reaktioenergian

Reaktioenergia, or reaction energy, describes the energy change that accompanies a chemical reaction. In thermodynamics it is commonly taken as the change in enthalpy (ΔH) at constant pressure or, when volume changes are important, the change in internal energy (ΔU). The sign of the energy change indicates whether energy is released to the surroundings (exothermic, ΔH < 0) or absorbed from the surroundings (endothermic, ΔH > 0).

Under standard conditions, the standard enthalpy of reaction, ΔH°rxn, can be estimated from standard enthalpies of formation using Hess’s law. The concept is often measured experimentally by calorimetry: constant-pressure calorimetry yields ΔH, while bomb calorimetry yields ΔU. Reaction energy is a thermodynamic quantity that tells about energy storage or release, but it does not determine how fast a reaction occurs.

Kinetics are governed by activation energy (Ea), the energy barrier that must be overcome for reactants to transform into products. The overall spontaneity of a reaction at a given temperature is described by Gibbs free energy change, ΔG = ΔH − TΔS; a negative ΔG indicates a thermodynamically favorable process, which may still proceed slowly if Ea is large.

Example: the combustion of methane, CH4 + 2 O2 → CO2 + 2 H2O(l), has a large negative ΔH°rxn (about −890 kJ/mol), reflecting substantial energy release. In Finnish usage, reaktioenergia refers to this general concept of energy change in chemical reactions, with the genitive form reaktioenergian used in phrases describing the energy change of a reaction.