Changing the pressure on the reaction
4 marks · NormalState whether changing the pressure has a significant effect on the rate of reaction for solids or liquids.
Describe the arrangement of particles in solids and in liquids.
Explain, using your answer to (b), why pressure does not change the rate much for solids and liquids.
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(a) Changing pressure has little or no effect on the rate for solids or liquids.
(b) In solids, particles are tightly packed and arranged in a regular pattern. In liquids, particles are closely packed but can move past each other.
(c) Because the particles are already close together, increasing the pressure does not significantly decrease the space between them, so the frequency of collisions does not increase.
Common mistakes: Incorrectly stating that rate increases for solids and liquids; Ignoring particle arrangement
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Marking points
B1 (a) · 1 mark · States correctly that changing pressure has little/no effect for solids or liquids.
B2 (b) · 1 mark · Describes the arrangement of particles in solids: tightly packed/regular.
B3 (b) · 1 mark · Describes the arrangement of particles in liquids: closely packed but able to move.
B4 (c) · 1 mark · Explains that since particles are already close together, pressure does not significantly increase collisions.
Worked solution
a) Changing the pressure has little or no effect on the rate of reaction for solids or liquids.
b) In solids, particles are tightly packed in a fixed, regular pattern. In liquids, particles are still close together but have no regular arrangement and can move past one another.
c) Because solids and liquids have particles already close together, increasing the pressure does not noticeably decrease the space between particles, so collision frequency and the rate of reaction stay almost the same.