Explain the term energy changes in a chemical change. Give an example with a balanced equation, for each of the following reactions:

(a) exothermic reaction

(b) endothermic reaction

(c) photochemical reaction

(d) electrochemical reaction.

The difference between the chemical energy of the reactants and the products is known as the energy changes in a chemical change or reaction.

Example of :

(a) exothermic reaction:
     2H₂ + O₂ $\xrightleftharpoons{\enspace\Delta\enspace}$ 2H₂O + ΔT

(b) endothermic reaction:
     N₂ + O₂ $\xrightleftharpoons{\enspace\Delta\enspace}$ 2NO - ΔT

(c) photochemical reaction:
     H₂ + Cl₂ $\xrightarrow{\text{sunlight}}$ 2HCl
     Slow reaction in diffused light
     Explosive reaction in direct sunlight

(d) electrochemical reaction:
     $\underset{\text{[acidified]}}{2\text{H}$2\text{O}} \xrightarrow[\text{Current}]{\text{Electric}} \underset{\text{[cathode]}}{2\text{H}2} + \underset{\text{[anode]}}{\text{O}_2}