Higher Tier: predict the response to an equilibrium disturbance
| English | 中文 | Pinyin |
|---|---|---|
| Le Chatelier’s Principle | 勒沙特列原理 | lēi shā tè liè yuán lǐ |
| equilibrium shift | 平衡移动 | píng héng yí dòng |
What would explain this observation?
- Higher Tier: An equilibrium mixture responds when a condition changes. Its response opposes the disturbance, but does not necessarily restore the original amounts exactly.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Equilibrium amounts depend on reaction conditions. Le Chatelier’s Principle 勒沙特列原理 states that when a system at equilibrium is disturbed, it responds to counteract the change. After a change of concentration, temperature or reacting-gas pressure, the system adjusts until forward and reverse rates are equal again. Predict qualitatively which direction is favoured using the equation and the changed condition. This entire source section is Higher-only.
- Le Chatelier’s Principle: An equilibrium system responds to a disturbance in a direction that counteracts it; equilibrium shift 平衡移动: A net composition adjustment toward a new equilibrium after conditions change.
After a disturbance, which statement describes a new equilibrium?
Begin with the system already at equilibrium, identify one imposed change and select the appropriate response rule. Increased reactant concentration favours its consumption; removing product favours forming more. Warming favours the endothermic direction. Compression of a gaseous equilibrium favours the side with fewer gas molecules. These are composition responses, not the same as saying every faster reaction makes a larger final yield.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Begin with the system already at equilibrium, identify one imposed change and select the appropriate response rule. Increased reactant concentration favours its consumption; removing product favours forming more. Warming favours the endothermic direction. Compression of a gaseous equilibrium favours the side with fewer gas molecules. These are composition responses, not the same as saying every faster reaction makes a larger final yield.
- Use supplied equations, forward energy direction, states and before/after data. Mark a shift arrow as net adjustment toward a new equilibrium, not cessation of the other reaction. Keep other conditions specified. A suitable catalyst increases both rates and reduces time to equilibrium without shifting the final equilibrium composition at fixed conditions. No equilibrium-constant expression or numerical constant calculation is required by this GCSE section.
Which two habits make the investigation or model in this case more defensible?
Use supplied equations, forward energy direction, states and before/after data. Mark a shift arrow as net adjustment toward a new equilibrium, not cessation of the other reaction. Keep other conditions specified. A suitable catalyst increases both rates and reduces time to equilibrium without shifting the final equilibrium composition at fixed conditions. No equilibrium-constant expression or numerical constant calculation is required by this GCSE section.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a supplied one-to-one mixture initially contains 40 A and 60 B particles at equilibrium. After an imposed change and re-equilibration it contains 30 A and 70 B. Net B increase is ten, so the observed response favoured products. Total remains 100 in this stated closed conversion model. These data show a direction of shift but alone do not identify which condition was changed.
A supplied mixture changes from 55 to 68 product particles after re-equilibration. Find the product-count increase. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A supplied mixture changes from 55 to 68 product particles after re-equilibration. Find the product-count increase.
The result is 13 particles. Known: a supplied one-to-one mixture initially contains 40 A and 60 B particles at equilibrium. After an imposed change and re-equilibration it contains 30 A and 70 B. Net B increase is ten, so the observed response favoured products. Total remains 100 in this stated closed conversion model. These data show a direction of shift but alone do not identify which condition was changed.
Check the conclusion and its limits
- Do not describe counteracting as entirely cancelling the original disturbance. A catalyst does not count as a concentration addition of reactant. A shift toward products does not mean all reactants are consumed, and equal rates at the new equilibrium do not require the old mixture ratio. Different perturbations can produce the same direction of observed composition change.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Counteracting a disturbance always restores every original concentration exactly. This claim is false: Do not describe counteracting as entirely cancelling the original disturbance. A catalyst does not count as a concentration addition of reactant. A shift toward products does not mean all reactants are consumed, and equal rates at the new equilibrium do not require the old mixture ratio. Different perturbations can produce the same direction of observed composition change.
Higher Tier: predict the response to an equilibrium disturbance: Begin with the system already at equilibrium, identify one imposed change and select the appropriate response rule. Increased reactant concentration favours its consumption; removing product favours forming more. Warming favours the endothermic direction. Compression of a gaseous equilibrium favours the side with fewer gas molecules. These are composition responses, not the same as saying every faster reaction makes a larger final yield.
Counteracting a disturbance always restores every original concentration exactly.
Do not describe counteracting as entirely cancelling the original disturbance. A catalyst does not count as a concentration addition of reactant. A shift toward products does not mean all reactants are consumed, and equal rates at the new equilibrium do not require the old mixture ratio. Different perturbations can produce the same direction of observed composition change.
An equilibrium system responds to a disturbance in a direction that counteracts it: write the technical term.
Le Chatelier’s Principle means An equilibrium system responds to a disturbance in a direction that counteracts it.