SACE Chemistry — Stage 2
Chemical Equilibrium — SACE Chemistry — Stage 2 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.
Sample Flashcards
Q1: Using equilibrium principles, explain why a lower ocean pH makes it harder for marine organisms to form calcium carbonate shells.
Shells and exoskeletons are made of CaCO₃, which is in equilibrium with its ions: CaCO₃(s) ⇌ Ca²⁺(aq) + CO₃²⁻(aq). Extra H₃O⁺ from dissolved CO₂ reacts with carbonate ions: CO₃²⁻(aq) + H₃O⁺(aq) → HCO₃⁻(aq) + H₂O(l). This lowers [CO₃²⁻], so by Le Châtelier’s principle the dissolution equilibrium shifts to the right: less CaCO₃ can form and existing shells can dissolve.
Q2: How does ion exchange (ion chromatography) work, using equilibrium principles?
An ion-exchange resin has charged sites holding exchangeable ions. In a cation exchanger, ions in solution swap with ions on the resin, for example: 2R–Na⁺ + Ca²⁺(aq) ⇌ R₂–Ca²⁺ + 2Na⁺(aq). Ions with a higher charge are generally held more strongly, so Ca²⁺ and Mg²⁺ displace Na⁺. The resin is regenerated by passing a concentrated NaCl solution through it, which by Le Châtelier’s principle shifts the equilibrium back to the left.
Sample Quiz Questions
Q1: Lower ocean pH reduces the concentration of carbonate ions, shifting the CaCO₃ dissolution equilibrium to the right.
Answer: TRUE
H₃O⁺ reacts with CO₃²⁻ to form HCO₃⁻. Removing CO₃²⁻ shifts CaCO₃(s) ⇌ Ca²⁺(aq) + CO₃²⁻(aq) to the right, so carbonate shells dissolve more readily.
Q2: A cation-exchange resin can be regenerated by passing a concentrated sodium chloride solution through it.
Answer: TRUE
A high [Na⁺] shifts the exchange equilibrium back towards the Na⁺ form of the resin, releasing the bound Ca²⁺ and Mg²⁺.
Revision Tip
Le Chatelier is pattern recognition — drill a Revizi deck of 15+ equilibrium scenarios covering every type of disturbance until predictions become automatic.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards · 2 sample quiz questions