What is an exothermic reaction? Give some examples?

Hint: Enthalpy of a reaction is an important term here. It is defined as heat energy change that takes place when reactants convert to products. If heat is absorbed during a reaction, its enthalpy change will be positive but if heat is released during a reaction, its enthalpy change will be negative.

Complete step by step answer:
In a chemical reaction, chemical bonds are either formed or broken. When chemical bonds are formed, heat is released and when chemical bonds break, they require energy so heat is supplied or absorbed. Molecules tend to want to stay together, so the formation of chemical bonds between molecules need less energy as compared to the breaking of bonds between molecules, which need more energy and results in heat being absorbed from the surroundings.
The reactions which are exothermic in nature have positive enthalpy change and heat is released in the process due to bond breaking or bond making. Here some examples are given below.
In a combustion reaction, methane combines with an oxygen molecule and yields carbon dioxide with water. As the formation of bonds occurs, heat is released in the reaction and thus it is an exothermic reaction. The reaction is shown below,
\[C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O\]
We know that acids react with strong alkali metals like sodium and potassium that react with organic acids to produce strong basic salts along with the release of hydrogen gas. In the reaction given above, ethanoic acid reacts with sodium metal that results in the formation of sodium acetate or sodium ethanoate, and hydrogen gas is released. This reaction is highly exothermic in nature.
\[C{H_3}COOH + Na \to C{H_3}COONa + {H_2}\].

Note:Exothermic reactions give products which have less energy than reactants. When the energy of a compound is low, stability increases. Moreover, CuO decomposes to release oxygen when heated and serves as an oxidizer in reactive composites and chemical looping combustion.