Explain how a problem can be solved using Hess's Law.

Hess's law gives a way of calculating the total change in
enthalpy for an equation if the reaction can be split into a number of steps and the
enthalpy change for each step is known.

Stated differently,
it means that enthalpy changes are additive in nature. And the difference in the
enthalpy change in the formation of products and the enthalpy change in the formation of
the reactants is the enthalpy change of the reaction. As an example consider the
following reaction:

PCl5 --> PCl3 +
Cl2

We need to know the enthalpy change in the reaction. We
are given that

P4 + 6Cl2 --> 4 PCl3, enthalpy change
= -2439 kJ
and 4PCl5 --> P4 + 10 Cl2, enthalpy change = 3438
kJ

Adding the two equations we get:
P4 + 6Cl2 +
4PCl5 --> 4 PCl3 + P4 + 10 Cl2

Eliminate the common
terms and simplify
PCl5 --> PCl3 + Cl2

The
enthalpy change is (-2439 + 3438)/4 = 249.75 kJ

The
original reaction PCl5 --> PCl3 + Cl2 has an enthalpy change of 249.75 kJ, the
same as what we got.