Homogeneous and Heterogeneous Equilibria

In-depth explanation of Homogeneous and Heterogeneous Equilibria

A homogeneous equilibrium is one in which all reacting species are in the same phase, such as all gases or all species dissolved in one solution. A heterogeneous equilibrium involves species in more than one phase. In the equilibrium expression, pure solids and pure liquids have constant activity equal to one, so they are omitted from the expression for K. Only gases and dissolved species, whose concentrations or pressures vary, appear in it. The omission is valid as long as the solid or liquid is present in excess and its composition does not change appreciably. Careful handling of phases is essential when writing K for heterogeneous systems.

Going deeper: Homogeneous and Heterogeneous Equilibria

Key ideas: Exclude pure solids and pure liquids, including water as a solvent in dilute solutions. Include gases and aqueous solutes. For heterogeneous equilibria K depends on the gas partial pressures or solute concentrations only, so the amount of solid does not matter as long as some is present. Decomposition of CaCO3 gives a fixed CO2 pressure at each temperature.

Common mistakes and connections: Students include solids in K, or insist that more solid shifts the equilibrium. Adding more solid does nothing as long as some remains. Another error is forgetting to note that if no solid remains, the equilibrium simply does not exist and the expression is not valid. Related to solubility product, vapour pressure and the effect of pressure on gas equilibria.

Visualization of Homogeneous and Heterogeneous Equilibria

For CaCO3(s) to CaO(s) + CO2(g), K equals P(CO2) alone because solids are omitted; it rises with temperature to 1 atm near 898 C (values approximate). The interactive lesson shows this as a live animation or chart.

Worked example and use case

For CaCO3(s) <-> CaO(s) + CO2(g) the equilibrium constant is simply K = P(CO2), since both solids are omitted.

Where Homogeneous and Heterogeneous Equilibria fits in Kinetics & Equilibrium

Homogeneous and Heterogeneous Equilibria is topic 49 of 91 in Kinetics & Equilibrium.

Related topics

• Kc and KpIntermediate
• Reaction QuotientIntermediate
• Magnitude of KIntermediate
• Manipulating KIntermediate
• ICE TablesIntermediate
• Le Chatelier's PrincipleBeginner

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