Week3.PropertiesofPureSubstances

GENESYSLaboratory

Objectives1. Introducetheconceptofapuresubstance2. Discussthephysicsofphase-changeprocesses3. IllustratetheP-v,T-v,andP-Tpropertydiagrams andP-v-Tsurfacesofpuresubstances4. Demonstratetheproceduresfordeterminingthermodynamicpropertiesofpuresubstancesfromtablesofpropertydata5. Describethehypotheticalsubstance“idealgas”andtheideal-gasequationofstate6. Applytheideal-gasequationofstateinthesolutionoftypicalproblems7. Introducethecompressibilityfactor,whichaccountsforthedeviationofrealgasesfromideal-gasbehavior8. Presentsomeofthebest-knownequationsofstateGENESYSLaboratory

Phases of a Pure Substance• Puresubstance:AsubstancethathasafixedchemicalcompositionEx)water,nitrogen,helium,andcarbondioxide,air.• Amixtureofvariouschemicalelementsorcompoundsalsoqualifiesasapuresubstancesaslongasthemixtureishomogeneous• Airisamixtureofseveralgases,butitisoftenconsideredtobeapuresubstancebecauseithasauniformchemicalcomposition(mostly79%ofNitrogenand21%ofOxygen)• Solid:Asmalldistancesbetweenmoleculesinasolidmaketheattractiveforcesofmoleculesoneachotherlargeandkeepthemoleculesatfixedpositions• Liquid:Themoleculesarenolongeratfixedpositions• Gas:Themoleculesarefarapartfromeachother,andamolecularorderisnonexistent

Phase

GENESYSLaboratory

Summary

Homogeneous HeterogeneousHomogeneous

GENESYSLaboratory

T-vdiagramfortheheatingprocessofwateratconstantpressure

Phase-Change Processes of a Pure Substances• CompressedLiquidandSaturatedLiquid- State1:compressedliquid(=subcooled liquid):Aliquidthatisnotabouttovaporize- State2:saturatedliquid:phase-changeprocessfromliquidtovaporisabouttotakeplace

compressedliquid saturatedliquidGENESYSLaboratory

T-vdiagramfortheheatingprocessofwateratconstantpressure

Phase-Change Processes of a Pure Substances II• Saturated MixtureandSaturatedVapor- State3:SaturatedLiquid-VaporMixture:Aliquidandavaporcoexist- State4:saturatedVapor:Avaporisabouttocondense

saturatedliquid-vapormixture saturatedvaporGENESYSLaboratory

T-vdiagramfortheheatingprocessofwateratconstantpressure

Phase-Change Processes of a Pure Substances III• SaturatedVaporandSuperheatedVapor- State5:superheatedvapor:Avaporisnotabouttocondense(Temperature>Tsat)

superheatedvaporGENESYSLaboratory

Summary

GENESYSLaboratory

Saturation Temperature and Saturation Pressure• Thetemperatureatwhichwaterstartsboilingdependsonthepressure;therefore,ifthepressurefixed,soistheboilingtemperature- Saturationtemperature(Tsat ):atagivenpressure,thetemperatureatwhichapuresubstancechangesphase- Saturationpressure(Psat ):atagiventemperature,thepressureatwhichapuresubstancechangesphaseEx)At101.325kPa(1atm):Tsat=99.97℃At100℃ :Psat =101.42kPa (fromITS-90)• Latentheat- Latentheatoffusion:theamountofenergyabsorbedduringmelting=theamountofenergyreleasedduringfreezing- Latentheatofvaporization:theamountofenergyabsorbedduringvaporization=theamountofenergyreleasedduringcondensation

( )satsat PfT =

Liquid-vaporsaturationcurveofapuresubstance(water)GENESYSLaboratory

T-v Diagram• Asthesystempressureincreases,1. Waterstartsboilingatamuchhighertemperature2. v ofthesaturatedliquidislargerandvofthesaturatedvaporissmaller3. Thehorizontallineconnectingthesaturatedliquidandsaturatedvaporstatesisgettingshorterandfinallybecomesapointwherethesaturatedliquidandvaporstatesareidentical(i.e.criticalpoint)• T,P,v atthecriticalpointTcr :criticaltemperaturePcr :criticalpressure

vcr :criticalspecificvolume• Atsupercriticalpressures(P>Pcr )thereisnodistinctphase-changeprocess

T-v diagram of constant pressurephase-change process ofa pure substance at various pressureGENESYSLaboratory

T-v Diagram (Continue)• Thesaturatedliquidstatescanbeconnectedbyasaturatedliquidline• Thesaturatedvaporstatescanbeconnectedbyasaturatedvaporline• Thesetwolinesmeeteachotheratthecriticalpoint,formingadome• Allthecompressedliquidstatesarelocatedintheregiontotheleftofthesaturatedliquidline,anditiscalledthecompressedliquidregion• Allthesuperheatedvaporstatesarelocatedtotherightofthesaturatedvaporlinewhichiscalledthesuperheatedvaporregion• Allthestatesthatinvolvebothphasesinequilibriumarelocatedunderthedomewhichiscalledthesaturatedliquid-vapormixtureregion,orthewetregion.

Singlephase

SinglephaseBothphasesinequilibrium

GENESYSLaboratory

P-v Diagram

Thepressureinapiston-cylinderdevicecanbereducedbyreducingtheweightofthepistonGENESYSLaboratory

P-T Diagram• Triplepoint:allthreephasesofapuresubstancecoexistinequilibrium

GENESYSLaboratory

Enthalpy-A Combination Property• The combination u+Pv is frequently encountered in the analysis of control volumes

(kJ) or

(kJ/kg)

PVUH

Pvuh

+=

+=

• enthalpy←fromGreekword“enthalpien”,whichmeanstoheat

Total energy Total energy Change in the totalentering the system leaving the system energy of the system

= (kJ)in out system

mass

E E E

W Q E U KE PE

æ ö æ ö æ ö- =ç ÷ ç ÷ ç ÷

è ø è ø è ø

- D

D + D + D = D + D + D

v Remind1st lawofthermodynamics

GENESYSLaboratory

SummaryEmass=PV

Emass=PVGENESYSLaboratory

Summary

Emass=PVGENESYSLaboratory

1st law of Thermodynamics & Energy Balance

Controlmass(ClosedSystem) Controlvolume(OpenedSystem)

W Q KE PU ED + D = + D + DD massW Q KE PEW Q KE P

EE

UH

D + D + = + D + D

D + D = + DD + D

DD

IfyoursystemisastationarysystemUW QD + D = D ( )mass boundaryW Q UE

HWE

QD + D + =

D

D

+ D

D

= D

D

Emass=PVMovingboundary(closedSystem)

GENESYSLaboratory