DJ

Student'sSheet
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Robert-Bosch-Breite10 Tel:+49551604-0 [email protected]
D-37079Göttingen Fax:+49551604-107 www.phywe.com
Difficulty
Intermediate
PreparationTime
10Minutes
ExecutionTime
10Minutes
RecommendedGroupSize
2Students
HeatcapacityofgaseswithCobra4(ItemNo.:P2320261)
CurricularRelevance
AdditionalRequirements:
PC,Windows®XPorhigher
ExperimentVariations:
Keywords:
Equationofstateforidealgases,1stlawofthermodynamics,Universalgasconstant,Degreeoffreedom,Molevolumes,Isobars,Isotherms,
Isochors,Adiabaticchangesofstate
Overview
Shortdescription
Principle
Heatisaddedtoagasinaglassvesselbyanelectricheaterwhichisswitchedonbriefly.Thetemperatureincreaseresultsina
pressureincrease,whichismeasuredwithamanometer.Underisobaricconditionsatemperatureincreaseresultsinavolume
dilatation,whichcanbereadfromasyringe.Themolarheatcapacities and arecalculatedfromthepressureandvolume
changerespectively.
Fig.1:Experimentalset-upforthedeterminationofCv.
AreaofExpertise:
Chemistry
EducationLevel:
University
Topic:
PhysicalChemistry
Subtopic:
Thermochemistry,
Calorimetry
Experiment:
Heatcapacityof
gaseswithCobra4

Student'sSheet
Printed:03/12/201713:46:54|P2320261
Robert-Bosch-Breite10 Tel:+49551604-0 [email protected]
D-37079Göttingen Fax:+49551604-107 www.phywe.com
Safetyinstructions
Equipment
PositionNo. Material OrderNo. Quantity
1 Cobra4USB-Link 12610-00 1
2 Cobra4Sensor-UnitEnergy:Current,voltage,work,power 12656-00 1
3 SoftwareCobra4-multi-userlicence 14550-61 1
4 Precisionmanometer 03091-00 1
5 PHYWEPowersupplyDC:0...12V,2A/AC:6V,12V,5A 13505-93 1
6 Mariotteflask,10l 02629-00 1
7 Weathermonitor,6linesLCD 87997-10 1
8 TripodbasePHYWE 02002-55 1
9 On/offswitch 06034-01 1
10 Stopcock,3-way,t-sh.,capil.,glass 36732-00 1
11 Nickelelectrode,d3mm,w.socket 45231-00 2
12 Chrome-nickelwire,d.0,1mm,100m 06109-00 1
13 Stopcock,1-way,straight,glass 36705-00 1
14 Scissors,straight,blunt,l140mm 64625-00 1
15 Rub.stop.d=59.5/50.5mm,1hole 39268-01 1
16 Connectingcord,32A,500mm,red 07361-01 2
17 Connectingcord,32A,250mm,blue 07360-04 1
18 Connectingcord,32A,500mm,blue 07361-04 2
19 Syringe10ml,Luer,10pcs 02590-03 1
20 Rubberstopper26/32,3holes7mm+2x1,5mm 39258-14 1
21 Siliconetubing,innerdiameter3mm 39292-00 1
22 Rubbertubing,i.d.6mm 39282-00 2
23 Tubingadaptor,ID3-5/6-10mm 47517-01 1
Additional
material
PC,Windows®XPorhigher
Tasks
1. Determinethemolarheatcapacitiesofairatconstantvolume .
2. Determinethemolarheatcapacitiesofairatconstantpressure .
Setupandprocedure

Student'sSheet
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Performtheexperimentalset-upaccordingtoFigs.1and3respectively.
Insertthetwonickelelectrodesintotwoholesofthethreeholerubberstopperandfixtheterminalscrewstothelower
endsoftheelectrodes.
Screwtwopiecesofchromenickelwire,whichareeachabout15cmlong,intotheclampsbetweenthesetwoelectrodes
sothattheyareelectricallyconnectedinparallel.Thewiresmustnottoucheachother.
Inserttheone-waystopcockintothethirdholeofthestopperandinsertthethuspreparedstopperintheloweropeningof
thebottle.Givespecialattentiontothewireswhichhavetoprotrudeintothemiddleofthebottle.
Insertthesecondstopper,whichhasbeenequippedwiththethree-waystopcock,intotheupperopeningofthebottle(Fig.
1)andconnecttheprecisionmanometertothebottlewithapieceoftubing.
Themanometermustbepositionedexactlyhorizontally.
Itisequippedwithaspiritleveltofacilitatethecorrectadjustment.Usetheadjustingscrewsofthetripodbasetoalignthe
manometercompletelyhorizontally.
Themanometermustbefilledwiththeoilwhichissuppliedwiththedevice.
ThescaleisnowcalibratedinhPa.
Youcanchoosethescaleofeither2hPaor4hPabyalteringtheinclinationangleofthemanometer.Forthese
measurements2hPaaresufficientsojustleaveithorizontal.
CombinetheCobra4SensorUnitEnergywiththeCobra4USBLink.
ConnectthepowersupplyandthenickelelectrodeswiththeCobra4SensorUnitEnergyasshowninFig.2.
Fig.2:ConnectionbetweentheCobra4SensorUnit
Energyandthepowersupply(“in”)andtheheating
coil(“out”).
StartthePCandconnecttheCobra4USBLinkwiththecomputerviaaUSBcable.
Callupthe“Measure”programmeandboottheexperiment“HeatcapacitywithCobra4”(experiment>openexperiment).
Themeasurementparametersforthisexperimentareloadednow.
NowthesensorisautomaticallyrecognizedandanIDnumber(01)isallocatedtothesensor,whichisindicatedinthe
displayoftheCobra4USBLink.
Todetermine connectthesyringetothebottleviathetree-waystopcock(compareFig.3).

Student'sSheet
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D-37079Göttingen Fax:+49551604-107 www.phywe.com
Fig.3:Experimentalset-upforthedeterminationofCp.
Foreachtaskperformatleasttenmeasurements.
Therisetubeofthemanometermustbewellwettedbeforeeachmeasurement.
Determinetheairpressure,whichisrequiredforthecalculations,withtheaidoftheweatherstation.
Startthemeasurementwith
Task1:
Startandstopthemeasuringprocedurebyoperatingtheon/offswitch.
Themeasuringprocedureshouldbeasshortaspossible(lessthantwoseconds).
Thethree-waycockmustbepositionedinsuchamanner,thatitconnectsthebottlewiththeprecisionmanometer.
Uponheatingthepressureinthebottlewillstarttorise.
Readthemaximumpressureincreaseimmediatelyaftercessationoftheheatingprocess.
Aftereachmeasurementwaitasufficienttimeuntilthegasinthevolumecooleddownagaintoroomtemperaturethereby
regainingambientpressure.
Theelectricalcurrentwhichflowsduringthemeasurementsmustnotbetoostrong,i.e.itmustbesufficientlyweakto
limitthepressureincreaseduetotheheatingofthegastoamaximumof1hPa.
Forthisreasonitmaybenecessarytouseonlyoneheatingwireortoreducetheelectricalcurrentatthepowersupply.
Stopthemeasurementbypressing
Sendalldatato„measure“andsavethemeasurement(File>Savemeausrementas…).
Task2:
Startandstopthemeasuringprocedurebyoperatingtheon/offswitch.

Student'sSheet
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Robert-Bosch-Breite10 Tel:+49551604-0 [email protected]
D-37079Göttingen Fax:+49551604-107 www.phywe.com
Themeasuringprocedureshouldbeasshortaspossible(lessthantwoseconds).
Whilemeasuring,thethree-waycockmustbepositionedinsuchamannerthatitconnectsthesyringeandthe
manometerwiththebottle.
Uponheatingthepressureinthebottlewillstarttorise.
Asyouwanttodeterminetheheatcapacityatconstantpressureyouhavetocompensatethepressurerisebyincreasing
thevolumeviathesyringe.
Youcanholdthesyringeinyourhandanduseyourthumbtogentlypushtheplunger.
Whentheheatingstopped,thevolumeofthegasinthebottlewillstillincreaseforamoment.
Becarefultonoticetheturningpointwhenthevolumestartsdecreasingagainbecausethegasstartscoolingdown.Inthis
momentthepressureshouldhaveitsinitialvalueandstartfallingwhileyouhavealreadystoppedincreasingthevolume.
Youcanreadthevolumeincreasedirectlyfromthesyringe’sscale.Youmayneedsomepracticeuntilyouareabletokeep
thepressurefairlyconstantduringthewholemeasurementandrecognizetheturningpointcorrectly.
Aftereachmeasurementresettheinitialvolumeandwaituntilthegascooleddownagaintoroomtemperature.
Beforestartinganewmeasurementboththevolumeinthesyringeandthepressureshouldhaveregainedtheirinitial
values.
Theoryandevaluation
Theory
Thefirstlawofthermodynamicscanbeillustratedparticularlywellwithanidealgas.Thislawdescribestherelationshipbetween
thechangeininternalenergy theheatexchangedwiththesurroundings andtheworkperformedbythesystem
generallyspeaking.Inourcasetheworkbeingperformedisthepressure-volumeworkresultsintoavolumeincrease
keepingconstantthepressure .
(1)
Themolarheatcapacity ofasubstanceresultsfromtheamountofabsorbedheat andthetemperaturechange per
molewhere isthenumberofmoles:
(2)
Onedistinguishesbetweenthemolarheatcapacityatconstantvolume andthemolarheatcapacityatconstantpressure
.accordingtoequations(1)and(2)andunderisochoricconditions( =const., =0),thefollowingholdstrue:
(3)
Underisobaricconditions( =const., =0):
(4)
Itisobviousformequation(3)thatthemolarheatcapacity isafunctionoftheinternalenergyofthegas.Theinternal
energycanbecalculatedwiththeaidofthekineticgastheorywiththenumberofdegreesoffreedom andtheuniversalgas
constant :