Endothermic reaction

An endothermic reaction is a chemical or physical process that absorbs heat energy from its surroundings. Because energy enters the system, the temperature of the surrounding environment typically drops while the reaction proceeds. The term derives from Greek: endo (within) and therme (heat).

Endothermic reactions are the opposite of exothermic reactions, which release heat and raise temperature. Both types belong to the field of thermochemistry, which studies the relationship between changes in matter and the flow of energy.

Why energy is absorbed

In any reaction, chemical bonds between atoms are broken and new bonds are formed. Breaking bonds requires energy; forming new bonds releases energy. When the energy required to break bonds exceeds the energy released from forming new bonds, the difference is drawn from the surroundings—and that constitutes an endothermic reaction.

Thermodynamic measurement

Endothermic reactions show a positive enthalpy change (ΔH > 0) at constant pressure. This means the system has absorbed energy from outside. Whether a reaction proceeds on its own (spontaneously) depends not only on endothermic absorption but also on changes in disorder (entropy) and temperature, as described by Gibbs free energy. Therefore many endothermic reactions do proceed spontaneously if disorder increases sufficiently.

Examples

Common examples include dissolving certain salts such as ammonium nitrate in water, which makes the container cold; ice melting and water evaporating; and photosynthesis, in which plants use light energy to produce sugar. The decomposition of calcium carbonate when heated is also an endothermic process.

Applications

Endothermic reactions are used in cooling applications. For instance, instant cold packs used in medicine work by mixing a salt and water, lowering temperature to reduce pain from injury. In industry, many endothermic reactions require continuous heat input, which affects the energy cost of the process.