Energy and Phase Changes

Energy Requirements for State Changes

• To change the state of matter, energy must be added or removed.

Endothermic process• Solid to a Liquid Melting (Fusion)

• particles overcome attractive forces and move around & past other particles

• Solid to a Gas Sublimation• occurs only at conditions far from normal MP

• Liquid to a Gas Vaporization• particles are very spread out – requires a lot of

energy• evaporation – vaporization at the surface of a

liquid

Exothermic processes

• Gas to a Liquid Condensation(equal and opposite of vaporization)

• Liquid to a solid Solidification(equal and opposite of melting)

Gas to a solid Deposition(equal and opposite of sublimation)

Heating curveA heating curve • illustrates the changes of state as a

solid is heated to a gas.• uses sloped lines to show an increase

in temperature.• uses plateaus (flat lines) to indicate a

change of state.• A cooling curve shows the opposite

process

Heat of fusionThe heat of fusion • is the amount of heat released when 1 gram of

liquid freezes (at its freezing point). • is the amount of heat needed to melt 1 gram of

a solid (at its melting point).• For water (at 0°C) =

334 J 1 g water

The heat needed to freeze (or melt) a specific mass of water (or ice) is calculated using the heat of fusion.

Heat = g water x 334 J 1 g water

Example: How much heat is needed to melt 15.0 g of water?

15.0 g water x 334 J = 5.01 kJ

1 g water

Heat of vaporizationThe heat of vaporization is the amount of heat• absorbed to vaporize 1 g of a liquid to gas at the boiling

point.• released when 1 g of a gas condenses to liquid at the

boiling point.

Boiling Point of Water = 100°C

Heat of Vaporization (water) = 2260 J 1 g water

• The heat needed to vaporize (or boil) a specific mass of water (or water vapor/steam) is calculated using the heat of vaporization.

Heat = g water x 2260 J 1 g water

Example: How much heat is needed to boil 12.0 g of water?

12.0 g water x 2260 J = 27.1 kJ

1 g water

Practice• Calculate the heat of vaporization of

25 g of water (in kJ).

• From this calculate the heat of vaporization of water in kJ/mol.

Heating curve calculations

• For heating (same phase): q = C.m ΔT• For phase changes: heat of fusion or

vaporization

Example• How much heat do you need to melt

10 g of ice and then heat that to 10 oC?

• Melting: 10 g x 334 J/g = 3340 J• Heating: 10 g x 4.184 J/(g.oC) x 10 oC =

418.4 J• Total heat needed: 3340 J + 418 J =

3758 J