Thermodynamics and Equilibrium
What you must know
Distinguish exothermic and endothermic processes, and understand enthalpy, entropy and Gibbs free energy: ΔG = ΔH − TΔS. Know that ΔG < 0 indicates spontaneity. Apply Le Chatelier's principle to predict shifts with changes in concentration, pressure, temperature and catalysts. Write equilibrium constants Kc and Kp, and understand that K depends only on temperature. Recognise the relationship between ΔG° and K.
Traps
- Assuming an exothermic reaction is always spontaneous (entropy matters).
- Forgetting that a catalyst does not change K or the equilibrium position.
- Mishandling pressure changes: only gaseous moles matter.
- Confusing K with the reaction quotient Q; compare Q with K to predict direction.
Worked example
For N₂ + 3H₂ ⇌ 2NH₃ (exothermic), increasing pressure shifts equilibrium to the right (fewer gas moles, 4 → 2). Increasing temperature shifts it left, lowering yield. Adding a catalyst speeds attainment but leaves the yield unchanged. Rule of thumb: if Q < K, the reaction proceeds forward; if Q > K, it proceeds backward.