AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
A catalyst does not affect the position of equilibrium.
Full specification wording
Chemical equilibria and Le Chatelier's principle: A catalyst does not affect the position of equilibrium.
A catalyst lowers the activation energy of the forward and the reverse reactions by the same amount, so both are speeded up equally.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A catalyst lowers the activation energy of the forward and the reverse reactions by the same amount, so both are speeded up equally. Equilibrium is therefore reached sooner, but it is the same equilibrium: the position is unmoved, the composition at equilibrium is identical, the equilibrium constant is unchanged and the yield is exactly what it would have been. This follows from the definition of equilibrium as equal forward and reverse rates, since multiplying both rates by the same factor leaves them equal at the same composition. It is also why catalysts matter industrially. They let a plant work at a lower temperature, where the yield of an exothermic reaction is better, without the reaction becoming uneconomically slow.
What earns the marks
- Stating that a catalyst increases the rates of the forward and reverse reactions equally.
- Concluding that the position of equilibrium is unchanged.
- Stating that the equilibrium constant is unaffected.
- Identifying the benefit as equilibrium being reached more quickly, or a lower operating temperature being usable.
Common mistakes and how to put them right
Saying a catalyst increases the yield of product.
Claiming it speeds the forward reaction more than the reverse.
Asserting that it changes the value of the equilibrium constant.
Stating that it makes the forward reaction more exothermic.
Assuming a catalyst must be a solid in every equilibrium system.
Exam tips
- Answer yield questions and rate questions separately; a catalyst belongs only in the rate answer.
- If a multiple choice option says a catalyst has no effect on the equilibrium constant, that statement is correct.
- Mention the industrial payoff, a lower temperature at an acceptable rate, when a question asks why a catalyst is used.
Try the lesson
The start of the Chemical equilibria and Le Chatelier's principle: A catalyst does not affect the position of equilibrium. lesson, made from checked teaching notes for this specification point.
Have a go first
A reversible reaction reaches equilibrium in a closed container. A catalyst is then added at constant temperature. What happens to the position of equilibrium?
Listen
A catalyst lowers the activation energy of the forward and the reverse reactions by the same amount, so both are speeded up equally. Equilibrium is therefore reached sooner, but it is the same equilibrium: the position is unmoved, the composition at equilibrium is identical, the equilibrium constant is unchanged and the yield is exactly what it would have been. This follows from the definition of equilibrium as equal forward and reverse rates, since multiplying both rates by the same factor leaves them equal at the same composition. It is also why catalysts matter industrially. They let a plant work at a lower temperature, where the yield of an exothermic reaction is better, without the reaction becoming uneconomically slow. A strong answer earns marks for these points. stating that a catalyst increases the rates of the forward and reverse reactions equally. concluding that the position of equilibrium is unchanged. stating that the equilibrium constant is unaffected. identifying the benefit as equilibrium being reached more quickly, or a lower operating temperature being usable. Exam tip: Answer yield questions and rate questions separately; a catalyst belongs only in the rate answer.
See it
Key words
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equal effect on both directions
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position of equilibrium unchanged
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equilibrium constant unchanged
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equilibrium reached sooner
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See every definition
- equal effect on both directions
- A catalyst lowers the activation energy of the forward and the reverse reactions by the same amount, so both are speeded up equally.
- position of equilibrium unchanged
- The equilibrium is the same, with the position unmoved and the composition at equilibrium identical.
- equilibrium constant unchanged
- The equilibrium constant is unaffected by a catalyst.
- equilibrium reached sooner
- Both reactions are speeded up equally, so equilibrium is reached sooner.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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