HSC Chemistry — Module 5
Le Chatelier’s Principle — Flashcards & Quiz
Le Chatelier’s principle states that a system at equilibrium will shift to partially counteract any change imposed on it. In HSC Chemistry Module 5, you must predict how changes in concentration, temperature, pressure, and volume affect equilibrium position. Understanding that catalysts do not shift equilibrium (they only speed up attainment) is a common exam trap. Questions frequently present industrial scenarios like the Haber process and ask you to explain optimal conditions using Le Chatelier’s principle.
Key Points
- Le Chatelier's principle: when a system at equilibrium is disturbed, it shifts to partially counteract the disturbance and restore equilibrium.
- Concentration: increasing [reactant] shifts right (toward products); removing product shifts right.
- Temperature: for an exothermic reaction, increasing T shifts left (away from heat); for endothermic, increasing T shifts right.
- Pressure (gases only): increasing P shifts to the side with fewer moles of gas; if moles are equal, no shift.
- Catalysts do NOT shift position — they speed up forward and reverse reactions equally, reaching equilibrium faster.
- Exam trap: Le Chatelier only describes direction of shift, not extent. Use Kc to quantify the new equilibrium state.
Common Mistakes to Avoid
- Claiming catalysts shift equilibrium position — they don't. They only speed up how fast equilibrium is reached.
- Stating that "heating always shifts right" — it depends on whether the reaction is exothermic (heat as product) or endothermic (heat as reactant).
- Changing pressure on a reaction where both sides have equal moles of gas and predicting a shift — no shift occurs.
- Adding inert gas at constant volume and predicting a shift — inert gas doesn't change partial pressures of reactants, so no shift.
- Using Le Chatelier to predict the NEW Kc value — Le Chatelier predicts direction of shift only; temperature is the only factor that changes Kc.
Exam Strategy
HSC Module 5 Le Chatelier questions are a staple: you're given a change (pressure, temperature, concentration, catalyst) and asked to predict and justify the shift. Structure: (1) identify the change, (2) state the principle, (3) predict direction with a reason (e.g. "the system shifts to the side with fewer moles of gas"), (4) if temperature, note whether Kc increases or decreases. Diagrams or equations often make responses clearer than prose.
Sample Flashcards
Q1: State Le Chatelier's principle.
If a system at equilibrium is subjected to a change in concentration, temperature or pressure, the system will shift to partially counteract the change and establish a new equilibrium position.
Q2: How does changing concentration affect equilibrium?
Increasing the concentration of a reactant shifts equilibrium toward the products (right). Increasing the concentration of a product shifts equilibrium toward the reactants (left). The system consumes the added substance to partially restore the original ratio.
Q3: How does changing temperature affect equilibrium?
Increasing temperature favours the endothermic direction. Decreasing temperature favours the exothermic direction. Temperature is the ONLY factor that changes the value of Kc.
Q4: How does changing pressure affect equilibrium in gaseous systems?
Increasing pressure shifts equilibrium toward the side with FEWER moles of gas (to reduce pressure). Decreasing pressure shifts toward the side with MORE moles of gas. Pressure changes only affect equilibria involving different moles of gas on each side.
Q5: What happens to equilibrium when an inert gas is added at constant volume?
Adding an inert gas at constant volume does NOT affect equilibrium. The concentrations of reactants and products remain unchanged because the volume hasn't changed, so partial pressures of the reacting gases are unaffected. No shift occurs.
Sample Quiz Questions
Q1: Adding more reactant to a system at equilibrium shifts the equilibrium to the right.
Answer: TRUE
Increasing reactant concentration shifts equilibrium toward the products (right) to partially counteract the increase.
Q2: Increasing temperature always shifts equilibrium to the right.
Answer: FALSE
Increasing temperature shifts equilibrium toward the ENDOTHERMIC direction — this could be right (if forward is endothermic) or left (if forward is exothermic).
Q3: Increasing pressure shifts equilibrium toward the side with more moles of gas.
Answer: FALSE
Increasing pressure shifts equilibrium toward the side with FEWER moles of gas, to reduce the total pressure.
Revision Tip
Le Chatelier is a pattern-recognition topic — drill a Revizi deck of 20+ scenarios (one per disturbance type) until you can call the shift direction in seconds.
Related Concepts
Last updated: March 2026 · 7 flashcards · 6 quiz questions