Exothermic reaction

Exothermic reaction

An exothermic reaction is a chemical reaction that releases heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:

reactants → products + energy


An exothermic reaction is a chemical reaction accompanied by the release of heat. In other words, the energy needed for the reaction to occur is less than the total energy released. As a result of this, the extra energy is released, usually in the form of heat.

When using a calorimeter, the change in heat of the calorimeter is equal to the opposite of the change in heat of the system. This means that when the medium in which the reaction is taking place gains heat, the reaction is exothermic.

The absolute amount of energy in a chemical system is extremely difficult to measure or calculate. The enthalpy change, ΔH, of a chemical reaction is much easier to measure and calculate. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a combustion reaction. Measured and calculated ΔH values are related to bond energies by:

ΔH = energy used in bond breaking reactions − energy released in bond making products

by definition the enthalpy change has a negative value:

ΔH < 0

For an exothermic reaction, this gives a negative value for ΔH, since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction). For example, when hydrogen burns:

2H2 + O2 → 2H2O
ΔH = −483.6 kJ/mol of O2

Examples of exothermic reactions

Key points

  • The concept and its opposite number endothermic relate to the enthalpy change in any process not just chemical reaction.
  • In endergonic reactions and exergonic reactions it is the sign of the Gibbs free energy that count and not enthalpy. the related concepts endergonic and exergonic apply to all physical processes.
  • the conceptually related endotherm and exotherm are concepts in Animal physiology.
  • In quantum numbers, when any exited energy level goes down to its original level for example: when n=4 fall to n=2, energy is released so, it is exothermic.


Heat production or absorption in either a physical process or chemical reaction are measured using calorimetry. One common laboratory instrument is the Differential scanning calorimeter or DSC, where a small sample is slowly heated in a controlled way and the heat flow into or from the sample chamber is monitored. The technique can be used to follow chemical reactions as well as physical processes, such as melting and boiling (both of which are endothermic). Crystallization and condensation, the reverse processes, are both exothermic.

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