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EXPERIMENTB2:ENTHALPYOFVAPORIZATION
LearningOutcomes
Uponcompletionofthislab,thestudentwillbeableto:
1) Measurethepressureinsideasealedvesselcontainingavolatileliquid
overarangeoftemperatures.
2) Determinetherelationshipbetweenpressureandtemperatureofthe
volatileliquid.
3) Calculatetheheatofvaporizationoftheliquid.
Introduction
Whenaliquidisplacedinacontainer,andthecontainerissealedtightly,aportion
oftheliquidwillevaporate.Thenewlyformedgasmoleculesexertpressureinthe
container,whilesomeofthegascondensesbackintotheliquidstate.Ifthe
temperatureinsidethecontainerisheldconstant,thenatsomepointequilibrium
willbereached.Atequilibrium,therateofcondensationisequaltotherateof
evaporation.Thepressureoftheliquidatequilibriumiscalledthevaporpressure,
andwillremainconstantaslongasthetemperatureinthecontainerdoesnot
change.
Inmathematicalterms,therelationshipbetweenthevaporpressureofaliquidand
temperatureisdescribedintheClausius-Clayperonequation:
ΔH vap ⎛ 1 ⎞
ln P = −
⎜ ⎟ + constant R ⎝T⎠
Intheaboveequation,lnPisthenaturallogarithmofthevaporpressure(inkPa),
ΔHvapistheheatofvaporization(inkJ/mol),Ristheuniversalgasconstant(8.31
€
J/mol•K),Tistheabsolute,orKelvin,temperature.
Thus,theClausius-Clayperonequationnotonlydescribeshowvaporpressureis
affectedbytemperature,butitrelatesthesefactorstotheheatofvaporizationofa
liquid.
ΔHvapistheamountofenergyrequiredtoevaporateonemoleofliquidatconstant
pressure.
ExperimentalDesign
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Inthisexperiment,theenthalpyofvaporization,ΔHvapofethanolwillbe
determined.Ethanolwillbeplacedinsideatightlysealedcontainerandthevapor
pressureofethanolwillbemeasuredasafunctionoftemperature.AplotoflnPvs.
1/Twillbeanalyzedtoobtaintheenthalpyofvaporization.
Thesealedcontainernotonlycontainsvaporsofethanolbutalsotheairpresent
insideit.Thereforethemeasuredvalueofthepressure,whichwouldbethetotal
pressure(PTotal),isthesumofthevaporpressureoftheethanol(Pvap)andthe
partialpressureofair(Pair)[NOTE:Dalton’slawofpartialpressureswaspreviously
discussedinExperimentB1(Molarvolumeofagas)].
PTotal=Pair+Pvap
ReagentsandSupplies
Ethanol
(SeepostedMaterialSafetyDataSheets)
VernierLabProkitwithagaspressuresensorandtemperatureprobe,hotplate
(stockroom)laptopcomputer(lab)
Procedure
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1. Obtainahotplate,a600-mLbeaker,andVernierkitwithagaspressuresensor
fromthestockroom.
2. Obtainalaptopcomputerfromtheinstructor.
3. Usethehotplatetoheatabout200mLofwaterina600mLbeakerorobtain
about200mLofhotwaterfromthefaucet.
4. Preparearoomtemperaturewaterbathina600-mLbeaker.Thebathshouldbe
deepenoughtoreachthebottomofthewhitestopper,oncetheapparatusis
insertedintothewaterbath.
5. Settinguptheequipment:
a. Connectthegaspressuresensortochannel1oftheLabProinterface
box.Connectthetemperatureprobetochannel2oftheLabPro
interfacebox.
b. ConnecttheLabProinterfaceboxtothepoweroutletwiththepower
cord.TheLabProwillbeeptoindicateitisready.
c. ConnecttheLabProinterfaceboxtothelaptopcomputerusingthe
USBcable.
d. ConnectthecomputertotheEthernetusingtheEthernetcableand
theportlocatedonthelabbench.
6. Preparethecomputertomonitortemperatureandpressurebyopeningthefile
“VaporPressure”fromtheChemistry1Adesktopfolder.
7. UsethecleartubingtoconnectthewhiterubberstoppertotheGasPressure
Sensor.(Aboutone-halfturnofthefittingswillsecurethetubingtightly.)Twist
thewhitestoppersnuglyintotheneckoftheErlenmeyerflasktoavoidlosing
anyofthegasthatwillbeproducedastheliquidevaporates(seeFigure1).
Important:Openthevalveonthewhitestopper.
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FIGURE1
8. ConditiontheErlenmeyerflaskandthesensorstothewaterbath.
a. PlacetheTemperatureProbeintheroomtemperaturewaterbath.
b. PlacetheErlenmeyerflaskinthewaterbath.Holdtheflaskdowninto
thewaterbathsothatthewaterlevelinthebeakerisjustbelowthe
bottomofthewhitestopper.
c. After30seconds,closethevalveonthewhitestopper.
d. Whenthepressureandtemperaturereadingsstabilize,recordthese
valuesinthedatatableasInitialairpressure(IPair)andInitial
temperature(ITair).
9. Draw3mLofethanolintothe20mLsyringethatispartoftheGasPressure
Sensoraccessoriesdirectlyfromthestockbottleprovided.Threadthesyringe
ontothevalveonthewhitestopper(seeFigure1).
10. Addethanoltotheflask.
a. Openthevalvebelowthesyringecontainingthe3mLofethanol.
b. Pushdownontheplungerofthesyringetoinjecttheethanol.
c. Quicklypulltheplungerbacktothe3-mLmark.Closethevalve
belowthesyringe.
11. Gentlyrotatetheflaskinthewaterbathforafewseconds,usingamotion
similartoslowlystirringacupofcoffeeortea,toacceleratetheevaporationof
theethanol.
12. Monitorandcollecttemperatureandpressuredata.
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a. Click
tobegindatacollection.
b. Holdtheflasksteadyonceagain.
c. Monitorthepressureandtemperaturereadings.
d. Whenthereadingsstabilize,click
.
13. Usingbeakertongsaddasmallamountofhotwater,fromthebeakeronthehot
plate,towarmthewaterbathby3-5°C.Stirthewaterbathslowlywiththe
TemperatureProbe.Monitorthepressureandtemperaturereadings.Whenthe
readingsstabilize,click
.
14. RepeatStep13untilatotaloffivetrialshavebeencompleted.Addenoughhot
waterforeachtrialsothatthetemperatureofthewaterbathincreasesby3-5°C,
butdonotwarmthewaterbathbeyond40°Cbecausethepressureincrease
maypopthestopperoutoftheflask.Ifwaterinthebathmustberemoved,then
doitcarefullysoasnottodisturbtheflask.
15. Afterthefifthsetofreadingshasbeenrecorded,openthevalvetoreleasethe
pressureintheflask.Removetheflaskfromthewaterbathandtakethestopper
offtheflask.Disposetheethanolasdirectedbytheinstructor.
16. Click
toendthedatacollection.Recordthefirstpressurereading,Ptotal,
andthefirsttemperaturereadingasTrial1inthedatatable.Recordthe
additionalpressureandtemperaturereadingsasTrials2-5.
17. DonotexittheLoggerProprogramuntilsteps1-4ofthedataanalysissection
havebeencompleted.
18. Onthesecondlabperiodrepeattheexperimenttocollectasecondsetofdata.
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DataTable
DAY1
InitialairpressureIPair(fromStep8d)=__________________________________
InitialtemperatureITair(fromStep8d)=________________________________
Trial1
Trial2
Trial3
Trial4
Trial5
Ptotal(kPa)
Temperature
(°C)
DAY2
InitialairpressureIPair(fromStep8d)=__________________________________
InitialtemperatureITair(fromStep8d)=________________________________
Trial1
Trial2
Trial3
Trial4
Trial5
Ptotal(kPa)
Temperature
(°C)
DataAnalysis
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1. Thetotalpressureinsidetheflask,whichisthemeasuredpressurePTotal.
PTotal=Pvap+Pair
Sincethetemperatureoftheairinsidetheflaskischanging,thePairneedstobe
calculatedateachofthetemperatures.
2. UsethegaslawrelationshipshownbelowtocalculatethePairfortrials2-5.
RememberthatallgaslawcalculationsrequireKelvintemperature.UsetheIPair
andITairfromthedatatabletocalculatethePairforeachtrialusingthe
correspondingKelvintemperatureforeachtrial.
IPair Pair(trial1,2,3,etc...)
=
ITair T(trial1,2,3,etc...) 3. CalculateandrecordthePvapforeachtrial:Pvap=PTotal–Pair
4. Calculateandrecordln(Pvap)and1/T(K)inthetablebelow.
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DAY1
Trial1
Trial2
Trial3
Trial4
Trial5
Pair(kPa)
(calculated)
Pvap(kPa)
(calculated)
Temperature(°C)
(measured)
Temperature(K)
Ptotal(kPa)
(measured)
Ln(Pvap)
1/T(K)
SampleCalculationsforPairandPvap
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5. Inordertodeterminetheheatofvaporization,ΔHvap,aplotofLn(Pvap)vs.the
reciprocalofabsolutetemperature(1/TinK)isneeded.
6. UseanExcelspreadsheettoobtaintheplotofLn(Pvap)vs.1/T.
a. Enterthedata(1/TandLn(Pvap))incolumnsAandB.UseRow1for
columnheadings.
A
B
1 1/T Ln(PVap)
2
3
4
5
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b. Todrawagraph,selectthexandydata,whichwouldbedatainfields
A:2-6andB:2-6.
c. Click“Insert”andthen“Chart”.Choose“XY”scatterandselect“Marked
Scatter”
d. Whenthegraphisdisplayed,clickonanydatapointonthechartand
fromthetoolbar,select“Chart”andthen“InsertTrendline”.
e. Fromthepop-upbox,selectthe“Options”tabandchecktheboxes:1)
Displayequationand2)DisplayR-squaredvalueandclickOK.
f. Fromtheequationofthestraightline,obtaintheslopeandsetthatequal
ΔH
to
.UsingthefactthatR=8.314J/molK,calculatethevalueofΔHvap.
R
7. TheacceptedvalueoftheΔHvapofethanolis42.32kJ/mol.Calculatethepercent
errorforyourexperimentallydeterminedvalueofΔHvapwiththeacceptedvalue.
8. RepeatalltheabovecalculationsusingthesecondsetofdatafromDay2
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DAY2
Trial1
Trial2
Trial3
Trial4
Trial5
Pair(kPa)
(calculated)
Pvap(kPa)
(calculated)
Temperature(°C)
(measured)
Temperature(K)
Ptotal(kPa)
(measured)
Ln(Pvap)
1/T(K)
SampleCalculationsforPairandPvap