AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The qualitative effect of a change in the pressure of a gas on collision…
Full specification wording
Effect of concentration and pressure: The qualitative effect of a change in the pressure of a gas on collision frequency.
For gases, pressure plays the role concentration plays in solution.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
For gases, pressure plays the role concentration plays in solution. Squeezing a fixed amount of gas into a smaller volume packs the same number of molecules into less space, so the concentration of every gaseous reactant rises and collisions become more frequent. The result is a greater frequency of successful collisions and a faster rate, with the proportion of successful collisions unchanged because the temperature and therefore the energy distribution are the same. The reasoning depends on the molecules being brought closer together, which is why pumping in an unreactive gas at constant volume does not speed the reaction up: the total pressure rises, but the partial pressures of the reactants, and so their concentrations, are untouched.
What earns the marks
- Stating that increasing the pressure forces the same number of molecules into a smaller volume.
- Identifying the resulting increase in concentration of the gaseous reactants.
- Concluding that collisions, and therefore successful collisions, become more frequent.
- Recognising that adding an inert gas at constant volume leaves the reactant concentrations, and the rate, unchanged.
Common mistakes and how to put them right
Claiming a pressure increase gives the molecules more energy and so a greater proportion of successful collisions.
Applying a pressure argument to a reaction between substances in solution.
Assuming any rise in total pressure must speed the reaction up, whatever causes it.
Writing more collisions without the word frequent or successful.
Confusing the effect of pressure on rate with its effect on the position of an equilibrium.
Exam tips
- Convert pressure into concentration in your first sentence; every later mark follows from that step.
- Check whether the reactants are gases before you use a pressure argument at all.
- Watch for the inert gas trap, where the total pressure rises at constant volume.
Try the lesson
The start of the Effect of concentration and pressure: The qualitative effect of a change in the pressure of a gas on collision frequency. lesson, made from checked teaching notes for this specification point.
Have a go first
A gaseous reaction mixture in a sealed container is compressed so that its pressure increases at constant temperature. What happens to the frequency of collisions between the gas particles?
Listen
For gases, pressure plays the role concentration plays in solution. Squeezing a fixed amount of gas into a smaller volume packs the same number of molecules into less space, so the concentration of every gaseous reactant rises and collisions become more frequent. The result is a greater frequency of successful collisions and a faster rate, with the proportion of successful collisions unchanged because the temperature and therefore the energy distribution are the same. The reasoning depends on the molecules being brought closer together, which is why pumping in an unreactive gas at constant volume does not speed the reaction up: the total pressure rises, but the partial pressures of the reactants, and so their concentrations, are untouched. A strong answer earns marks for these points. stating that increasing the pressure forces the same number of molecules into a smaller volume. identifying the resulting increase in concentration of the gaseous reactants. concluding that collisions, and therefore successful collisions, become more frequent. recognising that adding an inert gas at constant volume leaves the reactant concentrations, and the rate, unchanged. Exam tip: Convert pressure into concentration in your first sentence; every later mark follows from that step.
Key words
Say what each word means before you turn the card.
pressure and gas concentration
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molecules closer together
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collision frequency
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inert gas at constant volume
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See every definition
- pressure and gas concentration
- Squeezing a fixed amount of gas into a smaller volume packs the same number of molecules into less space, so the concentration of every gaseous reactant rises.
- molecules closer together
- The reasoning depends on the molecules being brought closer together, which raises the concentration of every gaseous reactant.
- collision frequency
- Collisions become more frequent, giving a greater frequency of successful collisions and a faster rate.
- inert gas at constant volume
- Pumping in an unreactive gas at constant volume does not speed the reaction up, because the partial pressures of the reactants, and so their concentrations, are untouched.
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