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# Exothermic reaction

The total energy in bond breaking is less than the total energy released in bond making.

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

$reactants \rightarrow \; product + energy$

## Overview

In an exothermic reaction, the total energy absorbed in bond breaking is less than the total energy released in bond making. In other words, the energy needed for the reaction to occur is less than the total energy provided. 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 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:

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

by definition the enthalpy change has a negative value:

$\Delta\;\!\!H < 0$

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:

$2H_2 + O_2 \rightarrow \; 2H_2O$
$\Delta\;\!\!H = -483.6\ kJ/mol\ of\ O_2$

## Examples of exothermic reactions

• Combustion reactions
• Neutralization reactions for instance direct reaction of acid and base
• Adding concentrated acid to water
• Adding water to anhydrous copper(II) sulfate
• The Thermite reaction
• Reactions taking place in a self-heating can based on lime and aluminum.

## Key points

• The concepts exothermic 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.