Chemical Equilibrium

How the model works

The model reaction is A ⇌ B, with first-order forward and reverse steps:

rate_f = k_f [A], rate_r = k_r [B]
at equilibrium: rate_f = rate_r ⇒ K = [B]/[A] = k_f / k_r

Equilibrium is dynamic: both reactions keep going, at equal rates. The reaction quotient Q = [B]/[A] tells you which way the system will move. If Q < K, the forward reaction wins; if Q > K, the reverse wins.

Temperature changes K through the van 't Hoff relation. For an exothermic forward reaction (ΔH < 0), heating lowers K, so the mixture shifts back towards A. Concentration changes don't alter K; they change Q, and the system responds until Q = K again. This is Le Chatelier's principle in quantitative form.

Idealised single-step reaction in a closed, well-mixed solution with constant volume. K at 298 K and ΔH are adjustable model parameters, not data for a specific reaction.

Guided experiments

1. Reach equilibrium from either side. Start with pure A, then with pure B. Prediction: both runs end at the same [B]/[A] = K. Equilibrium doesn't depend on the starting side. Start from pure B
2. Add more A. At equilibrium, add 0.5 mol/L of A. Prediction: Q drops below K, the forward rate jumps, and some of the added A turns into B. The final ratio returns to K. Equilibrate, then add A
3. Heat an exothermic reaction. ΔH = −40 kJ/mol, raise T from 298 K to 350 K. Prediction: K falls by e^(ΔH/R × (1/350 − 1/298)) ≈ 0.09×, so the mixture shifts strongly back to A. Heat it

Challenge

K = 4 at 298 K. You start with [A] = 1.0 mol/L and no B. What are the equilibrium concentrations?

References

• OpenStax, Chemistry 2e, §13.1–13.3 “Chemical Equilibria”, “Equilibrium Constants” and “Shifting Equilibria: Le Châtelier's Principle”.