Phase Changes

Bubble, bubble, toil and trouble...

Boiling

Converting liquid to vapor within liquid and at surface

Equilibrium vapor pressure equals atmospheric pressure

Boiling point: temperature at which equilibrium vapor pressure equals atmospheric pressure

Molar Heat of Vaporization

Defined: amount of energy required to convert 1 mole of liquid to solid at boiling point

Added energy does not increase temperature, but separates liquid particles

Liquid with strong attractive force has high molar heat of vaporization

Freezing

Change from liquid to solid Temperature at which liquid phase of substance

is in equilibrium with solid at one atmosphere is freezing point Freezing point = melting point

Energy loss during freezing converted to order of particles in solid

Molar Heat of Fusion

Defined: amount of heat required to melt one mole of substance at normal melting point

Temperature does not change during melting Added energy overcomes attractive forces of

solid and decreases order of particles

Example: Vaporization

Calculate the molar heat of vaporization of a substance if 0.433 mole of the substance absorbs 36.5 kJ energy when it is vaporized.

Known: 0.433 mole 36.5 kJ energy Molar heat of vaporization = 36.5 kJ

0.433 mole So ΔHv = 84.2956 kJ/mole