School of Chemistry, UNSW

Why should you write energy changes next to chemical equations in kJ/mol?

When we are working with energy or enthalpy changes we usually write down the energy or enthalpy change next to the chemical equation for the reaction we are interested in. For example,

CH₄(g) + 2O₂(g) CO₂(g) + 2H₂O(l) H° = -890 kJ mol^-1

This chemical equation can be interpreted in several ways. For example we could say it represents one molecule of methane (CH₄) reacting with two molecules of oxygen to form one molecule of carbon dioxide and two molecules of water. But chemists rarely work with individual molecules, so we might choose to count the molecules in larger groups, say millions of molecules. In this case we could say the chemical equation tells us that 1 million methane molecules will react with 2 million oxygen molecules to form 1 million carbon dioxide molecules and 2 million water molecules.

Even a million molecules still represents a very small amount of material, so chemists usually count molecules in groups called moles, where one mole of objects is approximately 6.022 x 10²³ objects. A mole is just a certain number of objects, just like a dozen objects is 12 objects.

These different ways of interpreting a chemical equation become important when you specify the energy or enthalpy change associated with the equation, because the size of the energy or enthalpy change depends upon the amount of material undergoing the change. The enthalpy change for one molecule of methane reacting with oxygen is smaller than when a dozen molecules of methane react with oxygen, and that is much, much smaller than when one mole of methane molecules react with two moles of oxygen.

Energy and enthalpy changes are usually quoted where each of the reactant and product molecules are counted in groups of moles, so the energy or enthalpy change is listed with units of kJ mol^-1.

If you would like to make any comments about this article, please e-mail the author, Dr Ron Haines.

Tutorials for High School Students

Why should you write energy changes next to chemical equations in kJ/mol?