Ch. 10 – Changes of Ch. 10 – Changes of StateState

Ch. 10 – Changes of Ch. 10 – Changes of StateState

(p. 324 – 330)

PhasesPhases

The phase of a substance is determined by three things.

• The temperature.

• The pressure.

• The strength of intermolecular forces.

A. Phase ChangesA. Phase Changes

A. Phase ChangesA. Phase Changes

EvaporationEvaporation• molecules at the surface gain enough

energy to overcome IMF (intermolecular forces)

VolatilityVolatility• measure of evaporation rate• depends on temp & IMF

A. Phase ChangesA. Phase Changes

Kinetic Energy

# o

f P

art

icle

s

p. 477

Boltzmann Distribution

temp

volatility

IMF

volatility

A. Phase ChangesA. Phase Changes

EquilibriumEquilibrium• trapped molecules reach a balance

between evaporation & condensation

CondensationCondensation

Change from gas to liquid. Achieves a dynamic equilibrium with

vaporization in a closed system. What is a closed system? A closed system means matter

can’t go in or out. Put a cork in it. What the heck is a “dynamic equilibrium?”

When first sealed the molecules gradually escape the surface of the liquid

As the molecules build up above the liquid some condense back to a liquid.

Dynamic EquilibriumDynamic Equilibrium

As time goes by the rate of vaporization remains constant

but the rate of condensation increases because there are more molecules to condense.

Dynamic EquilibriumDynamic Equilibrium

Equilibrium is reached when

Rate of Vaporization = Rate of Condensation

Molecules are constantly changing phase “Dynamic”

The total amount of liquid and vapor remains constant “Equilibrium”

Dynamic equilibriumDynamic equilibrium

Vapor PressureVapor Pressure

The pressure above the liquid at equilibrium.

Liquids with high vapor pressures evaporate easily.

They are called volatile.

Vapor pressureVapor pressure

Decreases with increasing intermolecular forces.

• Bigger molecules (bigger LDF)

• More polar molecules (dipole-dipole) Increases with increasing temperature.Easily measured in a barometer.

Vapor PressureVapor Pressure

Vaporization - change from liquid to gas at boiling point.

Evaporation - change from liquid to gas below boiling point

Heat (or Enthalpy) of Vaporization

(Hvap )- the energy required to

vaporize

1 mol at 1 atm.

Vaporization is an endothermic process - it requires heat.

Energy is required to overcome intermolecular forces.

Responsible for cool beaches.Why we sweat.

Temperature EffectTemperature Effect

Kinetic energy

# of

mol

ecu

les

T1

Energy needed to overcome intermolecular forces

Kinetic energy

# of

mol

ecu

les

T1

Energy needed to overcome intermolecular forces

T1

T2

At higher temperature more molecules have enough energy - higher vapor pressure.

Energy needed to overcome intermolecular forces

A. Phase ChangesA. Phase Changes

Vapor PressureVapor Pressure• pressure of vapor above

a liquid at equilibrium

IMF v.p.temp v.p.

• depends on temp & IMF• directly related to volatility

p.478

temp

v.p

.

Boiling PointBoiling Point

Reached when the vapor pressure equals the external pressure.

Normal boiling point is the boiling point at 1 atm pressure.

Superheating - Heating above the boiling point.

Supercooling - Cooling below the freezing point.

A. Phase ChangesA. Phase Changes

Boiling Point• temp at which v.p. of liquid

equals external pressure

IMF b.p.Patm b.p.

• depends on Patm & IMF

• Normal B.P. - b.p. at 1 atm

Which has a higher m.p.?• polar or nonpolar?• covalent or ionic?

A. Phase ChangesA. Phase Changes

Melting Point• equal to freezing point

polar

ionic

IMF m.p.

Melting PointMelting Point

Melting point is determined by the vapor pressure of the solid and the liquid.

At the melting point the vapor pressure of the solid = vapor pressure of the liquid

Solid Water

Liquid Water

Water Vapor Vapor

Solid Water

Liquid Water

Water Vapor Vapor

If the vapor pressure of the solid is higher than that of the liquid the solid will release molecules to achieve equilibrium.

Solid Water

Liquid Water

Water Vapor Vapor

While the molecules of condense to a liquid.

This can only happen if the temperature is above the freezing point since solid is turning to liquid.

Solid Water

Liquid Water

Water Vapor Vapor

If the vapor pressure of the liquid is higher than that of the solid, the liquid will release molecules to achieve equilibrium.

Solid Water

Liquid Water

Water Vapor Vapor

Solid Water

Liquid Water

Water Vapor Vapor

While the molecules condense to a solid.

The temperature must be below the freezing point since the liquid is turning to a solid.

Solid Water

Liquid Water

Water Vapor Vapor

If the vapor pressure of the solid and liquid are equal, the solid and liquid are vaporizing and condensing at the same rate. The Melting point.

Solid Water

Liquid Water

Water Vapor Vapor

Changes of stateChanges of state

The graph of temperature versus heat applied is called a heating curve.

The temperature a solid turns to a liquid is the melting point.

The energy required to accomplish this change is called the Heat (or Enthalpy) of Fusion Hfus

A. Phase ChangesA. Phase Changes

Sublimation

• solid gas

• v.p. of solid equals external pressure

EX: dry ice, mothballs, solid air fresheners

Deposition

• gas solid

• EX: frost, snow

Heating Curve for Water

IceWater and Ice

Water

Water and Steam

Steam

B. Heating CurvesB. Heating Curves

Melting - PE

Solid - KE

Liquid - KE

Boiling - PE

Gas - KE

n

B. Heating CurvesB. Heating Curves

Temperature Change• change in KE (molecular motion)

B. Heating CurvesB. Heating Curves

Phase Change• change in PE (molecular arrangement)• temp remains constant

Heat of Fusion (Hfus)

• energy required to melt 1 gram of a substance at its m.p.

B. Heating CurvesB. Heating Curves

Heat of Vaporization (Hvap)

• energy required to boil 1 gram of a substance at its b.p.

• usually larger than Hfus…why?

EX: sweating, steam burns, the drinking bird

Phase Diagrams.Phase Diagrams.

A plot of temperature versus pressure for a closed system, with lines to indicate where there is a phase change.

C. Phase DiagramsC. Phase Diagrams

Show the phases of a substance at different temps and pressures.

Temperature

SolidLiquid

Gas

1 Atm

AA

BB

CCD

D D

Pre

ssur

e

D

SolidLiquid

Gas

Triple Point

Critical Point

Temperature

Pre

ssur

e

SolidLiquid

Gas

This is the phase diagram for water.

The density of liquid water is higher than solid water.

Temperature

Pre

ssur

e

Solid Liquid

Gas

1 Atm

This is the phase diagram for CO2

The solid is more dense than the liquid The solid sublimes at 1 atm.

Temperature

Pre

ssur

e