Liquids & Vapor Pressure

Vapor

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

• Some evaporation occurs at all temperatures.

• Generally, the easier a substance evaporates, the weaker the intermolecular forces.

Fig a: open systemopen system. Evaporation continues until all liquid is gone. Fig b: closed systemclosed system. Eventually, rate of evaporation = rate of condensation. System appears static.

At equilibrium, the rate of evaporation is equal to the rage of condensation.

At equilibrium, the vapor pressure is constant.

How does evaporation occur?

The molecules in the liquid phase have a range of kinetic energies. Some of them have enough kinetic energy to “escapeescape” from the liquid phase.

Vapor Pressure

• Pressure exerted by a vapor over its liquid.

• How is vapor pressure affected by temperature?

• The higher the temperature, the higher the vapor pressure.

Measuring vapor pressure

For all liquids, vapor pressure increases as temperature increases.

Vapor Pressure

• Vapor pressure does NOT depend on how much liquid is present.

• As long as some liquid is present, can be a teaspoon or a gallon.

• Vapor pressure depends only on temperature.

Which has the highest v.p.?

90

50C

20C70C

90C

Intermolecular Forces in Liquids

WEAK FORCES

• High vapor pressure• High rate of

evaporation• Low boiling point

• Small Hv

STRONG FORCES

• Low vapor pressure• Low rate of

evaporation• High boiling point

• Large Hv

Boiling Point

• Temperature at which the vapor pressure of a liquid is equal to the external pressure.

Temperature where: VPliq = Pext

• Usually the external pressure is atmospheric pressure.

Temperature where: VPliq = Patm

• Atmospheric pressure varies from day to day.

Boiling Point

• Temperature at which the vapor pressure of a liquid = external or atmospheric pressure.

• Normal Boiling PointNormal Boiling Point is temperature at which vapor pressure of the liquid = 1 atm.

• Substances with high boiling points have strong molecular interactions & low vapor pressures.

Normal Boiling Point

• Temperature at which the vapor pressure of a liquid is equal to exactly 1 atmosphere or 760 torr or 101.3 kPa.

Boiling So, what are 2 ways to get a liquid to boil?

Heat it up or reduce the air pressure above it’s surface

Pext = Patm

Raise the vapor pressure of the liquid by heating it up.

Reduce the external pressure using a vacuum pump.

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 you increase the external pressure (say you are camping in Death Valley), the boiling point is ________ than 100oC.

• If you decrease the external pressure (say you are eating Raman noodles at the top of Mt. Whitney), the boiling point is ________ than 100oC.

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Boiling vs. Evaporation

BOILING• Occurs throughout the

liquid• Occurs at a definite

temperature• Occurs at a constant

temperature• Only spontaneous when

the vapor pressure equals the external pressure

EVAPORATION• Occurs at the surface of

the liquid (or solid)• Occurs to some extent at

all temperatures• The liquid cools off as

some of it evaporates• Spontaneous all the time

Can you explain this …

• Drinking Bird

Phase Changes

• Melting• Boiling (occurs throughout, constant

temperature)– Evaporation (occurs at surface, temperature

drops)

• Sublimation• Freezing• Deposition• Condensation

Phase Changes are accompanied by energy changes.

• Which phase changes absorb energy?

• Which phase changes release energy?

Melting, boilingMelting, boiling

Condensation, FreezingCondensation, Freezing

It’s always from the system’s perspective.

• Endothermic Process: Energy is absorbed

• Exothermic Process: Energy is released

Melting Point

• = temperature at which the liquid phase and solid phase of a substance can coexist at equilibrium.

• The higher the melting point, the stronger the molecular interactions.

Freezing Point

• Temperature at which a liquid is converted to a crystalline solid.

• How does freezing point compare to melting point?

They’re the same.