Define exothermic and endothermic reactions. Give two examples in each case.

A chemical reaction in which heat is given out is called an exothermic reaction. It causes a rise in temperature.

Examples:

1. Combustion of methane gives out large amount of energy.
   CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

2. When carbon burns in oxygen to form carbon dioxide, lot of heat is produced.
   C + O₂ ⟶ CO₂ ↑ + Heat

A chemical reaction in which heat is absorbed is called an endothermic reaction. It causes a fall in temperature.

Example :

1. When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.
   C + 2S $\xrightarrow \Delta$ CS₂

2. Calcium carbonate decomposes to carbon dioxide and calcium oxide when heated to 1000°C
   CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)