6.7 Changing conditions: Le Chatelier's principle
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State Le Chatelier's principle.
Mark scheme
if a system at equilibrium is subjected to a change in conditions, the system responds to counteract / oppose the change
Model answer
If a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to counteract the change.
N2 + 3H2 ⇌ 2NH3. The forward reaction is exothermic.
Explain the effect of increasing the pressure and of increasing the temperature on the yield of ammonia.
Mark scheme
- higher pressure: equilibrium shifts right / yield increases (1)
- because fewer gas molecules on the right / 4 → 2 (1)
- higher temperature: equilibrium shifts left / yield decreases (1)
- because the reverse reaction is endothermic / shifts in endothermic direction (1)
Model answer
Increasing the pressure shifts the equilibrium to the right, the side with fewer gas molecules (2 instead of 4), so the yield of ammonia increases. Increasing the temperature shifts the equilibrium in the endothermic direction, which is the reverse reaction, so the yield of ammonia decreases.
Explain why adding a catalyst does not change the yield of a reversible reaction.
Mark scheme
- speeds up forward and reverse reactions equally (1)
- so position of equilibrium does not change / only reached faster (1)
Model answer
A catalyst speeds up the forward and reverse reactions by the same amount, so the position of equilibrium does not change; equilibrium is just reached faster.
H2 + I2 ⇌ 2HI (all gases). Explain why changing the pressure has no effect on the position of this equilibrium.
Mark scheme
- same number of gas molecules on each side / 2 and 2 (1)
- so neither side reduces the pressure (1)
Model answer
There are two molecules of gas on each side, so shifting either way would not change the pressure, and the position stays the same.
2NO2 (brown) ⇌ N2O4 (colourless). The forward reaction is exothermic. Predict what you would see when the mixture is cooled, and explain why.
Mark scheme
- becomes paler / less brown (1)
- equilibrium shifts right / in the exothermic direction (1)
Model answer
The mixture becomes paler, because cooling shifts the equilibrium in the exothermic direction, to the right, making more colourless N2O4.
In a reaction at equilibrium, ammonia is removed as it forms. Explain the effect on the amount of ammonia made.
Mark scheme
- equilibrium shifts to the right (to replace the ammonia) (1)
- more ammonia is made (1)
Model answer
Removing ammonia shifts the equilibrium to the right to replace it, so more nitrogen and hydrogen react and more ammonia is made overall.
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