Steve Marsden’s

When sodium and chlorine react, the products are sodium chloride--an ionic compound--and lots of energy. We've seen that reaction up-close-and-in-person. The notion of energy as one of the participants in a chemical reaction may seem a little strange. But as we will see in this unit, energy input and output are just as characteristic of chemical change as color change, precipitates and the like. What's more, there is a certain predictive power in those energy changes just as there is a kind of predictive power in the solubility rules and the activity series.

Hydrogen and oxygen react to produce ordinary water and also a large amount of energy. Although not all reactions produce energy, there are energy changes involved in all reactions. Why?

To answer that question we need to understand something about chemical bonding or how atoms and ions are held together in compounds.