Endothermic and Exothermic Reactions
The chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. Heat evolved is expressed in Joules (J) or kilo Joules (kJ).
All combustion reactions are exothermic. These reactions proceed with the evolution of heat energy. An important exothermic reaction occurs in our body cells which is nothing but respiration. During respiration, glucose in food burns in oxygen and gives out heat energy. Another example is formation of ammonia. When nitrogen combines with hydrogen in the presence of iron catalyst to form ammonia, a lot of heat is produced. Thus, the formation of ammonia is an exothermic reaction
N2(g) + 3H2(g) →2NH3(g) + Heat
The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions.
Why heat is evolved or absorbed in chemical reactions?
Chemical reaction involves the rearrangement of atoms. During the reaction, certain bonds are broken while certain new bonds are formed between the atoms. If the energy required to break the bonds is more than the energy released during the formation of bonds, then there is net absorption of energy and the reaction is endothermic. Both exothermic and endothermic reactions are due to the internal (intrinsic) energy that is stored within the structural units of every substance. When nitrogen and oxygen are heated to a very high temperature, they combine to form nitrogen monoxide and a lot of heat is absorbed in this reaction. Thus, the formation of nitrogen monoxide is an endothermic reaction.
N2(g) + O2(g) + Heat ←→2NO(g)
What is impact of Temperature on Exothermic and Endothermic Reactions?
Increase in temperature favours endothermic reactions and decrease in temperature favours the exothermic reactions.
