AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The effects of changing conditions on a system at equilibrium can be…
Full specification wording
The effect of changing conditions on equilibrium (HT only): The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
Le Chatelier’s Principle states that when a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change.
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- Point 3 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Le Chatelier’s Principle states that when a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure shifts equilibrium to the right because there are fewer gas molecules on the product side, partially reducing the pressure rise. Increasing temperature shifts equilibrium in the endothermic direction, so for this exothermic forward reaction it shifts left. Adding a catalyst does not change the position of equilibrium; it only speeds up the attainment of equilibrium. The principle applies to changes in concentration, pressure and temperature, and predictions must state both the direction of shift and the resulting effect on yield.
What earns the marks
- State that Le Chatelier’s Principle predicts that a system at equilibrium responds to a change by shifting in the direction that opposes the change.
- Apply the principle to a change in concentration by stating that increasing a reactant shifts equilibrium to the right, increasing product yield.
- Apply the principle to a change in pressure by comparing the number of gas molecules on each side; increasing pressure shifts equilibrium towards the side with fewer gas molecules.
- Apply the principle to a change in temperature by identifying the exothermic and endothermic directions; increasing temperature shifts equilibrium in the endothermic direction.
- Explain that a catalyst does not change the position of equilibrium because it speeds up the forward and reverse reactions equally.
- Use a balanced equation to justify a prediction, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has four gas molecules on the left and two on the right.
Common mistakes and how to put them right
Thinking that increasing pressure always shifts equilibrium to the right.
Put it right: The correct approach is to compare the number of gas molecules on each side of the equation.
Believing that a catalyst increases yield by shifting equilibrium.
Put it right: A catalyst only increases the rate at which equilibrium is reached.
Confusing the effect of temperature with the effect of pressure.
Put it right: Temperature changes alter the equilibrium position according to whether the forward reaction is exothermic or endothermic, not according to gas molecule numbers.
Exam tips
- Always quote the balanced equation and count gas molecules on each side before predicting a pressure change.
- State the direction of the shift and then link it explicitly to whether the yield of the named substance increases or decreases.
- For temperature changes, identify whether the forward reaction is exothermic or endothermic before predicting the shift.
Try the lesson
The start of the The effect of changing conditions on equilibrium (HT only): The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle. lesson, made from checked teaching notes for this specification point.
Have a go first
A gas-phase equilibrium is endothermic in the forward direction. According to Le Chatelier’s Principle, what is the effect of raising the temperature on the position of equilibrium?
Listen
Le Chatelier’s Principle states that when a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure shifts equilibrium to the right because there are fewer gas molecules on the product side, partially reducing the pressure rise. Increasing temperature shifts equilibrium in the endothermic direction, so for this exothermic forward reaction it shifts left. Adding a catalyst does not change the position of equilibrium; it only speeds up the attainment of equilibrium. The principle applies to changes in concentration, pressure and temperature, and predictions must state both the direction of shift and the resulting effect on yield. A strong answer earns marks for these points. State that Le Chatelier’s Principle predicts that a system at equilibrium responds to a change by shifting in the direction that opposes the change. Apply the principle to a change in concentration by stating that increasing a reactant shifts equilibrium to the right, increasing product yield. Apply the principle to a change in pressure by comparing the number of gas molecules on each side; increasing pressure shifts equilibrium towards the side with fewer gas molecules. Apply the principle to a change in temperature by identifying the exothermic and endothermic directions; increasing temperature shifts equilibrium in the endothermic direction. Exam tip: Always quote the balanced equation and count gas molecules on each side before predicting a pressure change.
See it
Key words
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Le Chatelier’s Principle
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Effect of concentration, pressure and temperature
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Industrial processes such as the Haber process
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See every definition
- Le Chatelier’s Principle
- When a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change.
- Effect of concentration, pressure and temperature
- The principle applies to changes in concentration, pressure and temperature, and predictions must state both the direction of shift and the resulting effect on yield.
- Industrial processes such as the Haber process
- In the Haber process, increasing pressure shifts equilibrium to the right because there are fewer gas molecules on the product side, partially reducing the pressure rise.
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