AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: use the Maxwell–Boltzmann distribution to…
Full specification wording
Effect of temperature on reaction rate: Students should be able to: use the Maxwell–Boltzmann distribution to explain why a small temperature increase can lead to a large increase in rate.
The puzzle is that a 10 K rise, which changes the mean molecular energy by only a few per cent, can double the rate.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The puzzle is that a 10 K rise, which changes the mean molecular energy by only a few per cent, can double the rate. The distribution explains it. Warming the gas shifts the whole curve to the right and flattens it, but the important change is out in the tail. Because the tail falls away steeply, a small rightward shift moves a disproportionately large slice of molecules past Eₐ: the shaded area beyond the barrier might double while the mean energy rose by three per cent. Since the rate depends on the number of molecules with energy greater than or equal to Eₐ, not on the mean, the rate roughly follows that shaded area. Collision frequency also rises slightly, but it is the change in the area beyond Eₐ that dominates.
What earns the marks
- The higher temperature curve having a lower peak shifted to the right and crossing the first curve once.
- Stating that at the higher temperature many more molecules have energy greater than or equal to Eₐ.
- Concluding that the frequency of successful collisions, and therefore the rate, increases greatly.
- Noting that the increase in collision frequency alone is too small to account for the change.
Common mistakes and how to put them right
Attributing the whole rate increase to more frequent collisions, which the mark scheme explicitly refuses.
Saying the mean energy rises above Eₐ, rather than that more molecules lie beyond Eₐ.
Stating that the activation energy is lowered by heating.
Drawing the hotter curve with a higher peak, so the area under it is no longer constant.
Writing more successful collisions without linking it to the area beyond Eₐ on the curve.
Exam tips
- Shade the two areas beyond Eₐ on your sketch and refer to them by name in the explanation.
- Use the word proportion or fraction of molecules beyond Eₐ, then convert that to frequency of successful collisions.
- A three mark version of this question usually wants: curve shifts right, many more molecules exceed Eₐ, so more frequent successful collisions.
Try the lesson
The start of the Effect of temperature on reaction rate: Students should be able to: use the Maxwell–Boltzmann distribution to explain why a small temperature increase can lead to a large increase in rate. lesson, made from checked teaching notes for this specification point.
Have a go first
Using the Maxwell–Boltzmann distribution, which statement best explains why a small increase in temperature causes a large increase in the rate of a chemical reaction?
Listen
The puzzle is that a 10 K rise, which changes the mean molecular energy by only a few per cent, can double the rate. The distribution explains it. Warming the gas shifts the whole curve to the right and flattens it, but the important change is out in the tail. Because the tail falls away steeply, a small rightward shift moves a disproportionately large slice of molecules past Eₐ: the shaded area beyond the barrier might double while the mean energy rose by three per cent. Since the rate depends on the number of molecules with energy greater than or equal to Eₐ, not on the mean, the rate roughly follows that shaded area. Collision frequency also rises slightly, but it is the change in the area beyond Eₐ that dominates. A strong answer earns marks for these points. the higher temperature curve having a lower peak shifted to the right and crossing the first curve once. stating that at the higher temperature many more molecules have energy greater than or equal to Eₐ. concluding that the frequency of successful collisions, and therefore the rate, increases greatly. noting that the increase in collision frequency alone is too small to account for the change. Exam tip: Shade the two areas beyond Eₐ on your sketch and refer to them by name in the explanation.
Key words
Say what each word means before you turn the card.
shape of the energy tail
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area beyond Eₐ
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successful collision frequency
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mean energy versus tail
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See every definition
- shape of the energy tail
- The tail falls away steeply, so a small rightward shift moves a disproportionately large slice of molecules past Eₐ.
- area beyond Eₐ
- The shaded area beyond the barrier represents the molecules with energy greater than or equal to Eₐ, and the rate roughly follows it.
- successful collision frequency
- The frequency of successful collisions, and therefore the rate, increases greatly at the higher temperature.
- mean energy versus tail
- The rate depends on the number of molecules with energy greater than or equal to Eₐ, not on the mean, which rises by only a few per cent.
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