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QLDChemistryQuick questions
Unit 3: Equilibrium, acids and redox reactions
Quick questions on Le Chatelier's principle and equilibrium shifts (QCE Chemistry Unit 3)
13short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is concentration changes?Show answer
For any equilibrium A + B leftrightarrow C + D:
What is temperature changes?Show answer
Temperature is the only disturbance that actually changes Kc. Whether the equilibrium shifts forward or back depends on the thermicity of the forward reaction.
What is pressure and volume changes (gaseous systems only)?Show answer
These two are linked: at constant temperature, decreasing volume increases pressure and vice versa. The shift direction depends on the relative moles of gas on each side of the equation.
What is catalysts?Show answer
A catalyst speeds the forward and reverse reactions equally. The equilibrium position does not shift and Kc does not change. The system reaches equilibrium faster.
What is visualising the shifts?Show answer
A useful approach is the "what would the system do to undo the change?" test:
What is sample concentration-vs-time graph after a disturbance?Show answer
A typical IA1 stimulus shows a system at equilibrium, then a sudden disturbance (e.g. extra reactant injected), then re-equilibration.
What is adding an inert gas at constant volume?Show answer
Total pressure rises but partial pressures of reacting species are unchanged. No shift.
What is adding an inert gas at constant pressure?Show answer
The container expands; partial pressures of reacting species fall. Equivalent to a volume increase; shift toward more moles of gas.
What is saying "the system shifts to make the change go away"?Show answer
It only partially opposes; the affected species does not return to its original concentration.
What is applying pressure logic to aqueous systems?Show answer
Pressure changes affect only gases. The iron(III) thiocyanate equilibrium is aqueous; pressure has no effect.
What is forgetting to count moles of gas carefully?Show answer
Solids and liquids do not count. For 2SO2(g) + O2(g) leftrightarrow 2SO3(g), reactant side has 3 mol gas, product side has 2 mol gas; increasing pressure shifts right.
What is confusing kinetics and thermodynamics?Show answer
A catalyst affects how fast equilibrium is reached, not where it sits. Temperature is the only condition that changes Kc.
What is treating heat as a thermal reservoir that absorbs without shifting?Show answer
Heat is a participant in the equation; raising T shifts in the endothermic direction by adding energy as if it were a reactant or product.