Changing the pressure on the reaction
4 marks · NormalThe table below shows the time taken for a reaction between two gases at different pressures.
| Pressure (kPa) | Time to complete reaction (s) |
|---|---|
| 50 | 40 |
| 100 | 20 |
| 200 | 10 |
Describe how the time taken changes as the pressure increases.
Calculate how many times faster the reaction is at 200 kPa compared to 50 kPa.
Explain why the reaction is faster at higher pressures.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) As pressure increases, the time taken for the reaction decreases.
(b) 4
(c) Gas particles are closer together and collide more frequently at higher pressures.
Common mistakes: Mixing up times and rates; Thinking rate changes linearly with pressure
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Marking points
B1 (a) · 1 mark · Correctly describes: time decreases as pressure increases.
M1 (b) · 1 mark · Identifies that rate is proportional to 1/time.
A1 (b) · 1 mark · Arrives at correct factor: 4 times faster (40/10 = 4). (follow-through)
B2 (c) · 1 mark · Explains that at higher pressure, particles are closer so more collisions occur.
Worked solution
a) As the pressure increases, the time taken for the reaction decreases.
b) At 50 kPa, the reaction takes 40 s; at 200 kPa, it takes 10 s. The factor increase in rate = 40/10 = 4. The reaction is 4 times faster at 200 kPa compared to 50 kPa.
c) This is because, at higher pressures, gas particles are forced closer together, resulting in more frequent collisions.