Changing the pressure on the reaction
4 marks · NormalA student increases the pressure in a chemical reaction involving only solids. Predict what effect, if any, this has on the rate of reaction.
Explain why changing the pressure has little effect on the rate of reaction for solids. Use particle arrangement in your answer.
Would increasing the pressure have a similar effect on a reaction involving only liquids? Explain your answer.
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(a) No effect or almost no effect on the rate.
(b) Particles in solids are already closely packed, so increasing pressure does not significantly decrease their volume or increase particle collisions. Thus, the number of collisions per second does not change much.
(c) No, increasing pressure does not significantly affect the rate of reaction for liquids, for the same reason as solids—liquid particles are already close together, so pressure changes make little difference.
Common mistakes: Assuming pressure affects solids/ liquids like it does gases.; Not mentioning particle arrangement.
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Marking points
B1 (a) · 1 mark · Correctly predicts there is little or no effect on the rate of reaction.
B2 (b) · 1 mark · Mentions that particles in solids are closely packed together.
B3 (b) · 1 mark · Explains that increasing pressure does not significantly increase the number of collisions because there is little space between particles.
B4 (c) · 1 mark · States that increasing pressure does not significantly affect rate for liquids, with correct reasoning.
Worked solution
a) Increasing pressure has little or no effect on the rate of reaction for solids.
b) In solids, particles are closely packed together with very little space between them. Increasing the pressure does not significantly decrease the volume or allow for more collisions because the particles cannot get much closer.
c) The effect is similar for liquids, as liquid particles are also close together. Increasing pressure has little effect on the rate of reaction involving only liquids.