AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to: use a Maxwell–Boltzmann distribution to help…

    Full specification wording

    Catalysts: Students should be able to: use a Maxwell–Boltzmann distribution to help explain how a catalyst increases the rate of a reaction involving a gas.

    The catalyst question and the temperature question look alike but need opposite diagrams.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 3 of 3
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The catalyst question and the temperature question look alike but need opposite diagrams. Adding a catalyst does not change the temperature, so the distribution curve is not redrawn at all: a single curve is used and a second, lower activation energy is marked to the left of the original. The shaded area beyond that new barrier is far larger than the area beyond the original one, meaning many more molecules now have energy greater than or equal to the catalysed activation energy. The proportion of successful collisions rises and so does the rate, even though the mean energy of the molecules and the collision frequency are both unaltered. Marking two barriers on one unchanged curve is what the mark scheme is looking for.

    What earns the marks

    • A single correctly shaped distribution curve with both axes labelled.
    • Two activation energies marked on the energy axis, the catalysed one lower.
    • Stating that more molecules have energy greater than or equal to the catalysed activation energy.
    • Concluding that the proportion, and hence the frequency, of successful collisions increases so the rate rises.

    Common mistakes and how to put them right

    • Drawing a second curve for the catalysed reaction, as though the temperature had changed.

    • Shifting the distribution to the right instead of moving the activation energy to the left.

    • Saying the catalyst gives the molecules more energy.

    • Marking the catalysed Eₐ to the right of the original one.

    • Explaining the effect as an increase in collision frequency.

    Exam tips

    • Ask yourself first whether the temperature changed; if it did not, use one curve only.
    • Label the two barriers Eₐ and Eₐ(catalysed) and shade both tails so the comparison is visible.
    • Finish with a sentence naming the proportion of successful collisions, which is the explanation mark.

    Try the lesson

    The start of the Catalysts: Students should be able to: use a Maxwell–Boltzmann distribution to help explain how a catalyst increases the rate of a reaction involving a gas. lesson, made from checked teaching notes for this specification point.

    Have a go first

    How does a Maxwell-Boltzmann distribution show that a catalyst increases the rate of a gas-phase reaction?

    Listen

    The catalyst question and the temperature question look alike but need opposite diagrams. Adding a catalyst does not change the temperature, so the distribution curve is not redrawn at all: a single curve is used and a second, lower activation energy is marked to the left of the original. The shaded area beyond that new barrier is far larger than the area beyond the original one, meaning many more molecules now have energy greater than or equal to the catalysed activation energy. The proportion of successful collisions rises and so does the rate, even though the mean energy of the molecules and the collision frequency are both unaltered. Marking two barriers on one unchanged curve is what the mark scheme is looking for. A strong answer earns marks for these points. a single correctly shaped distribution curve with both axes labelled. two activation energies marked on the energy axis, the catalysed one lower. stating that more molecules have energy greater than or equal to the catalysed activation energy. concluding that the proportion, and hence the frequency, of successful collisions increases so the rate rises. Exam tip: Ask yourself first whether the temperature changed; if it did not, use one curve only.

    See it

    A top-down flowchart. At the top is a single Maxwell–Boltzmann distribution curve. From it, two branches: one marks the original activation energy Eₐ, the other marks the catalysed activation energy Eₐ(catalysed), which is lower and to the left. The Eₐ branch leads to the area beyond Eₐ, representing molecules with energy greater than or equal to Eₐ. The catalysed branch leads to the area beyond Eₐ(catalysed), which is far larger. That leads to more molecules having energy greater than or equal to the catalysed activation energy, which leads to the proportion of successful collisions increasing, which leads to the rate of reaction rising. A separate branch from the curve states that the mean energy and collision frequency are unchanged.
    It shows how marking a lower catalysed activation energy on one unchanged Maxwell–Boltzmann curve leads to a larger area beyond the barrier, more molecules with sufficient energy, a higher proportion of successful collisions and a faster rate.

    Key words

    Say what each word means before you turn the card.

    one curve, two barriers

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    lower catalysed activation energy

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    area beyond Eₐ

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    unchanged temperature

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    See every definition
    one curve, two barriers
    A single distribution curve is used and a second, lower activation energy is marked to the left of the original.
    lower catalysed activation energy
    The catalysed barrier is marked lower on the energy axis, to the left of the original activation energy.
    area beyond Eₐ
    The shaded area beyond the new barrier is far larger than the area beyond the original one.
    unchanged temperature
    Adding a catalyst does not change the temperature, so the distribution curve is not redrawn at all.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top is a single Maxwell–Boltzmann distribution curve. From it, two branches: one marks the original activation energy Eₐ, the other marks the catalysed activation energy Eₐ(catalysed), which is lower and to the left. The Eₐ branch leads to the area beyond Eₐ, representing molecules with energy greater than or equal to Eₐ. The catalysed branch leads to the area beyond Eₐ(catalysed), which is far larger. That leads to more molecules having energy greater than or equal to the catalysed activation energy, which leads to the proportion of successful collisions increasing, which leads to the rate of reaction rising. A separate branch from the curve states that the mean energy and collision frequency are unchanged.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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