An exothermic reaction is a chemical reaction that releases energy by light
or heat. It is the opposite of an endothermic reaction.. Expressed in a
chemical equation: reactants → products + energy
An exothermic reaction occurs when the energy used to break the bonds in the reactants (the starting
stuff) is less than the energy released when new bonds are made in the products (the stuff you end up
with). This extra energy is given off as heat and there is a temperature rise around the surroundings of the
Combustion is an example of an exothermic reaction- you can feel the heat given off if
you get too close!
In reversible reactions, the reaction in one direction will be
exothermic and the reaction in the other direction will be
Some examples of exothermic reactions are burning neutralisation, reactions between
acids and alkalis and the reaction between water and calcium oxide.
Everything that lets out heat, even from striking a match, and burning fuels are
exothermic reactions, no matter how simple it may be.
Obviously, lots of chemical reactions give out energy as heat. Getting heat by
burning a fuel is a simple example, but you will probably have come across
lots of others in the lab.
Other reactions need a continuous supply of heat to make them work.
Splitting calcium carbonate into calcium oxide and carbon dioxide is a
simple example of this.
Any chemical reaction will involve breaking some bonds and making new ones. Energy is needed to
break bonds, and is given out when the new bonds are formed. It is very unlikely that these two
processes will involve exactly the same amount of energy - and so some energy will either be
absorbed or released during a reaction.
Because exothermic reactions give out heat, it may result in
a change of temperature in the surrounding areas,.
As exothermic reactions release heat energy, the product of the
reaction is the heat, with whatever the heat is caused by being
It's important to know how much energy fuels release on
combustion i.e. their calorific value. Its important to know the energy
released on burning petrol. diesel, coal or any other fossil fuel and
alternative fuels like hydrogen or biofuels (biomass fuels).
The same sort of data is important in knowing how much energy is
released on metabolising foods such as fats and carbohydrates.