Chapter 7

Phase Changes

Some things to think about

Two allotropes of Carbon

Dynamic Equilibrium

• A dynamic equilibrium can be established between either a (liquid and its vapor) or a (saturated solution and its solute).– L/V the # of particles leaving the liquid to

become a vapor equals the # of particles returning to the liquid

– SS/S the # of solid particles dissolving equals the # if dissolved particles reattaching to the solid

Boiling Point

• When the vapor pressure of a liquid reaches atmospheric pressure, the substance will boil.

• Normal boiling occurs at normal atmospheric pressure.– Remember normal atmospheric pressure can

be 1 atm, 760 mmHg, or 101.3 kPa

Boiling Point Curves

• A boiling point curve represents all the temperatures at which a liquid will boil at various pressures.– If you raise the pressure, the liquid will boil at

a higher temperature.– If you lower the pressure, the liquid will boil at

a lower temperature.

Volatile vs nonvolatile

Volatile Nonvolatile

Normal BP Relatively low Relatively high

Vapor pressure Relatively high Relatively low

Does it evaporate easily?

Yes No

Intermolecular forces

Weak Strong

Let’s look at a boiling pt curve

Heating curve

Heat of fusion

• Heat of fusion is the amount of energy needed to melt one gram of a substance.– Units are usually J/g

• The molar heat of fusion is the amount of energy needed to melt one mole of a substance.– Units are usually kJ/mol

• Heat of fusion is also used for freezing (although this is the energy given off)

Heat of vaporization

• Heat of vaporization is the amount of energy needed to boil one gram of a substance.– Units are usually J/g

• The molar heat of vaporization is the amount of energy needed to boil one mole of a substance.– Units are usually kJ/mol

• Heat of vaporization is also used for condensing (although this is the energy given off)

Determining energy involved for heating curve

• Heat the solid Q = msolid csolid ∆tsolid

• Melt the solid Q = m Hf

• Heat the liquid Q = mliquid cliquid ∆tliquid

• Boil the liquid Q = m Hv

• Heat the gas Q = mgas cgas ∆tgas

• #’s for water

Vapor Phase Diagrams

• Points of interest– Triple point– Critical point

• Critical temperature• Critical pressure

– Equilibrium lines– Normal bp and mp

Vapor Phase Diagram

Phase Changes

Phase change What changes ∆heat

Melting Solid to liquid Endothermic

Freezing Liquid to solid Exothermic

Boiling Liquid to gas Endothermic

Condensing Gas to liquid Exothermic

Subliming Solid to gas Endothermic

Depositing Gas to solid Exothermic

Energy needed to melt

ΔHfusion

Energy needed to boil

ΔHvaporization

Energy needed to sublime

ΔHsublimation

ΔHsub = ΔHfus + ΔHvap

What to know for test?

• Volatile vs nonvolatile

• Affect of strength of intermolecular forces on vapor pressure

• Be able to read diagrams

• Be able to do math regarding heating curve

• Dynamic equilibrium