Phase Changes

• Matter can change from one form to another. As this occurs, energy also changes.

*As one proceeds from ice to water to water vapor, there is an

increase

in kinetic energy.

* The changes of phase are

not chemical

, they are

physical

changes.

Heating and Cooling Curves

Heating Curve

If there is a change in the temperature there is a change in

kinetic energy

because there is a change in the

average motion of the particles.

During a phase change there is no change in temperature, therefore no change in

kinetic

energy. Instead, energy goes to breaking bonds so it is a

potential

energy change.

Along Energy Changes AB BC 

KE, no



PE no



KE,



PE

CD 

KE, no



PE

DE

no



KE,



PE

EF 

KE, no



PE

A cooling curve would be the opposite.

Gases • Gases (g): Transparent, compressible, expand without limit, have no shape/volume. **Take the shape and volume of their container.

Gases exert pressure: STP: defined as

standard temperature and pressure

*Found on Table A

101kPa or 1atm

*Pressure can also be

760

or

760

mm Hg torr

Liquids • Liquids:

no definite

shape/

but definite

volume with very

low

compressibility.

• Compressibility is the ability to occupy less space.

Boiling Point • The

temperature

at which the

vapor pressure

of a liquid reaches

atmospheric pressure

; therefore allowing particles to escape as a gas. *When vapor pressure of a liquid = atmospheric pressure

As atmospheric pressure

increases

one must

raise

the vapor pressure of the liquid by

increasing

its temperature.

Normal Boiling Point is measured at standard pressure For water it is:

100



C or 373K

• • Vapor Pressure See Table H The pressure exerted by the vapor evaporating off the surface of a liquid.

• Each liquid has its own vapor pressure.

• As the temperature of the liquid

increases

, the vapor pressure of that liquid

increases

.

Evaporation • The change of phase from

liquid

to

gas

.

Heat of Vaporization: The amount of heat energy required to

vaporize

a given mass of a liquid to

gas

at a constant

temperature

. This is an

endothermic

process, energy is being

absorbed

.

Each substance has its own Heat of Vaporization.

For water at its normal boiling temperature of 100  C and standard pressure the heat of vaporization is

2260

joules per gram. (Table B)

Condensation • The change of phase from

gas

to

liquid

. This is an

exothermic

process.

• The Heat of Condensation is the direct opposite of the heat of vaporization. The quantity of heat energy is the same as for the heat of vaporization, but instead of being

absorbed

the heat energy is being

released

.

• Solids

Definite

shape/

definite

volume.

***Regular Geometric Pattern***

 Melting (fusion): An

endothermic

process in which a

solid

becomes a

liquid

. For water at Standard Pressure the temperature at which melting occurs is

0



C / 273K

.

Freezing (Solidification) • Freezing is the direct opposite of melting, but instead of being an

endothermic

process where energy is

absorbed

, it is an

exothermic

process where energy is

released

. Water freezes at

0



C / 273K

Heat of Fusion • The amount of heat energy required to

change

a given mass of

solid

to

liquid

at a

constant temperature

.

Each substance has its own Heat of Fusion.

For water at standard pressure, this quantity of heat is

334 J/gK (Table B)

Heat of Solidification (Crystallization) • The direct opposite of the Heat of Fusion. Since solidification is an

exothermic

process, the heat energy is

released absorbed

.

instead of

Sublimation • The change of phase from

solid

to

gas

, completely skipping the

liquid

phase.

• This generally occurs only in solids with

high

vapor pressures and

weak

intermolecular forces of attractions.

• • Examples:

Dry ice (CO2), paradichlorobenzene (moth balls), I 2