Chemical Equilibrium

What Is Equilibrium

In a reversible reaction, equilibrium is established when the rates of the forward and reverse reactions become equal. It is dynamic: the reaction does not stop.

Equilibrium constant: for aA + bB ⇌ cC + dD, K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ).

• K: products / reactants.
• The concentrations of solids are not included.
• 2SO₂ + O₂ ⇌ 2SO₃: K = [SO₃]² / ([SO₂]²·[O₂]).

Le Chatelier's Principle

If a system at equilibrium is disturbed, the equilibrium shifts in the direction that reduces the effect of the disturbance.

Change	Shift of equilibrium
C(reactant) ↑	toward products →
P ↑	toward fewer molecules
T ↑	toward the endothermic direction
Catalyst	no shift

• If the concentration is increased: toward the direction in which it is consumed.
• If the pressure increases: toward fewer gas molecules.
• If the temperature is increased: toward the endothermic (−Q) reaction.

Important Examples

• A catalyst does not shift the equilibrium. It only speeds up how quickly equilibrium is reached.
• N₂ + 3H₂ ⇌ 2NH₃ + Q: to increase the yield of ammonia, raise the pressure, lower the temperature and remove NH₃ from the mixture.
• H₂ + I₂ ⇌ 2HI: pressure has no effect. The numbers of molecules are equal.
• When heated, an exothermic reaction shifts to the left.