AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    For the Higher Tier, students are also required to use quantity of…

    Full specification wording

    Calculating rates of reactions: For the Higher Tier, students are also required to use quantity of reactants in terms of moles and units for rate of reaction in mol/s.

    While basic rate calculations apply generally, Higher Tier candidates must also calculate rates using amounts in moles.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 2 of 2
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    While basic rate calculations apply generally, Higher Tier candidates must also calculate rates using amounts in moles. The amount in moles is found from the mass of a substance using moles = mass ÷ relative formula mass, or from a concentration and volume for a solution. The rate is then the change in the number of moles divided by the time taken, giving units of moles per second (mol/s). For example, if 0.20 mol of a reactant is used up in 40 s, the average rate is 0.20 mol ÷ 40 s = 0.005 mol/s. This approach is useful when comparing reactions involving different substances, because moles measure the number of particles rather than their mass. This specific skill is assessed at Higher Tier only.

    What earns the marks

    • Calculate the amount in moles from mass using the equation: moles = mass ÷ relative formula mass.
    • For a solution, calculate the amount in moles by multiplying concentration by volume.
    • Calculate the rate of reaction in mol/s by dividing the change in the number of moles by the time taken.
    • State that the unit mol/s represents the amount of substance in moles reacting or forming each second.
    • Explain that using moles allows a valid comparison between reactions involving substances with different relative formula masses.

    Common mistakes and how to put them right

    • Writing the rate unit as mol instead of mol/s.

      Put it right by ensuring the division by time is reflected in the final unit.

    • Forgetting to convert mass to moles before dividing by time.

      Put it right by calculating moles first using moles = mass ÷ relative formula mass.

    • Dividing time by the change in moles.

      Put it right by dividing the change in moles by the time taken.

    Exam tips

    • Write down the moles calculation as a separate step before the rate calculation so your working is clear for method marks.
    • Always check that the time is in seconds before dividing, converting minutes to seconds by multiplying by 60 if necessary.

    Try the lesson

    The start of the Calculating rates of reactions: For the Higher Tier, students are also required to use quantity of reactants in terms of moles and units for rate of reaction in mol/s. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A chemist measures the rate of a reaction by recording the quantity of a reactant consumed in moles and the time taken in seconds. What is the correct unit for the rate of this reaction?

    Listen

    While basic rate calculations apply generally, Higher Tier candidates must also calculate rates using amounts in moles. The amount in moles is found from the mass of a substance using moles = mass ÷ relative formula mass, or from a concentration and volume for a solution. The rate is then the change in the number of moles divided by the time taken, giving units of moles per second (mol/s). For example, if 0.20 mol of a reactant is used up in 40 s, the average rate is 0.20 mol ÷ 40 s = 0.005 mol/s. This approach is useful when comparing reactions involving different substances, because moles measure the number of particles rather than their mass. This specific skill is assessed at Higher Tier only. A strong answer earns marks for these points. Calculate the amount in moles from mass using the equation: moles = mass ÷ relative formula mass. For a solution, calculate the amount in moles by multiplying concentration by volume. Calculate the rate of reaction in mol/s by dividing the change in the number of moles by the time taken. State that the unit mol/s represents the amount of substance in moles reacting or forming each second. A common slip: Writing the rate unit as mol instead of mol/s. Instead, ensuring the division by time is reflected in the final unit. Exam tip: Write down the moles calculation as a separate step before the rate calculation so your working is clear for method marks.

    See it

    A flowchart. Two starting boxes lead to the amount in moles: one is the mass of substance, which goes through moles = mass ÷ relative formula mass; the other is the concentration and volume of solution, which goes through moles = concentration × volume. Both feed into a box for the amount in moles, which leads to the change in number of moles. Separately, a box for the time taken in seconds and the change in number of moles both lead into rate = change in moles ÷ time taken, which produces the rate in mol/s.
    Shows the two routes to finding the amount in moles and how the change in moles divided by time gives the rate in mol/s.

    Key words

    Say what each word means before you turn the card.

    Mole calculations

    Try answering aloud or in your head before turning the card.

    Rates of reaction

    Try answering aloud or in your head before turning the card.

    Units and dimensional reasoning

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    See every definition
    Mole calculations
    The amount in moles is found from the mass of a substance using moles = mass ÷ relative formula mass, or from a concentration and volume for a solution.
    Rates of reaction
    The rate is the change in the number of moles divided by the time taken, giving units of moles per second (mol/s).
    Units and dimensional reasoning
    The unit mol/s represents the amount of substance in moles reacting or forming each second.

    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 flowchart. Two starting boxes lead to the amount in moles: one is the mass of substance, which goes through moles = mass ÷ relative formula mass; the other is the concentration and volume of solution, which goes through moles = concentration × volume. Both feed into a box for the amount in moles, which leads to the change in number of moles. Separately, a box for the time taken in seconds and the change in number of moles both lead into rate = change in moles ÷ time taken, which produces the rate in mol/s.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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