Explain:
(i) Enthalpy (heat) of the formation.

(ii) Standard enthalpy of formation. How is it helpful in calculating the enthalpy of a reaction?

Enthalpy of formation: It is the enthalpy (heat) change accompanying the formation of one mole of a compound from its elements. It is denoted by ∆H_f e.g.


Hence enthalpy of formation of water i.e. ∆H_f (H₂O) = –285.9 kJ
Standard enthalpy of formation: When all the species of the chemical reaction are in their standard states, the enthalpy of formation is called standard enthalpy of formation. It is denoted by ∆_fH°. Standard enthalpy of formation of a compound is defined as the enthalpy change accompanying the formation of one mole of a compound from its constituent elements, all the substances being in their standard states (1 bar pressure and 298 K). For example

It should be noted that standard enthalpies of elementary substances are taken to be zero.
Calculation of enthalpy of reaction (∆H): By knowing the standard enthalpies of formation of various substances, we can calculate the standard enthalpy change of any reaction.
Standard enthalpy change of reaction = [Standard enthalpies of formation of all products]
                                                             - [Standard enthalpies of formation of all reactants]
   i.e.