chapter 12.3 : changes of state
DESCRIPTION
TRANSCRIPT
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.