Heterogeneous Equilibria

Homogeneous reactions have all reactants and products existing in the same phase.

• A homogeneous reaction at equilibrium is termed a homeogeneous equilibrium

Heterogeneous reactions will have at least one reactant or product in a different phase from the other components

• A heterogeneous reaction at equilibrium is termed a heterogeneous equilibrium

Calcium carbonate, CaCO₃ (a solid) can decompose to produce calcium oxide, CaO (another solid) and carbon dioxide, CO₂ (a gas)

• The system involves a gas and two solids in equilibrium:

• The concentration of any pure liquid or solid is equal to its density (mass per unit volume) divided by its molar mass (mass per mole)

• The density of a liquid or solid changes little with temperature, and is essentially a constant.
• The molar mass is a characteristic of the compound in question (i.e. is a constant).
• Therefore, the value of (density/molar mass), i.e. the concentration of a solid or liquid, is a constant, regardless of the amount of the solid or liquid present.

With this information, the equilibrium constant for the decomposition of calcium carbonate reduces to:

• The concentrations of gases (and solutes in solution) are included in the equilibrium expression because their concentrations can change

What does the heterogeneous equilibrium expression for the decomposition of calcium carbonate (given above) tell us?

• At a given temperature, an equilibrium mixture of calcium oxide, carbon dioxide and calcium carbonate will always result in the same concentration of CO₂(g). Since CO₂ is a gas, this means that the pressure of CO₂ at equilibrium will be the same value (at a given temperature)
• Since CaO and CaCO₃ are solids, the pressure of CO₂ at equilibrium is independent of the amounts of these compounds (but if one or both are missing, we won't have equilibrium, and the pressure of CO₂ won't be the expected equilibrium value)