Topic: Liquids & Vapor Pressure

Do Now: Compare I2 and F2

Why is F2 a gas at room temp. and I2 a solid

Click here and go over slides from Wisc-Online before talking notes!

Vapor

• Vapor is the gas phase of substance that is normally liquid at room temperature

• Some evaporation occurs at all temperatures• (like you saw with the ice cube)

• 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

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

How does evaporation occur?

Vapor Pressure

–Pressure exerted by vapor over its liquid

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

the VP

Vapor Pressure• VP does NOT depend on how much liquid

is present

• VP depends only on temperature

all liquids: as T ↑ the VP ↑

Boiling Point

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

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

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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IMF in Liquids

WEAK FORCES

• High VP• High rate evaporation• Low bp

STRONG FORCES

• Low VP• Low rate evaporation• High bp

Strong or Weak attractive forces?

• High vapor pressure

• High boiling point• Low boiling point• Low vapor pressure• Evaporates rapidly• Evaporates slowly

• Weak

• Strong• Weak• Strong• Weak• Strong