g)where ∆Hd = ∆He + ∆Hf + ∆Hg.Standard Enthalpy of Formation
Hess’s law is useful because measuring the change in enthalpy for a process directly can be
challenging experimentally, but by taking advantage of the fact that this quantity is independent of
path, one can calculate ∆H for any process if the values for certain other reactions are known. The
most common approach is to express the enthalpy change of a reaction in terms of the standard
enthalpies of formation of the products and the reactants.
The standard enthalpy of formation of a compound, ∆H°f , is the enthalpy change that would occur if
one mole of a compound were formed directly from its elements in their standard states. For
example, the standard enthalpy of formation of H 2 O is just the enthalpy change for the reaction:
H 2 (g) + 1/2 O 2 (g) → H 2 O (l)if the reaction were carried out under standard conditions. We have picked the gaseous forms of
hydrogen and oxygen because that is the most stable form in which they exist under such
conditions.
Note that ∆H°f of an element in its standard state is zero. The ∆H°f’s of most known substances are
tabulated. The enthalpy of formation is also often referred to as the heat of formation.
Standard Enthalpy of Reaction
The standard enthalpy (or heat) of a reaction, ∆H°rxn, is the hypothetical enthalpy change that would
occur if the reaction were carried out under standard conditions; that is, when reactants in their
standard states are converted to products in their standard states at 298 K. It can be expressed as: