What is defined as an exothermic reaction?

An exothermic reaction is characterized by the release of energy to its surroundings, often in the form of heat or light. This means that during the process, the products of the reaction have lower energy than the reactants. This energy difference is expelled into the environment, which can result in an increase in the temperature of the surroundings. Common examples of exothermic reactions include combustion reactions, such as burning fuels, or the process of respiration in living organisms, where glucose is oxidized to produce energy.

The other options do not correctly describe exothermic reactions. For instance, a reaction that absorbs energy is defined as endothermic, which is the opposite of what happens in exothermic reactions. Similarly, breaking down molecules does not inherently indicate whether energy is absorbed or released; this could occur in either type of reaction depending on the specifics of the process. Lastly, while some reactions require a catalyst to proceed, this does not determine whether the reaction is exothermic or endothermic, as catalysts influence the rate of a reaction, not the energy changes involved.