Heat & Changes of State
Changes of State

Solid to liquid

Liquid to solid

Liquid to gas

Gas to liquid
Heat & Changes of State

When a change of
state occurs by a gain
or loss of heat, the
temp remains
constant until the
change of state is
complete!
Molar Heat of Fusion (∆Hfus)



Fusion - when a solid absorbs heat at its
freezing point
Molar heat of fusion – heat absorbed by
1 mole of a solid as it melts into a liquid
(at constant temp)
∆Hfus for water: 1 mole = 6.01kJ
Molar Heat of Solidification (∆Hsolid)



Solidification - when a liquid releases
heat as it turns into a solid
Molar heat of solidification – heat
released by 1 mole of a liquid as it
becomes a solid (at constant temp)
∆Hsolid for water: 1 mole = - 6.01kJ
∆Hfus = - ∆Hsolid

The quantity of heat absorbed by a
melting solid is the same as the heat
released when a liquid solidifies
H2O
(s)
H2O
(l)
+ 6.01kJ → H2O
- 6.01kJ → H2O
(l)
(s)
Examples


How many grams of ice at 0°C will melt if
2.25 kJ of heat are added?
How many kilojoules of heat are
required to melt a 10g popcicle at
0°C? use molar mass of water
Molar Heat of Vaporization (∆Hvap)



Vaporization - when a liquid absorbs
heat at its boiling point to become a
vapor
Molar heat of vaporization – heat
absorbed by 1 mole of a liquid as it
vaporizes into a gas
∆Hvap for water: 1 mole = 40.7kJ
Molar Heat of Condensation (∆Hcond)



Condensation- when a vapor releases
heat as it turns into a liquid
Molar heat of condensation – heat
released by 1 mole of a vapor as it
becomes a liquid
∆Hcond for water: 1 mole = 40.7 kJ
∆Hvap = - ∆Hcond

The quantity of heat absorbed by a
vaporizing liquid is the same as the heat
released when a vapor condenses
H2O
(l)
+ 40.7kJ → H2O
(g)
H2O
(g)
– 40.7kJ → H2O
(l)
Examples

How much heat (in kJ) is absorbed when
24.8g H2O(l) at 100°C & 101.3kPa is
converted to steam at 100°C ?
How many kJ of heat are absorbed
when 0.46g of chloroethane (C2H5Cl,
bp 12.3°C) vaporizes at it normal
boiling point? Molar heat of
vaporization of chloroethane is 26.4
kJ/mol.