Question

Please select the best answer and click "submit."

Constants for water
Hvap 40.65 kJ/mol
Hf -285.83 kJ/mol
Hfusion 6.03 kJ/mol
specific heat 4.186 J/gC
molar mass 18.02 g
How much energy is generated from freezing 2.5 g water?

A. 2.5 g 1 mol/18.02 g (285.83) kJ/mol
B. 2.5 g 1 mol/18.02 g 4.186 kJ/mol
C. 2.5 g 1 mol/18.02 g 40.65 kJ/mol
D. 2.5 g 1 mol/18.02 g (-285.83) kJ/mol

Answer 1

When it comes to physical changes like phase changes, there are two types of heat energy: sensible heat and latent heat. Sensible heat is the heat absorbed/released when you heat the substance but it doesn't change phase. An example would be heating lukewarm water. The substance is liquid all throughout. Latent heat, on the other hand, is the heat absorbed/released when there is a phase change. An example would be boiling water, because it changes liquid to vapor.

Hence, for freezing liquid, you use the latent heat, specifically the heat of fusion. The answer should be

2.5 g * (1 mol/18.02 g) * 6.03 kJ/mol = 0.84 kJ/mol

The answer is not in the choices. You only use Hvap if you boil water.