Liquid State SB

Liquid state, Liquefaction of gases, Vapor pressure of liquids, Boiling point, Aerosol, Joule Thomson effect, adiabatic expansion, Equilibrium vapor pressure, Clausius Clapeyron equation

1. The LiquidState
General properties
Liquefaction of gases
Vapor pressure of liquids
Boilingpoint

2. Generalproperties
Liquids are denser than gases and occupy a definite volume
and density due to the presence of van der Waalsforces.
Liquidsarerelatively
incompressible.
Liquidarefluids(have
nodefiniteshape)like
gases.

3. Generalproperties
Unlikegases,liquidsdonotdispersetofillallthespaceof
acontainer.
Liquidshavetranslational
motioni.e.liquids
moveasawhole(the
moleculescanslide
overeachotherbutthey
cannotbreakawayfrom
the intermolecular
forceswhileinthe
liquidstate).

4. Liquefaction ofgases
Effect of temperature andpressure
•Thetransitionsfromagastoaliquidandfromaliquid
toasoliddependonbothtemperatureandpressure.
•Whenagasiscooled,itlosessomeofitskineticenergy
inandthevelocityofthemoleculesdecreases.
•Ifpressureisappliedtothegas,themoleculesare
broughtwithintherangeofthevanderWaalsforces
andpassintotheliquidstate.

5. Liquefaction ofgases
Critical temperature andpressure
•Critical temperature is the temperature above which it is
impossible to liquefy a gas regardless of the pressureapplied.
•Criticalpressureis
the pressure
requiredtoliquefy
agasatitscritical
temperature
•Thefurtheragasis
cooledbelowits
criticaltemperature,
thelesspressureis
requiredtoliquefy
it.

6. Liquefaction ofgases
Critical temperature andpressure
•Thecriticaltemperatureandpressureofwateris374°C
(647K)and218atmrespectively,whereasthe
correspondingvaluesforheliumare5.2Kand2.26atm.
•Thehighcriticalvaluesforwaterresultfromthestrong
hydrogenbondingbetweenthemolecules.
•Conversely,heliummoleculesareonlyattractedbythe
weakLondonforces,andtherefore;mustbecooledto
theextremelylowtemperatureof5.2Kbeforeitcanbe
liquefied.Abovethiscriticaltemperature,helium
remainsagasnomatterwhatthepressure.

7. Liquefaction ofgases
Joule-Thomson effect (adiabatic expansion)
•Ifweallowgastoexpandrapidly(insideavacuum
flask)sothatnoheatenterssystem,suchexpansionis
knownasadiabaticexpansion.
•Energytoexpandgascomesfromgasitself.Energy
spendtoovercomecohesiveforces.
•Asaresultofthattemperatureofgasreduces.This
coolingeffectisknownasJoule-Thomsoneffect
•Ifwerepeatthiscycleseveraltimes,liquefactionof
gascouldbeachieved

8. Liquefaction ofgases
Aerosols
•Inpharmaceuticalaerosolsa
drugisdissolvedorsuspended
inapropellant(amaterialthat
isliquidunderthehigh
pressureinsidethecontainer
butformsagasundernormal
atmosphericconditionse.g.
CFCs,N
2,CO
2).
•Partofthepropellantexistsasa
gasandexertsthepressure
necessarytoexpelthedrug,
whereastheremainderexistsas
liquidandprovidesasolutionor
suspensionvehicleforthedrug.

9. Liquefaction ofgases
Aerosols
•Bydepressingavalveonthe
container,someofthedrug–
propellantmixtureis
expelledowingtotheexcess
pressure insidethe
container.
•Outsidethecontainer,the
liquidpropellantrevertsto
gasandvaporizesoff,while
thedrugformsafinespray.

10. Vapor pressure ofliquids
Equilibrium vaporpressure
•Whenaliquidisplacedinaclosedcontainerata
constanttemperature,themoleculeswiththehighest
energiesbreakawayfromthesurfaceoftheliquidand
passintothegaseousstate(evaporate),andsomeofthe
moleculessubsequentlyreturntotheliquidstate
(condense).
•Whentherateofcondensationequalstherateof
vaporizationatadefinitetemperature,thevapor
becomessaturatedandadynamicequilibriumis
established.
•Thepressureofthesaturatedvaporabovetheliquidis
thenknownastheequilibriumvaporpressure.

11. Vapor pressure ofliquids
Clausius–Clapeyronequation
•Anypointononeofthe
curverepresentsa
conditioninwhichthe
liquidandthevapour
existtogetherin
equilibrium.
•Ifthetemperatureofany
oftheliquidsisincreased
whilethepressureisheld
constant,orifthe
pressureisdecreased
whilethetemperatureis
heldconstant,allthe
liquidwillpassintothe
vaporstate.

12. Vapor pressure ofliquids
Clausius–Clapeyronequation
Clausius–Clapeyronequationexpressestherelationship
betweenthevaporpressureandtheabsolutetemperature
ofaliquid:
??????�∆??????????????????�−??????�
??????og=
??????�2.303 ????????????�??????�
P
1 and P
2: vapor pressures at absolute temperatures T
1 andT
2.
ΔHv: the molar heat of vaporization (the heat absorbed by 1 mole of
liquid when it passes into the vaporstate).

13. Vapor pressure ofliquids
Clausius–Clapeyron equation:Example
Example:Computethevaporpressureofwaterat120°C.
Thevaporpressureofwaterat100°Cis1atm,and
ΔHvis9720cal/mole.
??????og
??????�
=
∆??????????????????�−??????�
??????�2.303 ????????????�??????�
??????og
??????�
=
9720393 −373
11.987 ×393 ×373
??????�= 1.95atm

14. Vapor pressure ofliquids
Clausius–Clapeyronequation
The Clausius–Clapeyron equation can be written in a more
generalform:
∆????????????�
??????og??????= +??????����??????��
2.303????????????
Aplotof??????og??????against1/T
results in a straightline.
The heat of vaporizationof the
liquid can be calculated
from the slope of theline.

15. Boiling point
Definition
Ifaliquidisplacedinanopencontainerandheateduntil
thevaporpressureequalstheatmosphericpressure,the
liquidstartstoboilandescapeintothegaseousstate.

16. Boilingpoint
Definition
The temperature at which the vapor pressure of the liquid
equals the external or atmospheric pressure isknown as
the boilingpoint.
Theabsorbedheatusedtochangetheliquidtovapor
(at constant temperature i.e., boiling point) is called
the latent heats ofvaporization.

17. Boilingpoint
Clausius–Clapeyronequation
•Thetemperatureatwhichthevaporpressureofthe
liquidequalsanatmosphericpressureof1atmiscalled
normalboilingpoint
•Athigherelevations,theatmosphericpressuredecreases
and the boiling point islowered.
•At a pressure of 700 mm Hg, water boils at 97.7°C; at
17.5 mm Hg, it boils at20°C.
•The change in boiling point with pressure can be
computed by using the Clausius–Clapeyronequation.

18. Boiling point
Clausius–Clapeyron equation:Examples
1.Determinenormalboilingpointofchloroformifitsheatof
vaporizationis31.4KJ/molandithasavaporpressureof
190mmHgat25°C.
2.Thenormalboilingpointofbenzeneis80.1°C;at26.1
°Cithasavaporpressureof100mmHg.Whatistheheatof
vaporizartion?
??????og
??????�
=
∆??????????????????�−??????�
??????�2.303????????????�??????�

19. Boiling point
Intermolecularforces
•Theboilingpointcanbeconsideredthetemperatureat
whichthermalagitationcanovercometheattractive
forcesbetweenthemoleculesofaliquid.
•Theboilingpointofacompound,liketheheatof
vaporizationandthevaporpressure,dependsonthe
magnitudeoftheattractiveforces.
•Nonpolarsubstanceshavelowboilingpointsandlow
heatsofvaporizationbecausethemoleculesareheld
togetherpredominantlybytheweakLondonforces.
•Polarmolecules(e.gwater)exhibithighboilingpoints
andhighheatsofvaporizationbecausetheyare
associatedthroughhydrogenbonds.

20. Boilingpoint
Intermolecularforces
Theboilingpointsofnormalhydrocarbons,simplealcohols,and
carboxylicacidsincreasewithmolecularweightbecausevander
Waalsforcesbecomegreaterwithincreasingnumbersofatoms.

21. Boilingpoint
Intermolecularforces
Branchingofthechainproducesalesscompactmolecule
withreducedintermolecularattraction,andadecreasein
theboilingpoint.

22. Boilingpoint
Intermolecularforces
Alcoholsboilatamuchhigher
temperaturethansaturated
hydrocarbonsofthesame
molecularweightbecauseof
associationofthealcohol
moleculesthroughhydrogen
bonding.
Theboilingpointsofcarboxylic
acidsarehigherthanthatof
alcoholsbecausetheacids
form dimersthrough
hydrogenbonding.

23. Reference
Sinko,P.J.M.A.N.2006.Martin'sphysicalpharmacyand
pharmaceuticalsciences