What characterizes an exothermic reaction?

An exothermic reaction is characterized by the release of heat to the surroundings. This means that during the reaction, the total energy of the system decreases, leading to a feeling of warmth in the environment or an increase in the temperature of the surroundings. Common examples of exothermic processes include combustion reactions, such as burning fuels, and many oxidation reactions.

The definition of an exothermic reaction is rooted in thermodynamics, where the change in enthalpy (ΔH) is negative, indicating that the energy of the products is lower than that of the reactants. This release of energy can be harnessed in various applications, such as in heat packs, engines, and even biological processes like cellular respiration. Understanding this concept is crucial for grasping broader principles in chemistry, including thermochemistry and the study of different types of chemical reactions.

The other options do not accurately describe exothermic reactions. For instance, the absorption of heat would indicate an endothermic reaction, while requiring constant temperature and only occurring in gaseous states are neither characteristics of exothermic reactions nor universally applicable to all types of reactions.