Heterogeneous equilibria

A heterogeneous equilibrium involves more than one phase. Examples: water in equilibrium with its vapour, H₂O(l) ⇌ H₂O(g); and the decomposition of limestone, CaCO₃(s) ⇌ CaO(s) + CO₂(g).

The 'concentration' of a pure solid or a pure liquid is fixed by its density, and does not change however much of it is present. So pure solids and liquids are left out of the equilibrium expression (their constant values are absorbed into K).

For CaCO₃(s) ⇌ CaO(s) + CO₂(g), only the gas remains: Kp = p(CO₂). At 1100 K this equals 2.00 bar: at that temperature, the CO₂ pressure over heated limestone always settles at 2.00 bar, whatever the amount of solid.

Another example: nickel reacts with carbon monoxide, Ni(s) + 4CO(g) ⇌ Ni(CO)₄(g). The solid nickel is left out: Kc = [Ni(CO)₄] / [CO]⁴.

But note: although the solid does not appear in K, it must still be present, however little, for the equilibrium to exist. Without any limestone, CO₂ cannot be produced.

Kp can be made dimensionless by dividing each pressure by the standard pressure, 1 bar.