Liquids & Vapor Pressure

Vapor

• Gas phase of substance that is normally liquid at room temperature

• Some evaporation occurs at all temperatures

• The easier a substance evaporates, the weaker the IMF

Fig a: open systemopen system Evaporation continues until all liquid is gone Fig b: closed systemclosed system Rate of evaporation = rate of condensation System appears static due to equilibrium

How does evaporation occur?

molecules in liquid phase have range of KE; some have enough KE to “escapeescape” from liquid phase

Vapor Pressure

–Pressure exerted by vapor over its liquid

–VP is affected by temperature:• higher the T, higher the VP

all liquids: VP ↑as T ↑

Vapor Pressure

• VP does NOT depend on how much liquid is present

• VP depends only on temperature

IMF in Liquids

WEAK FORCES

• High VP• High rate evaporation• Low bp

• Small Hv

STRONG FORCES

• Low VP• Low rate evaporation• High bp

• Large Hv

Phase Changes• Melting• Boiling

–Vaporization –occurs throughout liquid,T constant)

• Evaporation (occurs only at surface; T ↓)

• Sublimation• Freezing• Deposition• Condensation

Phase Changes are accompanied by energy changes

• Endothermic Process: Energy is absorbedEndothermic Process: Energy is absorbed

• Exothermic Process: Energy is releasedExothermic Process: Energy is released

• Which phase changes absorb energy (endothermic)?

• Which phase changes release energy (exothermic)?

Melting, Boiling (vaporization)Melting, Boiling (vaporization)

FreezingFreezing , CondensationCondensation

Boiling Point

• Temperature at which: VP liquid = external atmospheric P

• Normal Boiling PointNormal Boiling Point:

temp at which VP liquid = 1 atm

• Substances with high bp’s have strong IMF’s

Melting Point

• = temp at which liquid & solid phases of substance coexist at equilibrium

• higher the mp, the stronger the IMF

Freezing Point

• Temp at which liquid is converted to crystalline solid

• How does fp compare to mp?

They’re the same: for H2O = 0˚C

Strong or Weak attractive forces?

• High vapor pressure

• Large Hv

• High boiling point• Low boiling point• Low vapor pressure

• Small Hv

• Evaporates rapidly• Evaporates slowly

• Weak

• Strong• Strong• Weak• Strong• Weak• Weak• Strong

Boiling and Pressure

• If increase external pressure (ex: camping in Death Valley), boiling point is ____ than 100oC

• If decrease external pressure (ex: eating Raman noodles at top of Mt. Whitney), the boiling point is ____ than 100oC

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Boiling vs. Evaporation

BOILING• occurs throughout

liquid• occurs at definite T• occurs at constant T

• only spontaneous when VP = external pressure

EVAPORATION• occurs at surface of

liquid (or solid)• occurs at all T• liquid cools off as

evaporates• spontaneous all the

time