SACE Chemistry — Stage 2
Chemical Equilibrium — Flashcards & Quiz
Chemical equilibrium is the state where forward and reverse reaction rates are equal and concentrations of reactants and products remain constant. SACE Chemistry Stage 2 expects you to apply Le Chatelier's principle to predict the direction of shift when concentration, pressure, or temperature changes, and to interpret the equilibrium constant Kc. Industrial applications like the Haber process are common exam contexts.
Key Points
- At equilibrium, forward and reverse reaction rates are equal — dynamic, not static.
- Le Chatelier's principle: when a system at equilibrium is disturbed, it shifts to partially counteract the disturbance.
- Concentration: adding reactant shifts right (toward products); removing product shifts right.
- Pressure (gases only): increasing pressure shifts toward the side with fewer moles of gas.
- Temperature: for an exothermic reaction, increasing T shifts left; for endothermic, increasing T shifts right.
- Catalysts do NOT shift position — they speed up both forward and reverse equally, reaching equilibrium faster.
Common Mistakes to Avoid
- Claiming equilibrium means the reaction has stopped — it's dynamic, both directions continue.
- Saying catalysts shift equilibrium — they don't; they only change the rate of attainment.
- Applying pressure changes to reactions with equal moles on both sides — no shift occurs.
- Forgetting that exothermic and endothermic reactions respond oppositely to temperature changes.
- Using Le Chatelier to predict the NEW Kc — Kc changes only with temperature, not with concentration or pressure.
Exam Strategy
SACE Stage 2 equilibrium questions give you a disturbance (add reactant, change T or P) and ask you to predict the shift and justify. Method: (1) state Le Chatelier's principle, (2) identify the change, (3) predict the direction with reasoning, (4) explain the effect on Kc (only temperature changes it). Diagrams or equations help clarify responses.
Revision Tip
Le Chatelier is pattern recognition — drill a Revizi deck of 15+ equilibrium scenarios covering every type of disturbance until predictions become automatic.
Last updated: March 2026