Equilibria are classified by the phases (states) of the species taking part. The state symbols in the equation tell you which type an equilibrium is.A homogeneous equilibrium is one in which all the reactants and products are in the same phase. Examples:• all gases: H2(g) + I2(g) ⇌ 2HI(g)
• all in aqueous solution: Fe3+(aq) + SCN−(aq) ⇌ [FeSCN]2+(aq)
• all in one liquid mixture: CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l)A heterogeneous equilibrium is one in which the species are in more than one phase. Examples:• the thermal decomposition of calcium carbonate in a closed container: CaCO3(s) ⇌ CaO(s) + CO2(g)
• steam passed over hot carbon: C(s) + H2O(g) ⇌ CO(g) + H2(g)Note that (aq) and (l) count as different phases from (s) and (g). A single solid and a gas, or a solution and a solid precipitate, make an equilibrium heterogeneous.Do not confuse a heterogeneous equilibrium with a heterogeneous catalyst. In the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g), the solid iron catalyst is in a different phase, but it does not appear in the equation. All the species in the equation are gases, so the equilibrium is homogeneous.The type matters because it decides which species appear in the equilibrium constant expression, as the next lessons show.
• all in aqueous solution: Fe3+(aq) + SCN−(aq) ⇌ [FeSCN]2+(aq)
• all in one liquid mixture: CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l)A heterogeneous equilibrium is one in which the species are in more than one phase. Examples:• the thermal decomposition of calcium carbonate in a closed container: CaCO3(s) ⇌ CaO(s) + CO2(g)
• steam passed over hot carbon: C(s) + H2O(g) ⇌ CO(g) + H2(g)Note that (aq) and (l) count as different phases from (s) and (g). A single solid and a gas, or a solution and a solid precipitate, make an equilibrium heterogeneous.Do not confuse a heterogeneous equilibrium with a heterogeneous catalyst. In the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g), the solid iron catalyst is in a different phase, but it does not appear in the equation. All the species in the equation are gases, so the equilibrium is homogeneous.The type matters because it decides which species appear in the equilibrium constant expression, as the next lessons show.