Change of state
Change of state and energy
consumption
Fusion
 Heat of fusion is energy required to convert
the solid into the liquid at the same
temperature
 Originally called latent heat because no
temperature rise occurs
 Freezing releases same quantity of heat
Vapour pressure
 There is always a
certain pressure of a
gas in equilibrium with
the liquid phase, even
at temperatures well
below the boiling point
 Consequence of range
of energies of
molecular motion
Vapour pressure and boiling
 Molecules do not all have
the same energy
 High energy molecules
escape the liquid – vapour
pressure
 When vapour pressure =
atmospheric pressure
boiling occurs
 Sublimation is direct
transition of solid to gas
(dry ice)
Molecular energy and change of state
 Combining of molecules generates heat condensation
 Input of energy breaks molecular bonds vaporization
Sublimation
 Solid converts directly
to a gas
 “Dry ice” – solid carbon
dioxide sublimes to the
gas at room
temperature
Boiling
 Heat of vaporization is
energy required to
convert liquid into
vapor at the same
temperature
 Condensation releases
the same quantity of
energy
Heat, temperature and change of state
At phase
change, no
change in T
Heat input in
boiling much
greater
At phase
change, no
change in T
 Evaporation requires greater heat input than
melting because of the greater number of bonds
to be broken
 We lick our fingers to extinguish a burning candle
Energy calculations
 Calculate heat required to convert 15.0 g water at
25.0ºC to steam at 100ºC.
 Calculation is sum of two parts:
1. Heating the liquid
1. Mass water
2. Temperature change
3. SH water
2. Boiling the liquid
1. Heat vaporization water
2. Mass water
State change calculation
 Calculate heat required to convert 15.0 g water at
25.0ºC to steam at 100ºC.
 SH water = 4.18 J/gºC, ΔHvap = 2,259 J/g