Chemical Equilibrium, Law Of Mass-action : Heterogeneous Reactions

The law of mass-action applies only to homogeneous systems (gases; solutions), since it is an essential part of the theory of Berthollet that when solids, for example, are present their active masses are constant and they can intervene in reversible reactions only by their vapours or in solution. The vapour pressures and solubilities are constant when solid is present and in the expression for the law of mass-action for the gaseous (or homogeneous solution) phase, the concentrations of these substances are constant at a given temperature, and may be included in K, the equilibrium constant.

As an example, we may consider the action of steam on red-hot iron: 3Fe + 4H₂O <=> Fe₃O₄ + 4H₂.

In the gaseous phase, to which alone the law of mass-action applies, we may consider that in addition to H₂O and H₂ at measurable and variable pressures, there are present also the vapours of the iron and oxide of iron at immeasurably small and constant pressures. In

equlibrium constant for iron

since [Fe₃O₄] and [Fe] are constant, we can write:

K’ = [Fe₃O₄]/[Fe]³ x [H₂]⁴/[H₂O]⁴ = k[H₂]⁴/[H₂O]⁴

or K = [H₂]/[H₂O],

where

This shows that the ratio of the concentrations (or partial pressures) or hydrogen and steam will be constant, at a given temperature, independent of the amounts of solid iron or iron oxide present.

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