CHANGES OF STATEChapter 12.3

Objectives:

1. Explain the relationship between equilibrium and changes of state.

2. Predict changes in equilibrium using Le Chatelier’s principle.

3. Explain what is meant by equilibrium vapor pressure.

4. Describe the processes of boiling, freezing, melting, and sublimation.

5. Interpret phase diagrams.

Equilibrium Defined as: a dynamic condition in

which two opposing changes occur at equal rates in a closed system. Closed system: matter cannot enter or

leave, but energy can Analogy:

Public swimming pool Morning: more people entering than leaving Afternoon: number of people entering and

leaving are the same This would be equilibrium

Equilibrium and Changes of State Evaporation of water in a closed container

Vacuum over liquid Same temperature as surroundings ( 25oC) Water and container are the system

**Single Phase Liquid

**Evaporation begins at

constant rate**Some condensation occurs

**Rate of evaporation is equal to rate of condensationEquilibriu

m

Possible Phase ChangesChange of State Process ExampleSolid Liquid melting candleSolid Gas sublimation Rock showLiquid Solid freezing Lake

freezingLiquid Gas vaporization puddleGas Liquid condensation bathroomGas Solid deposition Frost

An Equilibrium Equationliquid + heat energy

vaporvapor liquid +heat energy

liquid + heat energyvapor

Reversible: vapor is being formed at the same RATE as liquid.

Le Chatelier’s Principle When a system at equilibrium is

disturbed by application of a stress, it attains a new equilibrium position that minimizes the stress.Stress:• Change in

1) Concentration2) Pressure3) temperature

These will cause a shift in equilibrium

liquid + heat energyvapor

• Will always adjust to move back to equilibrium

Example: 1. add liquid (increase concentration)

2. Start to create more vapor

3. Shift to the right

Shifts in Equilibrium

Change Shift _Add liquid RightRemove liquid LeftAdd Vapor LeftRemove Vapor RightDecrease Container Volume LeftIncrease Container Volume RightDecrease in Temperature LeftIncrease in Temperature Right

liquid + heat energyvapor

Equilibrium Vapor Pressure of a Liquid Defined as: the pressure exerted by

a vapor in equilibrium with its corresponding liquid at a given temperature

Volatile and Nonvolatile Liquids Volatile Liquids:

Liquids that evaporate readily Weak attractive forces

Nonvolatile Does not evaporate readily

Stronger attractive forces

Equilibrium vapor pressure Depends on

Nature of liquid Temperature

Boiling Conversion of a liquid to a vapor within the

liquid to a vapor within the liquid as well as at its surface.

Occurs when Equilibrium vapor pressure = atmospheric pressure

Boiling Point Temperature at which the equilibrium vapor

pressure of the liquid equals the atmospheric pressure

High elevations Lower atmospheric pressure Water boils at lower temperatures Food takes longer to cook

Molar Heat of Vaporization The amount of heat energy needed to

vaporize one mole of liquid at its boiling point.

Heat added (KJ)

Tem

pera

ture

(oC

)

0.0

100.0

Heat of vaporization

Freezing and Melting Freezing point

Temperature at which the solid and liquid are in equilibrium at 1 atm pressure.

Melting is the reverse of freezing.

liquid solid + heat energy

solid + heat energyliquid

solid + heat energyliquid

Molar Heat of Fusion The amount of heat energy required

to melt one mole of solid at its melting point.

Heat added (KJ)

Tem

pera

ture

(oC

)

0.0

100.0

Heat of fusion

Sublimation and Deposition Sublimation

Change of state from a solid directly to a gas

Deposition Change of state from a gas directly to a

solidsolid + heat energyvapor

Phase Diagrams Graph of pressure versus

temperature that shows the conditions under which the phases of a substance exist.

Triple Point• indicates the temperature

and pressure conditions at which the solid, liquid, and vapor of a substance can coexist at equilibrium.

Critical point• indicates the critical

temperature and critical pressure. Critical temperature

• temp. above which substance cannot exist as a liquid. Critical pressure

• lowest pressure at which the substance can exist as a liquid at the critical temperature.