(a) What do you understand by exothermic and endothermic reactions?
(b) Give one example of an exothermic reaction and one of an endothermic reaction.
(c) Which of the following are endothermic reactions and which are exothermic reactions?
(i) Burning of natural gas
(ii) Photosynthesis
(iii) Electrolysis of water
(iv) Respiration
(v) Decomposition of calcium carbonate

(a) Exothermic reaction is the chemical reaction that produces heat along with products.
Example: $\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\underset{}{\to }\mathrm{Ca}(\mathrm{OH}{)}_2+\mathrm{Heat}$

Endothermic reaction is the chemical reaction that consumes heat to form products.
Example: ${\mathrm{N}}_2+{\mathrm{O}}_2+\mathrm{Heat}\underset{}{\to }2\mathrm{NO}$

(b) Example of exothermic reaction: Zinc granules on reacting with dilute sulphuric acid produce zinc sulphate along with heat energy.
$\mathrm{Zn}+{\mathrm{H}}_2{\mathrm{SO}}_4\underset{}{\to }{\mathrm{ZnSO}}_4+{\mathrm{H}}_2$

Example of endothermic reaction: Barium hydroxide on reacting with ammonium chloride produces ammonia, water and barium chloride. It also results in decrease in temperature, as it consumes heat energy.
$\mathrm{Ba}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_4\mathrm{Cl}\underset{}{\to }{\mathrm{BaCl}}_2+2{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}$

(c)
(i) Burning of natural gas: Exothermic reaction
Burning of natural gas releases a huge amount of heat energy.

(ii) Photosynthesis: Endothermic reaction
Photosynthesis occurs by absorbing light energy from sunlight.

(iii) Electrolysis of water: Endothermic reaction
Electrolysis of water needs energy to break water into hydrogen and oxygen.

(iv) Respiration: Exothermic reaction
Respiration is an exothermic reaction in which glucose undergoes combustion to produce carbon dioxide, water and energy.

(v) Decomposition of calcium carbonate: Endothermic reaction
Decomposition of calcium carbonate is an endothermic reaction because it needs energy to decompose into carbon dioxide and calcium oxide.