Homogeneous Systems

In a homogeneous system all the species are in the same phase, so every reactant and product appears in the K_c expression.Example 1: the Haber process.N₂(g) + 3H₂(g) ⇌ 2NH₃(g)K_c = [NH₃]² / [N₂][H₂]³Units: (mol dm⁻³)² / (mol dm⁻³)⁴ = (mol dm⁻³)⁻² = dm⁶ mol⁻².Example 2: esterification. All four substances are liquids in one mixture, so water is included:CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)K_c = [CH₃COOC₂H₅][H₂O] / [CH₃COOH][C₂H₅OH]The units cancel, so K_c has no units.Worked example. For H₂(g) + I₂(g) ⇌ 2HI(g) at a certain temperature, the equilibrium concentrations are [H₂] = 0.10 mol dm⁻³, [I₂] = 0.10 mol dm⁻³ and [HI] = 0.70 mol dm⁻³.K_c = 0.70² / (0.10 × 0.10) = 0.49 / 0.010 = 49K_c has no units. Its value is well above 1, so the equilibrium lies to the right.Always use concentrations, not moles, unless the units cancel so that the volume makes no difference.