Everyone knows that constituent elements in a compound are bound by chemical bonds.
During a chemical change, when bonds between the constituents of the reactants are broken, energy is consumed; whereas energy is given out when the bonds are formed as in the case of products.
Thus, depending on whether overall energy is given out or consumed, the reaction is either exothermic or endothermic.
Exothermic reactions
(A) Chemical reactions in which energy is evolved (or given out) are known as Exothermic Reactions.
For Example —
$$\mathrm{C}(\mathrm{s})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{CO}_2(\mathrm{~g})+393.7 \mathrm{~kJ}$$
$(\Delta \mathrm{H}=-393.7 \mathrm{~kJ})$
This shows 1 mole of carbon reacts with 1 mole of oxygen and liberates $393.7 \mathrm{~kJ}$ of heat.
(B) Formation of ammonia from nitrogen and hydrogen is an Exothermic Reaction
$$\mathrm{N}_2(\mathrm{~g})+3 \mathrm{H}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{NH}_3(\mathrm{~g})+92 \mathrm{~kJ}(\Delta \mathrm{H}=-92 \mathrm{~kJ})$$
Endothermic Reactions :
(A) Reactions in which energy is absorbed are called Endothermic Reactions.
For Example —
When carbon reacts with sulphur, $92 \mathrm{~kJ}$ of heat is absorbed during the process.
$$\mathrm{C}(\mathrm{s})+2 \mathrm{~S}(\mathrm{~s}) \longrightarrow \mathrm{CS}_2(J)-92 \mathrm{~kJ} \quad(\Delta \mathrm{H}=+92 \mathrm{~kJ})$$
(B) Formation of nitric oxide (NO) is an endothermic reaction
$$\begin{array}{l}\mathrm{N}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{NO}(\mathrm{g})-180.5 \mathrm{~kJ} \\(\Delta \mathrm{H}=+180.5 \mathrm{~kJ})\end{array}$$
