Which reaction releases energy as heat?

The reaction that releases energy as heat is an exothermic reaction. In these reactions, the system gives off energy to its surroundings, typically in the form of heat, leading to an increase in temperature in the surrounding environment. This occurs because during an exothermic reaction, the total energy of the products is lower than the energy of the reactants. The difference in energy is released, often making exothermic reactions feel warm or hot.

In contrast, endothermic reactions absorb energy from the surroundings, causing a cooling effect. Combustive reactions can be exothermic, but not all are; for instance, some combustion reactions can be under controlled conditions. The term enthalpic refers to a broader thermodynamic context, relating to heat content but does not specifically indicate the direction of heat flow. Therefore, exothermic is the definitive term for reactions that release heat energy.