In chemistry, an exothermic reaction is one that releases heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:

$reactants\; \backslash rightarrow\; \backslash ;\; product\; +\; energy$

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 solution in which the reaction is taking place gains heat, the reaction is exothermic.

In an exothermic reaction, the total energy absorbed in bond breaking is less than the total energy released in bond making.

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:

$\backslash Delta\; \backslash ;\backslash !\backslash !H\; =\; energy\backslash ;used\backslash ;in\backslash ;bond\backslash ;breaking\backslash ;reactions\; -\; energy\backslash ;released\backslash ;in\backslash ;bond\backslash ;making\backslash ;products$.

For an exothermic reaction, this gives a negative value for ΔH as a larger value is subtracted from a smaller value. For example, when hydrogen burns:

$2H\_2\; +\; O\_2\; \backslash rightarrow\; \backslash ;\; 2H\_2O$

$\backslash Delta\backslash ;\backslash !\backslash !H\; =\; -483.6\backslash \; kJ/mol\backslash \; of\backslash \; O\_2$

Examples of exothermic reactions

• Combustion
• Neutralization
• Adding water to concentrated acid
• Adding water to anhydrous copper(II) sulfate
• Thermite

See also

• Endergonic
• Exergonic
• Endergonic reaction
• Exergonic reaction
• Exothermic
• Endothermic
• Endothermic reaction
• Endotherm
• Exotherm

External links

• GCSE Science: Reactions

Thermodynamics | Chemical reactions

تفاعل طارد للحرارة | Exoterm | Exotherme Reaktion | Eksotermiline reaktsioon | Réaction exothermique | תגובה אקסותרמית | Exoterm reakció | Егзотермна реакција | Exotherme reactie | Eksoterm reaksjon | Экзотермическая реакция | Exotermická reakcia | Eksoterminen reaktio | Exoterm | 放热反应