Homogeneous equilibria and Kp

In a homogeneous equilibrium, all the reactants and products are in the same phase: all gases, or all dissolved in one solution. N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is an example.

For gases, it is often easier to use partial pressures than concentrations. For aA + bB ⇌ cC + dD:

Kp = (p_C)ᶜ(p_D)ᵈ / (p_A)ᵃ(p_B)ᵇ, with the pressures in bar.

Since for an ideal gas p = cRT (c in mol L⁻¹), Kp and Kc are linked:

Kp = Kc (RT)^∆n, where ∆n = moles of gaseous products – moles of gaseous reactants, and R = 0.0831 bar L mol⁻¹ K⁻¹.

For N₂ + 3H₂ ⇌ 2NH₃: ∆n = 2 – 4 = –2, so Kp = Kc(RT)⁻². If ∆n = 0, as in H₂ + I₂ ⇌ 2HI (2 moles of gas on each side), then (RT)⁰ = 1 and Kp = Kc.

To find the partial pressure of a gas in a mixture, multiply its mole fraction (or its volume per cent ÷ 100) by the total pressure. In a mixture at 2 bar that is 25% nitrogen by volume, the partial pressure of nitrogen is 0.25 × 2 = 0.5 bar.