2.3 Chemical Properties and Reactions

Chemical Reactions

Energy and Chemical Reactions

Before a chemical reaction can occur, there must be enough energy to allow the reactants to change. The energy needed to get a chemical reaction started is called the activation energy. When you burn a piece of paper, for example, you apply enough heat to get the burning started. Once it has started, the paper will burn on its own.

In some chemical reactions, energy is released in the form of heat or light when the reactants change. These reactions are called exothermic reactions. Burning is an example of an exothermic reaction; the explosion of fireworks is a dramatic example of this type of reaction. 

In other chemical reactions, energy is absorbed when the reactants change. These reactions are called endothermic reactions. For example, when you bake a cake, the liquid batter absorbs heat energy and a solid cake is the product.

This table summarizes exothermic and endothermic reactions.