TCE Chemistry — Level 4
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. TCE Chemistry Level 4 asks you to write Kc expressions, apply Le Chatelier's principle to predict shifts, and analyse industrial processes like the Haber and Contact processes. The distinction between Kc (temperature-only sensitivity) and position shifts is a common exam trap.
Key Points
- Equilibrium is dynamic — forward and reverse reactions continue at equal rates, so concentrations stay constant.
- Kc = [products]^coefficients / [reactants]^coefficients. Exclude solids and pure liquids.
- Le Chatelier: when disturbed, the system shifts to partially counteract the disturbance.
- Temperature: exothermic shifts left on heating; endothermic shifts right. Temperature is the ONLY factor that changes Kc.
- Pressure (gases only): increasing P shifts toward fewer moles of gas.
- Catalysts do NOT shift position — they speed up forward and reverse rates equally, reaching equilibrium faster.
Common Mistakes to Avoid
- Claiming equilibrium means nothing is happening — it's dynamic, both directions continue.
- Saying catalysts shift equilibrium — they don't; they only reach it faster.
- Changing Kc for pressure or concentration changes — only temperature changes Kc.
- Including solids in Kc expressions.
- Using Le Chatelier to predict magnitudes — it only predicts direction.
Exam Strategy
TASC Level 4 equilibrium questions give you a reversible reaction and a disturbance, then ask you to predict the shift and its effect on Kc. Method: (1) identify the disturbance (concentration, pressure, temperature, catalyst), (2) apply Le Chatelier to determine direction, (3) note whether Kc changes (only with temperature), (4) justify using the principle explicitly.
Sample Flashcards
Q1: What is dynamic equilibrium?
Dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction. Macroscopic properties remain constant while reactions continue at the molecular level.
Q2: Write the general expression for Kc and explain what a large Kc indicates.
For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b. Large Kc (>>1) means products favoured; small Kc (<<1) means reactants favoured.
Q3: How does temperature affect equilibrium and Kc?
Temperature is the ONLY factor that changes Kc. Exothermic forward: increasing T shifts left, decreases Kc. Endothermic forward: increasing T shifts right, increases Kc.
Q4: How does pressure affect gaseous equilibrium?
Increasing pressure shifts towards fewer moles of gas. Decreasing pressure shifts towards more moles. Equal moles on both sides — no effect.
Q5: How does adding reactant affect equilibrium?
Increases concentration → Q < Kc → shifts right. Product concentrations increase; added reactant partially consumed.
Sample Quiz Questions
Q1: At dynamic equilibrium, all reactions have stopped.
Answer: FALSE
Forward and reverse reactions continue at equal rates.
Q2: Dynamic equilibrium requires a closed system.
Answer: TRUE
A closed system prevents reactants/products from escaping.
Q3: Pure solids are included in the Kc expression.
Answer: FALSE
Pure solids and liquids have constant concentration and are excluded.
Revision Tip
Le Chatelier is pattern recognition — drill a Revizi deck with 15+ equilibrium scenarios covering all disturbance types for exam fluency.
Last updated: March 2026 · 6 flashcards · 5 quiz questions