- The heat of fusion for water is 334 kJ/kg. How much thermal
energy is required to completely melt a 100-gram ice cube?
Here’s How to Crack It
The change in phase from solid to liquid requires the input of heat; in this case, Q =
mL = (0.1 kg)(334 kJ/kg) = 33.4 kJ.
- A beaker contains 0.1 kg of water, initially at room temperature
(20°C). If the specific heat of water is 4.2 J/kg • °C and the latent
heat of vaporization is 2,300 kJ/kg, how much thermal energy would
the water need to absorb to turn completely to steam?
Here’s How to Crack It
First we need to heat the water to the boiling point, 100°C; then we need to
continue to add heat until all the liquid is vaporized. The amount of thermal energy
required for the first step is
and the amount of energy required for the second step is
Q 2 = mwaterLvap = (0.1 kg)(2,300 kJ/Kg) = 230 kJTherefore, the total amount of thermal energy required is
Qtotal = Q 1 + Q 2 = 33.6 kJ + 230 kJ = 264 kJ