to break up the bonding forces between molecules by increasing the potential
energy content of the molecules so that a specific change of state can occur.
The amount of heat energy required to melt one mole of solid at its melting
point is called its molar heat of fusion. If the quantity of ice melted is one mole
(18 grams), then, for ice, it is 6.01 kJ/mole. Likewise, the amount of heat energy
required to vaporize one mole of liquid at its boiling point is called its molar
heat of vaporization. If the quantity of water vaporized is one mole, then its
molar heat of vaporization is 40.79 kJ/mole.
The following two problems are samples of calorimetry problems. The first
is solved by using the SI units of joules. The second is done by using calories but
is finally converted to joules.
Example 1
What quantity of ice at 273 K can be melted by 100. joules of heat?
Heat to fuse (melt) a substance = heat of fusion of the substance × mass of the
substance.
This quantity can be expressed by the following formula, where q denotes
the heat measurement made in a calorimeter:
q = m (mass) × C (heat of fusion)Solving for m, we getm = 29.9 × 10−2 g or 0.299 g of ice melted
Because heat is absorbed in melting, this is an endothermic action.Example 2
How much heat is needed to change 100.0 grams of ice at 273 K to steam at
373 K?
To melt 100. grams of ice at 273 K:
Use: m (mass) × C (heat of fusion) = q (quantity of heat)
To heat 100. grams of water from 273 K to 373 K:
Use: m × ΔT × specific heat = q