof simplicitywe ignore the fact that thedensity and theheatcapacityofwatervarywithtemperature;thus, ρcw=ρhw=ρw and Ccw=Chw=CwPluggingthesesubstitutionsintoEqn.6‐1andsolvingforCcalgives

�

Ccal = −ρwCw VhwΔThwΔTcw

⎛ ⎝ ⎜

⎞ ⎠ ⎟ +Vcw

⎡

⎣ ⎢

⎤

⎦ ⎥

Inserting the numerical values ρw = 1.00 g/mL and Cw =4.18J/(g·°C)gives

Experiment6∙Calorimetry 6‐4

�

Ccal = −4.18JmL⋅°C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vhw

ΔThwΔTcw

⎛ ⎝ ⎜

⎞ ⎠ ⎟ +Vcw

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐2a)

You use a slightly modified version of Eqn. 6‐2a to computeCcalofyourcalorimeter.ThemolarenthalpychangeofreactionΔH Atconstantpressure,theheatinoroutofasystemisequaltotheenthalpychangeΔH.Nospecialarrangement isneededtoguarantee that the experiment takes place at constant pres‐sure: theatmosphereautomaticallyprovidesa constantpres‐sureof1atm. Suppose that the system consists of a saltMB(s) that ab‐sorbsheatwhenitdissolvesinwaterinacalorimeter:

MB(s)+heat→MB(aq)Thefollowingrelationshipaccountsfortheheatexchanged:heatgainedby = heatlostby + heatlostbythesystem thesolution thecalorimeter(i.e.,bythesalt)Atconstantpressure,thisheatequationcanbeexpressedas qsys=–qsoln–qcal ΔHsys=–msolnCsolnΔT–CcalΔT (Eqn.6‐3a)whereΔHsysistheenthalpychangeofthesystem(i.e.,thesalt),msolnisthemassofthesolution,Csolnistheheatcapacityofthesolution,ΔT isthetemperaturechangeexperiencedbytheso‐lutionasaresultof thedissolutionof thesalt,andCcal is thecalorimeterconstant. Tomake themeasurementofΔHsysmoremeaningful, it iscustomarytoreportitsvalueonaper‐molebasis.Thus,wearereally interested in ΔHsys/nsys, where nsys is the number ofmolesofsaltthatdissolve.WewillgivethequantityΔHsys/nsysthesymbolΔH .BecausethenumberofmolesnofasubstanceisrelatedtothemassmofthatsubstanceandtoitsmolarmassMby

Experiment6∙Calorimetry 6‐5

n=m/M

dividingbothsidesofEqn.6‐3abynsysandmakingthesubsti‐tutionsΔHsys/nsys=ΔHandnsys=msys/Msysgives

�

ΔH = −msolnCsolnΔT +CcalΔT

msys /Msys (Eqn.6‐4)

Asbefore,themassmsolnofthesaltsolutioncanberelatedtoitsdensityanditsvolume:

msoln=ρsolnVsolnForsimplicity,weassumethat theaqueoussaltsolution issodilutethatitsdensityρsolnandheatcapacityCsolnarethesameas those of pure water, that is, ρsoln = ρw = 1.00 g/mL andCsoln=Cw=4.18J/(g·°C).SubstitutingthesevaluesintoEqn.6‐4gives

�

ΔH = −MsysΔTmsys

⎛

⎝ ⎜

⎞

⎠ ⎟

4.18JmL⋅°C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vsoln +Ccal

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐5)

You use Eq. 6‐5 to compute the value of themolar enthalpychangeΔHofavarietyofreactions.ProcedureDeterminationofCcalThefirstpartofthelabsessionisdevotedtoevaluatingCcalofyourcalorimeter.Youuseanaluminumvesselasacalorimeter.Thesolutionsthatyouarestudyingshouldbeplacedinthein‐sulatedinnercupofthecalorimeter.Thelidofthecalorimeteris equipped with a cap (which should remain in place at alltimes),athermometer,andastirringring(seeFigure6‐1).Itisessentialthatyouusethesamecalorimeterforallyourwork. Measure out about 50 mL of deionized water using agraduatedcylinder.ThisvolumecorrespondstoVcwinEqn.6‐

Figure6‐1Asee‐throughviewofthecalorimeter.Aninsulatedinnercup

holdsthesolutionsbeingstudied.Thelidofthe

vesselisequippedwithacap(whichshouldremain

inplaceatalltimes),athermometer,andastir‐

ringring.

19.6

Experiment6∙Calorimetry 6‐6

2a; record the volume in your notebook. Pour thewater intotheinnercupofthecalorimeter.Replacethelidandrecordthetemperatureofthewaterat30‐secintervalsuntilitstabilizes.Be sure that the thermometer is suspended near, but nottouching, the bottom or sides of the inner cup. Call the tem‐peratureatwhich thewater stabilizes the initial temperatureofthecoldwaterTcw,i;besuretorecordit. Using a Bunsen burner, bring about 100mL of deionizedwatertoafullboilinabeaker.Assumethatthetemperatureoftheboilingwater is100°C;call thistheinitialtemperatureofthe hotwaterThw,i. Do not attempt tomeasure the tempera‐ture of the boiling water because you may destroy the tem‐perature sensor in the thermometer in the process. Put onheavy‐dutygloves,extinguishtheburner,pourtheboilingwa‐terintothecalorimeter,replacethelid,andvigorouslyagitatethecontentsofthecalorimeterbymovingthestirringringupanddown.Recordthetemperature30secafterthehotwaterisaddedtothecalorimeterandat30‐secintervalsthereafterforatleast5min;agitatethecontentsofthecalorimeterthrough‐out the data acquisition period. You want to observe a tem‐peraturedecrease: if youdon’t, continue takingdatapast therecommended5mintimeperiod. Attheendofthedataacquisitionperiod,measurethetotalvolumeofwaterVtot inthecalorimeterusingagraduatedcyl‐inder.ThevolumeofhotwaterVhwinEqn.6‐2aequalstheto‐talvolumeofwaterinthecalorimeterminusVcw,thatis,Vhw=Vtot–Vcw.WorkinguptheCcaldataInyournotebookprepareaplotofyourCcaldata,plottingtimealongthexaxisandtemperaturealongtheyaxis.Callthetimewhenyouaddedthehotwatertothecoldwatert=0.Theplotshould look like Figure 6‐2. For maximum accuracy the plotshould take up a full page in your notebook.Donotwait topreparethisplotandtocalculateCcalathomeafterlab!IfyourCcal data is no good, youmust know so immediately sothatyoucantakeremedialaction. ThevalueofCcaliscomputedfromEq.6‐2a:

Donotboilthewatertoofar

aheadofthetimethatyouwill

bereadytomixitwiththe

coldwaterinthecalorimeter.

Ifyouaddsignificantlyless

than100mLofboilingwater,

youwillarriveatavalueof

Ccalthatisnegative,whichis

impossible.

Careful!Thecalorimetermay

bequitehot.Putonheavy

glovesbeforeattemptingto

removetheinnercupofthe

calorimeter.

Experiment6∙Calorimetry 6‐7

�

Ccal = −4.18JmL⋅°C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vhw

ΔThwΔTcw

⎛ ⎝ ⎜

⎞ ⎠ ⎟ +Vcw

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐2a)

YoualreadymeasuredVcw andVhw.WenowexplainhowthequantitiesΔTcwandΔThwaredetermined.Let’sdefine ΔTcw=Tcw,f–Tcw,i and ΔThw=Thw,f–Thw,iTcw,i andThw,i are the initial temperatures of the coldwaterandofthehotwater,respectively;youalreadymeasuredTcw,iandyouareassumingthatThw,i=100°C.Tcw,fandThw,farethefinaltemperaturesreachedbythecoldwaterandthehotwa‐ter, respectively. It’s clear that Tcw,f = Thw,f because the twobodiesofwaterwill havebeenmixed. Let’s call this commonfinaltemperatureofthemixtureTmix.Thus,thequantitieswearelookingforare ΔTcw=Tmix–Tcw,i and ΔThw=Tmix–Thw,i Usearulertodrawastraight linethroughthedatapointslyingonthedownwardslopingportionoftheplot.Makesurethatthelineinterceptstheverticaltemperatureaxiserectedatt=0 (seeFigure6‐2).ThetemperatureTmixcorrespondstothey‐interceptofthelineandrepresentsthetemperaturethatthe

Figure6‐2HypotheticaldatacollectedduringthemeasurementofCcal.Thehotwaterisaddedtothecoldwaterinthecalorimeteratt=0.ThelinedrawnthroughthedatapointsinterceptstheverticaltemperatureaxisatTmix.

Experiment6∙Calorimetry 6‐8

mixture would have exhibited if mixing and heat exchangewereinstantaneous. WeareatlastreadytocalculateCcalusingEqn.6‐2b,whichisamoredetailedversionofEqn.6‐2a:

�

Ccal = − 4.18JmL⋅° C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vhw

Tmix − 100°CTmix − Tcw,i

⎛

⎝ ⎜

⎞

⎠ ⎟ + Vcw

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

(Eqn.6‐2b)

Ccalmustbeapositivenumber.Ifyouarriveatanegativenumber,youdidsomethingwrong.Acommonmistakeisboil‐ingthewatertoofaraheadofthetime,leadingtotheadditionof significantly less than100mLofboilingwater.Checkyourcalculationsorrepeattheexperiment.IfonthesecondtryyouonceagainarriveataCcalthatisnegativeandyou’resurethatno errors have been committed, setCcal equal to zero joulesperdegreeCelsiusandcontinuewiththeexperiment.Experiment#1DeterminationofΔH ofNH4Cl(s)→ NH4+(aq)+Cl–(aq)Weigh out 3–4 g of ammonium chloride (NH4Cl(s)). In yournotebook record the exact mass taken. Measure out with agraduatedcylinderabout100mLofdeionizedwaterandplaceit in the calorimeter; record the exact volume in your note‐book. Cover the calorimeterwith the lid and record the tem‐peratureofthewaterinthecalorimeterat30‐secintervalsun‐tilitstabilizes.WewillcallthetemperatureatwhichthewaterstabilizestheinitialtemperatureTsoln,i;besuretorecordit. Once the temperature of thewater in the calorimeter hasstabilized,addtheNH4Cl(s)tothecalorimeter,replacethelid,andagitatethecontentsofthecalorimeterbymovingthestir‐ringringupanddown.Recordthetemperature30secaftertheNH4Cl(s) is added to the calorimeter and at 30‐sec intervalsthereafter for at least 5min; agitate the contents of the calo‐rimeterthroughoutthedataacquisitionperiod. WhenyouaddtheNH4Cl(s), thetemperatureof thewaterinthecalorimeterwillgodown.Youmusttakedatauntilyouobserveatemperatureincreaseofatleastafewtenthsofade‐gree; you may have to collect data past the recommended5mintimeperiod.

Experiment6∙Calorimetry 6‐9

Attheendofthedataacquisitionperiod,disposeoftheso‐lutioninahazardouswastereceptacle.Rinseoutthecalorime‐teranddisposeoftherinsesinahazardouswastereceptacle. Repeat the procedure in this section, but this timeweighout4–5gofNH4Cl(s):youwantdatafromtworuns.Experiment#2DeterminationofΔH ofKCl(s)→ K+(aq)+Cl–(aq)Weighout3–4gofpotassiumchloride(KCl(s)).Recordtheex‐actmass taken.Measureoutwitha graduated cylinder about100mLofdeionizedwaterandplaceitinthecalorimeter;re‐cordtheexactvolumeinyournotebook.Coverthecalorimeterwith the lid and record the temperature of the water in thecalorimeterat30‐sec intervalsuntil itstabilizes.Call thetem‐peratureatwhich thewater stabilizes the initial temperatureTsoln,i;besuretorecordit. Once the temperature of thewater in the calorimeter hasstabilized, add the KCl(s) to the calorimeter, replace the lid,andagitatethecontentsofthecalorimeterbymovingthestir‐ringringupanddown.Recordthetemperature30secaftertheKCl(s) is added and at 30‐sec intervals thereafter for at least5min; agitate the contents of the calorimeter throughout thedataacquisitionperiod. WhenyouaddtheKCl(s), the temperatureof thewater inthecalorimeterwillgodown.Youmusttakedatauntilyouob‐servea temperature increaseof at least a few tenthsof ade‐gree; you may have to collect data past the recommended5mintimeperiod. Attheendofthedataacquisitionperiod,disposeoftheso‐lutioninahazardouswastereceptacle.Rinseoutthecalorime‐teranddisposeoftherinsesinahazardouswastereceptacle. Repeat the procedure in this section, but this timeweighout4–5gofKCl(s):youwantdatafromtworuns.Experiment#3DeterminationofΔHofNH4Cl(s)+KCl(s)→ NH4+(aq)+K+(aq)+2Cl–(aq)Weighout3–4gofNH4Cl(s)and3–4gofKCl(s).Inyournote‐bookrecordtheexactmassofeachcompoundtaken.Measureoutwithagraduatedcylinderabout100mLofdeionizedwater

Experiment6∙Calorimetry 6‐10

andplaceitinthecalorimeter;recordtheexactvolumeinyournotebook. Cover the calorimeter with the lid and record thetemperatureofthewaterinthecalorimeterat30‐secintervalsuntilitstabilizes.Wewillcallthetemperatureatwhichthewa‐terstabilizestheinitialtemperatureTsoln,i;besuretorecordit. Once the temperature of thewater in the calorimeter hasstabilized,addtheNH4Cl(s)andtheKCl(s)tothecalorimeter,replace the lid, andagitate thecontentsof thecalorimeterbymovingthestirringringupanddown.Recordthetemperature30secaftertheNH4Cl(s)andtheKCl(s)areaddedtothecalo‐rimeter and at 30‐sec intervals thereafter for at least 5min;agitatethecontentsofthecalorimeterthroughoutthedataac‐quisitionperiod. When you add the NH4Cl(s) and the KCl(s), the tempera‐ture of thewater in the calorimeter will go down. Youmusttakedatauntilyouobserveatemperatureincreaseofatleastafew tenthsof adegree;youmayhave to collectdatapast therecommended5mintimeperiod. Attheendofthedataacquisitionperiod,disposeoftheso‐lutioninahazardouswastereceptacle.Rinseoutthecalorime‐teranddisposeoftherinsesinahazardouswastereceptacle. Repeat the procedure in this section, but this timeweighout4–5gofNH4Cl(s)and4–5gofKCl(s):youwantdatafromtworuns.DataanalysisExperiments#1and#2YouuseEq.6‐5 tocompute themolarenthalpychangeΔHofthereactionscarriedoutinExperiment#1and#2:

�

ΔH = −MsysΔTmsys

⎛

⎝ ⎜

⎞

⎠ ⎟

4.18JmL⋅°C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vsoln +Ccal

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐5)

ForExperiment#1thevariable MsysreferstothemolarmassofNH4Cl(M=53.49g/mol)andthevariablemsysreferstothemassofNH4Clused.ForExperiment#2thevariable Msysre‐ferstothemolarmassofKCl(M=74.55g/mol)andthevari‐

Experiment6∙Calorimetry 6‐11

ablemsysreferstothemassofKClused.ThevariableVsolnre‐ferstothevolumeofsolutioninthecalorimeter. ThevalueofΔT isdeterminedby thesametechniqueem‐ployedinthemeasurementofCcal.Timeisplottedalongthexaxisandtemperaturealongtheyaxis.CallthetimewhenyouaddedtheNH4Cl(s)ortheKCl(s)tothecalorimetert=0.Drawa straight line through the data points lying on the upwardsloping(i.e., increasingtemperature)portionoftheplot,mak‐ing sure that the line intercepts thevertical temperatureaxiserected at t = 0. The temperatureTmix corresponds to the y‐intercept of the line and represents the temperature that themixture would have exhibited if mixing and heat exchangewere instantaneous. In both experiments,ΔT =Tmix –Tsoln,i.Note that ΔT should be a negative number because Tmix <Tsoln,i.Experiment#3YouuseEq.6‐3atocomputetheenthalpychangeΔHofthere‐actioncarriedoutinExperiment#3: ΔH=–msolnCsolnΔT–CcalΔT (Eqn.6‐3a)Performing the substitutionmsoln = ρsolnVsoln and assumingthatρsoln=1.00g/mLandCsoln=4.18J/(g·°C)givesEqn.6‐3b,whichisamoredetailedversionofEqn.6‐3a:

�

ΔH = −ΔT 4.18JmL⋅° C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vsoln + Ccal

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐3b)

ThevalueofΔTisdeterminedbythesametechniqueemployedinthemeasurementofCcal.Timeisplottedalongthexaxisandtemperaturealongtheyaxis.CallthetimewhenyouaddedtheNH4Cl(s) and the KCl(s) to the calorimeter t = 0. Draw astraightlinethroughthedatapointslyingontheupwardslop‐ing (i.e., increasing temperature) portion of the plot, makingsure that the line intercepts the vertical temperature axiserected at t = 0. The temperatureTmix corresponds to the y‐interceptof the line.Note thatΔT =Tmix –Tsoln,i shouldbeanegativenumberbecauseTmix<Tsoln,i.

Experiment6∙Calorimetry 6‐12

Name___________________________________________________________LabDay__________LabTime_________Experiment6∙CalorimetryLabreportform Page1In(I.A),(II.A),(III.A),and(IV.A)youareaskedtosubmitplotssimilartoFigure6‐2.Preparea separateplot foreachrun.Giveeachplota truly informative title (i.e.,don’t just call it“Experiment1,Run1”),labeltheaxes,andincludeappropriateunitsanddivisionsofthoseaxes.DrawthestraightlinefromwhichthevalueofTmixisdeterminedandwritethevalueofTmixontheplot.Donotsubmitsmallplots:useawholesheetofpaper.Scalethehorizon‐talandverticalaxessothatthedatapointsoccupymostoftheareaoftheplot.(I.A)PrepareaplotofthedatacollectedinthemeasurementofCcal.(I.B)ReportthequantitiesneededforthecalculationofCcalaccordingtoEqn. 6‐2b:

�

Ccal = − 4.18JmL⋅° C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vhw

Tmix − 100°CTmix − Tcw,i

⎛

⎝ ⎜

⎞

⎠ ⎟ + Vcw

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

(Eqn.6‐2b)

Vhw[mL]

Vcw[mL]

Thw,i[°C]

Tcw,i[°C]

Tmix[°C]

Tmix–100°C[°C]

Tmix–Tcw,i[°C]

100°C (I.C)ShowthecalculationofCcalaccordingtoEqn.6‐2b.(I.D)WhataretheunitsofCcal?(II.A) Prepare plots of the data collected in the measurement of ΔH of NH4Cl(s) →NH4+(aq)+Cl–(aq).

Experiment6∙Calorimetry 6‐13

Name___________________________________________________________LabDay__________LabTime_________Experiment6∙CalorimetryLabreportform Page2(II.B) Report the quantities needed in the calculation of ΔH of NH4Cl(s)→ NH4+(aq) +Cl–(aq)accordingtoEqn.6‐5:

�

ΔH = −MsysΔTmsys

⎛

⎝ ⎜

⎞

⎠ ⎟

4.18JmL⋅°C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vsoln +Ccal

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐5)

CalculatethevalueofΔH ofeachofyourtworunsandthemeanvalueofΔH .Thevariable MsysreferstothemolarmassofNH4Cl(M=53.49g/mol);thevariablemsysreferstothemassofNH4Clused;Vsolnreferstothevolumeofsolutioninthecalorimeter.Run

msys[g]

Vsoln[mL]

Tsoln,i[°C]

Tmix[°C]

ΔT=Tmix–Tsoln,i[°C]

Ccal

ΔH

1 2 mean (II.C)ShowthecalculationofΔH forRun1.(II.D)WhataretheunitsofΔH ?(III.A)PrepareplotsofthedatacollectedinthemeasurementofΔH ofKCl(s)→K+(aq)+Cl–(aq).

Experiment6∙Calorimetry 6‐14

Name___________________________________________________________LabDay__________LabTime_________Experiment6∙CalorimetryLabreportform Page3(III.B)ReportthequantitiesneededinthecalculationofΔH ofKCl(s)→K+(aq)+Cl–(aq)according to Eqn. 6‐5. Calculate the value ofΔH of each of your two runs and themeanvalueofΔH .Thevariable MsysreferstothemolarmassofKCl(M=74.55g/mol);thevari‐ablemsysreferstothemassofKClused;Vsolnreferstothevolumeofsolutioninthecalo‐rimeter.Run

msys[g]

Vsoln[mL]

Tsoln,i[°C]

Tmix[°C]

ΔT=Tmix–Tsoln,i[°C]

Ccal

ΔH

1 2 mean (IV.A)PrepareplotsofthedatacollectedinthemeasurementofΔHofNH4Cl(s)+KCl(s)→NH4+(aq)+K+(aq)+2Cl–(aq).(IV.B) Report the quantities needed in the calculation of ΔH of NH4Cl(s) + KCl(s)→NH4+(aq)+K+(aq)+2Cl–(aq)accordingtoEqn.6‐3b.CalculatethevalueofΔHofeachofyourtworunsandthemeanvalueofΔH.

�

ΔH = −ΔT 4.18JmL⋅° C

⎛ ⎝ ⎜

⎞ ⎠ ⎟ Vsoln + Ccal

⎡

⎣ ⎢

⎤

⎦ ⎥

(Eqn.6‐3b)

Run

Vsoln[mL]

Tsoln,i[°C]

Tmix[°C]

ΔT=Tmix–Tsoln,i[°C]

Ccal

ΔH

1 2 mean (IV.C)WhataretheunitsofΔH?

Experiment6∙Calorimetry 6‐15

Name___________________________________________________________LabDay__________LabTime_________Experiment6∙CalorimetryLabreportform Page4Post­labquestions(1.a)LetΔHNH4ClrepresentthemeanvalueofΔH ofNH4Cl(s)→NH4+(aq)+Cl–(aq)re‐portedin(II.B),letΔHKClrepresentthemeanvalueofΔH ofKCl(s)→K+(aq)+Cl–(aq)re‐ported in (III.B), and let ΔH represent the mean value of ΔH of NH4Cl(s) + KCl(s)→NH4+(aq) + K+(aq) + 2 Cl–(aq) reported in (IV.B). Write a mathematical equation thatshowshowΔHNH4ClandΔHKClarerelatedtoΔH.(1.b)ConfirmwhetherthedatayoucollectedinExperiment#3fitstheequationyouwrotein (1.a) by completing the following table. Show the calculation of ΔH according to theequationyouwrotein(1.a)usingdatafromyourtworunsinExperiment#3andcalculatethepercenterrorusing

percenterror=100%×(ΔHcalc–ΔHreport)/ΔHreportRun

mNH4Cl[g]

mKCl[g]

ΔH

NH4Cl

ΔH

KCl

ΔHcalculatedbyeqn.in(1.a)

ΔHreportedin(IV.B)

%error

1 2 CalculationofΔHaccordingtotheequationin(1.a)forRun#1inExperiment#3:CalculationofΔHaccordingtotheequationin(1.a)forRun#2inExperiment#3:

Experiment6∙Calorimetry 6‐16

Name___________________________________________________________LabDay__________LabTime_________Experiment6∙CalorimetryLabreportform Page5(1.c)Regardlessofwhetheryourdatafurnishesagreementornot,whichexperimentalop‐erationsmightcontributemosttoanyobserveddiscrepancy?(2)InthisexperimentyoumeasureenthalpychangeΔH.YouarenotmeasuringstandardenthalpychangeΔHº.Whyaretheenthalpychangesyoumeasurenotstandard?(Consult‐ingyourlecturetextbookmayhelpinansweringthisquestion.)