Large molecules are broken down into smaller ones through catabolic reactions. Through these reactions, energy is given out, making them exergonic reactions.
A catabolic reaction is exergonic, meaning that as the molecules get broken apart, energy is released. One example of a catabolic reaction is the one that turns hydrogen peroxide into oxygen and water. Another one that is quite common is the conversion of glucose to water and carbon dioxide during the process of respiration. The opposite of catabolic reactions are anabolic reactions. They take energy because smaller molecules are building up to create large ones. An example is when sugar molecules join together to create dissacharides.