What are exothermic and endothermic reactions ?

Explanation

Exothermic reactions : Those chemical reactions which occur by the evolution of hert are called exothemic reactions, For example, when one mole of solid carbon burns in excess of oxygen at constant temperature and at constant vlome, 393.5 kj heat is given out.
`C(s) + O_(2) (g) to CO_(2) (g),Delta H = - 393.5 kj`
In exothermic reactions, the heat liberated is given by negative sign.
Endothermic reactions : Those chemical reactions which occur by absorption of heart are know as endothermic reactions. For example, when one mole of nitrogen reacts with one mole of `O_(2)` at constant volume 180 kj heat is absorbed.
`therefore` Reaction is represented as `N_(2) (g) + O_(2)(g) to 2NO(g),Delta H = + 180 kj`
In eddothermic reaction, heat absorted by the reacting substance is given by porisitive sign.