When a reversible reaction is carried out in a closed vessel, a stage is reached when the forward reaction and backward reaction proceed with the same speed, this stage is known as Chemical Equilibrium.
The reaction in which heat is evolved is called exothermic reaction.
N2 + 3H2 ⇌ 2NH3 + 23 Kcal (Exothermic)
The reaction in which heat is absorbed is called endothermic reaction.
N2 + O2 ⇌ 2NO – 43.2 Kcal (Endothermic)