Chemical Equilibria: Kc and Kp

Dynamic equilibrium

A reversible reaction in a closed system reaches dynamic equilibrium: the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products stay constant (though both reactions continue).

Le Chatelier's principle

If a change is made to a system at equilibrium, the position of equilibrium shifts to oppose the change:

• Increase concentration of a reactant → shifts to the products.
• Increase pressure → shifts to the side with fewer gas molecules.
• Increase temperature → shifts in the endothermic direction.
• A catalyst speeds up both directions equally → no shift (equilibrium reached faster).

The equilibrium constant Kc

For aA + bB ⇌ cC + dD:

Kc = [C]^c [D]^d ÷ ([A]^a [B]^b)

using equilibrium concentrations (mol dm⁻³).

• Kc > 1 → products favoured; Kc < 1 → reactants favoured.
• Only temperature changes Kc. Concentration, pressure and catalysts do not change Kc (the position shifts, but K is constant).

Kp — for gases

For gaseous equilibria, use partial pressures instead of concentrations:

Kp = (p_C^c × p_D^d) ÷ (p_A^a × p_B^b)

• Partial pressure of a gas = mole fraction × total pressure.
• Mole fraction = moles of that gas ÷ total moles of gas.

Effect of temperature on K

• For an exothermic forward reaction, increasing temperature decreases K (equilibrium shifts back to reactants).
• For an endothermic forward reaction, increasing temperature increases K.

Industrial compromise (e.g. Haber process)

Conditions are chosen to balance yield, rate and cost: e.g. the exothermic Haber process would give a higher yield at low temperature, but that's too slow — so a compromise temperature (~450 °C) with a catalyst and high pressure is used.

Worked example

For N₂ + 3H₂ ⇌ 2NH₃, what happens to the position of equilibrium if pressure is increased?

• The left has 4 gas moles, the right has 2. Increasing pressure shifts equilibrium to the side with fewer gas moles → towards ammonia (products), increasing yield. ✓

Common mistakes

• Saying a catalyst or pressure changes Kc/Kp — only temperature does.
• Forgetting Kc uses equilibrium concentrations (not initial ones).
• Getting the Kc expression upside down (products on top).

Exam tips

• Apply Le Chatelier by asking "which change is being opposed?"
• Write Kc/Kp correctly (products over reactants, raised to their coefficients) with units.
• Remember only temperature alters the value of K.

Key facts to remember

• Dynamic equilibrium: forward and reverse rates equal; Le Chatelier — the system opposes any change.
• Kc = [products]/[reactants] (equilibrium concentrations); Kp uses partial pressures; only temperature changes K.
• Increasing T shifts equilibrium in the endothermic direction; industrial conditions are a compromise of yield, rate and cost.