Adding a catalyst
3 marks · EasyNitrogen dioxide, NO₂, and dinitrogen tetroxide, N₂O₄, form a reversible reaction at equilibrium:
State what happens to the colour of the mixture when a catalyst is added.
Explain whether the ratio of NO₂ to N₂O₄ changes after the catalyst is added.
Give a reason for your answer to (b).
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(a) There is no permanent change to the colour of the mixture.
(b) The ratio of NO₂ to N₂O₄ stays the same.
(c) Because a catalyst increases both the forward and reverse reaction rates equally so the position of equilibrium is unchanged.
Common mistakes: Stating the colour changes permanently; Saying ratio changes
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Marking points
B1 (a) · 1 mark · Correctly states that there is no permanent change to colour.
B2 (b) · 1 mark · States that the ratio of NO₂ to N₂O₄ remains the same.
B3 (c) · 1 mark · Gives reason: catalyst increases rate equally for both directions, so equilibrium position is unchanged.
Worked solution
a) There is no lasting change in colour because the equilibrium mixture is unchanged.
b) The ratio of NO₂ to N₂O₄ does not change after adding a catalyst.
c) This is because a catalyst speeds up both directions of the reaction equally so the equilibrium composition remains the same.