Student'sSheet
Printed:30.03.201716:02:47|P3110900
TheChemicalFormulaforMethane,Ethaneand
Propane(ItemNo.:P3110900)
CurricularRelevance
AreaofExpertise:
Chemie
EducationLevel:
Hochschule
Topic:
AllgemeineChemie
Subtopic:
Stöchiometrie
Experiment:
Difficulty
PreparationTime
ExecutionTime
RecommendedGroupSize
Difficult
10Minutes
20Minutes
2Students
AdditionalRequirements:
ExperimentVariations:
Keywords:
Stoichiometry,Chemicalformula
Overview
Shortdescription
Principle
Aquiescenteudiometerisinsertedintotheglassjacketandtheglassjacketisfilledwithcoldwater.Gasmixturescomposedof
hydrocarbonsandoxygenareinjectedintotheeudiometerandburntherecontinuouslyattheconstantsparkingoftheignition
sparkgap.Thewaterformedbythereactioncondensesonthecoolwallsoftheeudiometer.Forthisreason,thevolume
betweenthemovingplungerandthefixedplungerwiththeignitionsystemaftercombustionissmallerthanthevolumeofthe
gasmixtureoriginallyinjected.Inasecondpartoftheexperiment,theeudiometerintheglassjacketisheateduptoover100
°C.Thewaterformedduringcombustioncannolongercondenseonthehoteudiometerwalls.Inmostcases,evenafter
conversiontostandardconditions,thevolumerecordedthenisgreaterthanthevolumeofthegasthatwasinjected.Theeasiest
waytoconvertthevolumestostandardconditionsiswithanomogram.Themeasurementdataobtainedinthiswaycanbeused
toderivetheempiricalformulasofgaseoushydrocarbonsexperimentally.
Robert-Bosch-Breite10
D-37079Göttingen
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Fax:+49551604-107
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Student'sSheet
Printed:30.03.201716:02:47|P3110900
Fig.1:Experimentalsetup.
Safetyinstructions
Thegasesmethane,ethaneandpropaneareeasilyflammableandformexplosivemixtureswithair.
Allflamesmustbeextinguishedbeforegassamplesaretaken.
Methane
H220:Extremelyflammablegas.
H280:Containsgasunderpressure;mayexplodeifheated.
P210:Keepawayfromheat,hotsurfaces,sparks,openflamesandotherignitionsources.Nosmoking.
P377:Leakinggasfire–donotextinguishunlessleakcanbestoppedsafely.
Ethane
H220:Extremelyflammablegas.
H280:Containsgasunderpressure;mayexplodeifheated.
P210:Keepawayfromheat,hotsurfaces,sparks,openflamesandotherignitionsources.Nosmoking.
P377:Leakinggasfire–donotextinguishunlessleakcanbestoppedsafely.
Propane
H220:Extremelyflammablegas.
H280:Containsgasunderpressure;mayexplodeifheated.
P210:Keepawayfromheat,hotsurfaces,sparks,openflamesandotherignitionsources.Nosmoking.
P377:Leakinggasfire–donotextinguishunlessleakcanbestoppedsafely.
Robert-Bosch-Breite10
D-37079Göttingen
Tel:+49551604-0
Fax:+49551604-107
[email protected]
www.phywe.com
Student'sSheet
Printed:30.03.201716:02:47|P3110900
Equipment
PositionNo.
Material
OrderNo.
Quantity
1
Glassjacket
02615-00
1
2
Sloweudiometer
02612-00
1
3
SupportbaseDEMO
02007-55
1
4
BaseplateforsupportbaseDEMO
02007-01
1
5
Supportrod,stainlesssteel,l=250mm,d=10mm
02031-00
2
6
Rightangleclamp
37697-00
2
7
Universalclamp
37715-00
2
8
Gasbar
40466-00
2
9
Rubbercaps,packof20
02615-03
1
10
Syringe20ml,Luer,10pcs
02591-03
1
11
Cannula0,45x13mm,Luer,20pcs
02598-04
1
12
Labthermometer,-10..+150C
38058-00
2
13
Magneticstirringbar30mm,cylindrical
46299-02
1
14
Magnet,d=10mm,l=200mm
06311-00
1
15
Heatingapparatusforglassjacketsystem
32246-93
1
16
Powerregulator,230V,withphasecontrolledmodulator
32286-93
1
17
GlassbeakerDURAN®,tall,250ml
36004-00
1
18
Graduatedvessel,1l,withhandle
36640-00
1
19
Cobra4hand-heldpressureandtemperaturemeasuringinstrument,Cobra4
Mobile-Link
12736-01
1
20
PHYWEHighvoltagesupplyunitwithdigitaldisplayDC:0...±10kV,2mA
13673-93
1
21
Connectingcord,30kV,1000mm
07367-00
2
22
Steelcylinderoxygen,2l,filled
41778-00
1
23
Reducingvalvef.oxygen
33482-00
1
24
Tablestandfor2lsteelcylinders
41774-00
1
25
Wrenchforsteelcylinders
40322-00
1
26
Compressedgas,methane,12l
41772-08
1
27
Compressedgas,ethane,14g
41772-09
1
28
Compressedgas,propane,7l
41772-10
1
29
Finecontrolvalve
33499-00
1
30
Hoseclip,diam.8-16mm,1pc.
40996-02
2
31
Funnel,glass,topdia.55mm
34457-00
1
32
Boilingbeads,200g
36937-20
1
33
Spoon,specialsteel
33398-00
1
34
Siliconetubingi.d.7mm
39296-00
3
35
Sodiumchloride,500g
30155-50
1
36
Water,distilled5l
31246-81
1
TasRs
Investigatethechemicalformulaformethane,ethaneandpropane.
Setupandprocedure
Robert-Bosch-Breite10
D-37079Göttingen
Tel:+49551604-0
Fax:+49551604-107
[email protected]
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Student'sSheet
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Procedure
PreparethegasbarbypouringsomuchwaterthroughthefunnelintothesmallgasometerthattheErlenmeyerflaskandthe
right-angledglasstubearefilledasairbubble-freeaspossible(Fig.1).Excesswaterrunsoutoftheglasstube,sothatthe
correctamountofwaterrequiredforfillingadjustsitselfautomatically.
Nowconnecttheglasstubetothesourceofgas(compressedgascontainer)withalengthofsiliconetubingandstartaslowflow
ofgas.ThegasforcesthewateroutoftheflaskupintothefunnelandisitselfcollectedintheErlenmeyerflask.Whenfillingis
complete,closetheendoftheglasstubewitharubbercap.Mounttheeudiometerintheglassjacketandfillthejacketwithan
aqueoussaltsolutionc(NaCl)=3mol/l(105gNaClin600mlwater),addingafewboilingstones(beads)(Fig.2).Usingthe20ml
syringe,draw15mlofoxygenand2mlofoneofthepurehydrocarbonsoutofthesmallgasometers.Switchonthehighvoltage
supplyunitandsetthevoltagetothemaximumvalue.Injectthehydrocarbonloxygenmixtureslowlythroughtherubbercap
intothesloweudiometer(roomtemperature),ensuringthatthereisapermanentignitionspark.Switchoffthehighvoltage
supplyunit,tapthemovableplungeroftheeudiometergentlyafewtimesandreadoffthevolumeshownbytheedgeofthe
plunger.Calculatethehydrogencontentofthehydrocarbonfromthedifferenceinvolumes.Thisresultallowsyoutoconstructa
hypotheticalcompoundandacorrespondingequationforthereactionwithoxygen.
Fig.2.
Totestthishypothesis,carefullydrytheinsideoftheeudiometerandheatthesaltsolutionwhichsurroundsituptoboiling
(approx.103°C)bymeansoftheheatingapparatus.Atthistemperature,switchonthehighvoltagesupplyunit(maximum
setting)andinjecttheappropriatestoichiometrichydrocarbon/oxygenmixtureintotheeudiometer.Readthevolume,convertit
tostandardconditionsandcheckyourhypothesis.
Table1
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D-37079Göttingen
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Student'sSheet
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Gas
VGas+
Vafter
VO2/ml Reaction/
ml
Methan 2+15
Ethan 2+15
Propan 2+15
13
12
11
4
5
6
02Theoreticalvolumeof
converted H2O-vapour/ml
toH20/
ml
2
4
3
6
4
8
VolumeofH2Ovapourper Hatomsper Possible
molecule
chemical
mlgas/VGas/ml
formula
2
3
4
4
6
8
CH4,C2H4
C2H6,C3H6
C3H8,C4H8
Table2
Gas Possiblechemical mixture
theor.CO2formula
hydrocarbon:02 Vol./ml
1 CH4
2:4
2
ReactionVol.
(103°C)/ml
-7.5
ReactionVol.
(20°C)/ml
5.8
theor.Volume
(20°C)/ml
6
2
C2H6
2:7
4
-13
10.1
10
3
C3H8
2:10
6
-18
14.0
14
Chemicalequationfor
thereaction
CH4+2O2→CO2+2
H2O
C2H6+3,5O2→2CO2
+3H2O
C3H8+5O2→3CO2+
4H2O
Theoryandevaluation
Results
Theinjectedgasmixtureburnscontinuouslyintheeudiometeratthepermanentsparkacrossthesparkgap.Watercondenses
onthecoldereudiometersurfaces.Thisisthereasonwhythevolumebetweenthemovableplungerandthefixedplungerwith
theignitiondeviceissmallerthanthatoftheoriginalgasmixture.Table1showssomeresultsobtained.
Inthesecondpartoftheexperiment,watercannotcondenseonthehoteudiometersurfaces.Thevolumereadis,after
conversiontostandardconditions,inmostcaseslargerthantheinjectedgasvolume.Table2showsdataobtainedfrom
measurementsonthecombustionofstoichiometricmixturesinthecoldandinthehoteudiometer.Conversionofthevolumesto
standardconditionswasmadeusingthenomogram40440.00.
Conclusions
Thecompletecombustionofahydrocarboninanexcessofoxygengives,acc.to:
carbondioxideandwater(steam).Assumingthatthegasesbehavesufficientlyideally,wecandeterminethechemicalformula
forthehydrocarbonusedfromthedecreaseinvolumefoundintheexperiment:AccordingtoAvogadro,whenonepartby
volume(orml)ofagaseoushydrocarbonreactswithonepartbyvolumeofoxygen,theygiveonepartbyvolumeofcarbon
dioxide:
sothatwitha„gaseous“
,acc.to
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D-37079Göttingen
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theinitialvolumeof
partsbyvolumeisreducedbythevolumeofthehydrocarbonusedtoypartsbyvolume.
Thehydrogencontainedinahydrocarbonburns,acc.to
towater.Inthesecondpartoftheexperiment,usingthehoteudiometer,thewaterisinvapourformandreducesthedecrease
involumeresultingfromthereactionfrom1partbyvolumeto1-x/4partsbyvolume.Undertheconditionsusedinthefirstpart
oftheexperiment,however,watercondensedontheeudiometersurfaces,resultinginanadditionaldecreaseinvolumeofx/4
partsbyvolume.
Forahydrocarbonofcomposition
,thereactioncanbedescribedbythefollowingequation:
Inthefirstpartoftheexperiment,thisreactionleadstoadecreaseinvolumeof
partsbyvolumeperpart
byvolumeofthehydrocarbonused.Thisallowsustodeterminethechemicalformulaformethanefromtheresultsgivenin
Table1,obtainedinthefirstpartoftheexperiment:
Adecreaseinvolume
of4mlwasfoundonthecombustionof2mlofmethane.Correctingthistoonepartbyvolumeof
hydrocarbonwehave4/2=1+x/4,giving
.
Itisclearthatthedecreaseinvolumefoundcomesfromthecombinationofthehydrocarbonvolumeconsumedandtheoxygen
volumewhichwasconvertedtowater.Thisallowsustocalculatethetheoreticalvolumeofsteamandtheproportionofsteamto
usedgas.Fromthiswehave,when2ml
wereconvertedtowater,atheoreticalsteamvolumeof4mlandasteamtogas
proportionof2,whichcorrespondsto4hydrogenatomspermolecule.
Thenumberofcarbonatomspermolecule(y)isnotgivenbytheseexperiments.AccordingtotheLawofconstantmassratios,
and mustberelatedtoeachotherinwholenumberproportionsandascarbonisnormallytetravalentinthecompoundsit
forms,theseconditionswouldbefulfilledby
and
,butalsofromexoticcompoundssuchas
Adecisiononthecorrectchemicalformulaisonlypossiblewhenfurtherinformationisavailable.
Tocheckonthehypotheticalformula
or
,astoichiometrichydrocarbon/oxygenmixtureiscombustedinthehot
eudiometer,accordingtotheequation:
Whenthegasvolumesarereducedtostandardconditions(°C,1013hPa),theyareingoodagreementwiththestoichiometric
equation.Adifferentiationbetweencompoundshavingthesamenumberofhydrogenatoms–forexample,
and
-isnotpossiblewiththismethod,becausethecombustionproductscontainunreactedgas(withnon-stoichiometric
mixtures)andthevolumesreadarethereforeapproximatelythesame.Theassignmentofthecorrectchemicalformulacanbe
madewhenadditionalinformation,suchasmolarmass,reactivityagainstbrominewateretc.isavailable.
Robert-Bosch-Breite10
D-37079Göttingen
Tel:+49551604-0
Fax:+49551604-107
[email protected]
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.