7.1.7—The Kc and Kp expressions to carry out

To calculate Kc or Kp, first write the correct expression, then substitute equilibrium concentrations or partial pressures with the required powers and units.

Keep brackets grouped, evaluate powers before division, and check whether omitted phases are pure solids or liquids. A large or small value describes composition, not reaction speed.

If Kc=[C]²/([A][B]) and [A]=0.20, [B]=0.10, [C]=0.30 mol dm⁻³, then Kc=0.09/(0.02)=4.5.

Do not use concentrations at the start of the reaction unless they are explicitly equilibrium values. K is defined at equilibrium.