Upon completion of this topic, learners will be able to:

• calculate change in enthalpy of a reaction using Hess’ Law.
• calculate change in standard enthalpy of a reaction using tables of change in enthalpy of formation.

Hess’s law can be stated as follows: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. Hess’s law is based on the fact that because H is a state function, ΔH depends only on the initial and final state (that is, only on the nature of reactants and products).

Let us understand the importance of this law with the help of an example. For example, we can think of the reaction of carbon with oxygen to form carbon dioxide as occurring either directly or by a two-step process. The direct process is written:

C(s) + O₂(g) → CO₂(g)       H⁰ = -393.5 kJ

In the two-step process, first carbon monoxide is formed:

Then, carbon monoxide reacts further to form carbon dioxide:

The equation describing the overall reaction is the sum of these two chemical changes. Because the CO produced in Step 1 is consumed in Step 2, the net change is:

C(s) + O₂(g) → CO₂(g)

According to Hess’s law, the enthalpy change of the reaction is equal to the sum of the enthalpy changes of the steps.

To apply Hess’s law, you can generally expect to do the following:

• Reverse certain equations and change the signs of their ΔH values.
• Multiply certain quantities and their ΔH values by appropriate factors. The factors may be whole numbers or fractions.