We have studied that the energy changes always occur during the chemical reactions. Depending upon the evolution or absorption of energy, the chemical reactions can be classified into two categories: exothermic and endothermic.
The chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. The heat energy produced during the reaction is indicated by writing +q or more precisely by giving the actual numerical value along with products. In general, exothermic reactions may be represented as:
A + B à C + D + q ( heat energy)
The heat evolved is expressed in the units of Joules (J) or kilo Joules (kJ).
Most of the reactions that we carry out in laboratory are processes carried out at constant temperature and constant pressure, the heat evolved is equal to enthalpy change (ΔH). In exothermic reactions, the enthalpy of products (ΣH p) becomes less than the enthalpy of reactants (ΣHR). Therefore, the enthalpy change for exothermic reaction is negative.
ΔH = ΣHP – ΣHR
As ΣH p < ΣHR
ΔH = -ve
For example, formation of one mole of water from hydrogen and oxygen gases is accompanied by the evolution of 286.0 kJ of heat. It can be expressed as:
H2(g) + O2(g) à H2O(l) + 286.0 kj
or H2 (g) + O2 (g) à H2O (l) ;ΔH = -286.0 KJ
The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions. The heat energy absorbed during the reaction can be indicated by writing +q (or the actual numerical value) with the reactants. It can be indicated by writing -q (or the actual numerical value) with the products. In general, an endothermic reaction can be represented as:
A + B + q (heat) à C + D
Or A + B à C + D – q (heat)
where q is the heat absorbed.
When, endothermic reaction is carried out at constant temperature and constant pressure, the heat absorbed increases the enthalpy of the products. In such a case the enthalpy change of the reaction is positive.
ΔH = ΣHP – ΣHP , As ΣHP > ΣHP
ΔH = + ve
For example, decomposition of 2 moles of mercuric oxide a constant temperature and pressure proceeds with the absorption of 180.4 kJ of heat. It can be represented as:
2HgO (l) à Hg (L) + O2 (g) + 180.4 KJ
2HgO(s) à 2Hg(l) +O2 (g) ; ΔH = – 180.4 KJ