AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: explain how a change in concentration or a…
Full specification wording
Effect of concentration and pressure: Students should be able to: explain how a change in concentration or a change in pressure influences the rate of a reaction.
Gas pressure increases act via increased concentration (particles per unit volume), raising collision frequency.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gas pressure increases act via increased concentration (particles per unit volume), raising collision frequency. Raising solution concentration or gas pressure brings reacting particles closer together. Collisions happen more often. The proportion of collisions with energy ≥ Eₐ is fixed by temperature. A greater collision frequency scales up successful collisions, increasing the rate. The energy distribution remains untouched, so the Maxwell–Boltzmann curve is unchanged. Example: Compressing a gas mixture halves its volume, doubling the concentration of gaseous reactants, which directly increases the frequency of successful collisions.
What earns the marks
- Increased gas pressure increases concentration (particles per unit volume), leading to higher collision frequency.
- A greater collision frequency increases the frequency of successful collisions.
- The proportion of successful collisions remains unchanged if temperature is constant.
- The rate of reaction increases directly in step with the frequency of successful collisions.
Common mistakes and how to put them right
Denying that concentration reasoning (particles per unit volume) applies to gas phase pressure changes.
Put it right by treating pressure as gas concentration.
Explaining a concentration change using activation energy.
Put it right by referencing collision frequency.
Stating particles collide harder.
Put it right by stating they collide more often.
Exam tips
- Treat pressure and concentration as acting on collision frequency, while temperature acts on the proportion of successful collisions.
- Always use the phrase 'per unit volume' when discussing concentration to secure the mark.
Try the lesson
The start of the Effect of concentration and pressure: Students should be able to: explain how a change in concentration or a change in pressure influences the rate of a reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
Hydrogen peroxide solution decomposes slowly at room temperature. When a more concentrated hydrogen peroxide solution is used at the same temperature, why does the rate of decomposition increase?
Listen
Gas pressure increases act via increased concentration (particles per unit volume), raising collision frequency. Raising solution concentration or gas pressure brings reacting particles closer together. Collisions happen more often. The proportion of collisions with energy ≥ Eₐ is fixed by temperature. A greater collision frequency scales up successful collisions, increasing the rate. The energy distribution remains untouched, so the Maxwell–Boltzmann curve is unchanged. Example: Compressing a gas mixture halves its volume, doubling the concentration of gaseous reactants, which directly increases the frequency of successful collisions. A strong answer earns marks for these points. Increased gas pressure increases concentration (particles per unit volume), leading to higher collision frequency. A greater collision frequency increases the frequency of successful collisions. The proportion of successful collisions remains unchanged if temperature is constant. The rate of reaction increases directly in step with the frequency of successful collisions. A common slip: Denying that concentration reasoning (particles per unit volume) applies to gas phase pressure changes. Instead, treating pressure as gas concentration. Exam tip: Treat pressure and concentration as acting on collision frequency, while temperature acts on the proportion of successful collisions.
Key words
Say what each word means before you turn the card.
Collision frequency
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Concentration and pressure equivalence
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Unchanged energy distribution
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See every definition
- Collision frequency
- Reacting particles brought closer together collide more often, and a greater collision frequency scales up successful collisions, increasing the rate.
- Concentration and pressure equivalence
- Gas pressure increases act via increased concentration, measured as particles per unit volume, so pressure is treated as gas concentration.
- Unchanged energy distribution
- The proportion of collisions with energy greater than or equal to the activation energy is fixed by temperature, so the energy distribution remains untouched and the Maxwell–Boltzmann curve is unchanged.
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