Question

All chemical changes are accompanied by chemical reactions. These reactions are described in sentence form with the help of chemical equations in terms of symbols and formulae of substances involved. Initially, the equation is in a skeletal form which is then balanced so that the total number of reactant species is the same as that of product species.

Why are some chemical reactions exothermic while some others are endothermic in nature?

Answer 1

Exothermic reaction:

1. A reaction in which heat is given away when reactants are transformed into the product is called an exothermic reaction.
2. The energy of the bonds present in the product is lower than that of the energy of the bonds present in the reactant.

Example:

Firing a Firecracker, Combustion of fuel, Respiration, etc.

Endothermic reaction:

1. A reaction in which heat is either absorbed from the surroundings or has to be supplied continuously from outside is called an endothermic reaction.
2. The energy of the bonds present in the product is greater than that of the energy of the bonds present in the reactant.

Example:

Evaporating liquid water., melting of an ice cube, etc.

Hence. endothermic and exothermic reactions are based on the energy absorbed or released during a chemical reaction.