Ncert Exercises & Solutions

What do you understand by exothermic reaction and endothermic reaction? Give one example of each type.

Exothermic reactions Reactions which are accompanied by evolution of heat are called exothermic reactions. The quantity of heat produced is shown either along with the products with a '+' sign or in terms of

∆ H

with a '-' sign. e.g.,

C (s) + O_{2} (g) \to {CO}_{2} (g) + 393.5 kJ

H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (l); ∆ H = - 285.8 kJ {mol}^{- 1}

Endothermic reactions Reactions which proceed with absorption of heat are called endothermic reactions. The quantity of heat absorbed is shown eighter along with the products with a '-' sign or in terms of

∆ H

with a '+' sign e.g.,

C (s) + H_{2} O (g) \to CO (g) + H_{2} (g) - 131.4 kJ

N_{2} (g) + 3 H_{2} (g) \to 2 {NH}_{3} (g); ∆ H = + 92.4 kJ {mol}^{- 1}

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