Consider the following reaction –1 (aq) (aq) (l) –1 2(g) 2(g) (l) 2(g) 2(g) (g) (l) –1 2(g) 2(g) 2(g) (l) –1 (i)H OH H O , H X kJ mol (ii)H 1/ 2O H O , H X kJ mol (iii)CO H CO H O , H X kJ mol (iv)C H 5 / 2O 2CO H O , H X kJmol Enthalpy of formation of H₂O(l) is (2007)

The amount of heat absorbed or released when 1 mole of a substance is directly obtained from its constituent elements is called the heat of formation or enthalpy of formation. Equation (i) represents neutralization reaction, (iii) represents hydrogenation reaction and (iv) represents combustion reaction. Thus, enthalpy of formation of H₂O(l) is –X2 kJ mol–1