What distinguishes exothermic reactions from endothermic reactions?

Exothermic reactions are characterized by the release of heat to the surroundings, leading to an increase in thermal energy in the environment. This heat release occurs because the products of the reaction have lower energy than the reactants, allowing the excess energy to be discharged in the form of heat. A common example of an exothermic reaction is combustion, where fuel reacts with oxygen, resulting in a significant release of heat.

On the other hand, endothermic reactions absorb heat from their surroundings, causing a decrease in the temperature of the environment. In these reactions, the products have higher energy compared to the reactants, necessitating the intake of heat to facilitate the reaction process. Photosynthesis in plants is a classic example of an endothermic reaction as it requires energy from sunlight to convert carbon dioxide and water into glucose.

The distinction lies primarily in the direction of heat flow: exothermic reactions discharge heat, whereas endothermic reactions take in heat. Therefore, stating that exothermic reactions release heat while endothermic reactions absorb it accurately captures the fundamental difference between these two types of reactions.