Calculating rates of reactions — AQA GCSE Chemistry
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Calculating rates of reactions explained
Calculating rates of reactions covers 2 specification points, including The units of rate of reaction may be given as… and For the Higher Tier, students are also….
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Open a point below for its explanation and revision guidance.
The units of rate of reaction may be given as g/s or cm /s.
Rate of reaction measures how quickly a reactant is used up or a product is formed. When the quantity measured is a mass, the rate is calculated by dividing the change in mass by the time taken, giving units of grams per second, written g/s. When the quantity measured is a volume of gas, the rate is calculated by dividing the change in volume by the time taken, giving units of cubic centimetres per second, written cm³/s. The same principle applies to any measured quantity: divide the change by the time. For example, if 24 cm³ of gas is collected in 8 s, the average rate is 24 cm³ ÷ 8 s = 3 cm³/s. Choosing the right unit depends on what is measured during the experiment.
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. 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.
Your focus
- Calculate the rate of reaction from a change in mass or volume and a time interval.
- Select and write the correct unit, g/s or cm³/s, for a calculated rate.
- Interpret the gradient of a rate graph as a measure of reaction rate.
Show all 6 objectives
- Calculate the amount in moles from a given mass or from a concentration and volume.
- Calculate the rate of reaction in mol/s from a change in moles and a specific time interval.
- Explain why using moles allows reactions involving different substances to be compared fairly.
Calculating rates of reactions exam tips
Marking Points
- Rate of reaction is calculated by dividing the change in the measured quantity by the time taken.
- When mass is measured, the rate may be expressed in g/s.
- When volume of gas is measured, the rate may be expressed in cm³/s.
- The unit must match the quantity measured: mass gives g/s and volume gives cm³/s.
- A rate calculated over the whole reaction is an average rate, whereas the gradient of a graph at a point gives the rate at that moment.
- 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.
Examiner Tips
- 💡Always include the unit with a calculated rate, and check it matches the quantity measured.
- 💡Show the division clearly, for example change in volume divided by time, before giving the final value.
- 💡If a graph is provided, remember that the gradient gives the rate, and a steeper gradient means a faster reaction.
- 💡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.
Common Mistakes
- Writing the volume unit as cm/s instead of cm³/s; correct this by using the cubed unit for volume.
- Dividing time by the change in quantity instead of dividing the change in quantity by time; correct this by putting the change in mass or volume on top.
- Using g/s for a volume measurement or cm³/s for a mass measurement; correct this by matching the unit to the quantity measured.
- Writing the rate unit as mol instead of mol/s; correct this by ensuring the division by time is reflected in the final unit.
- Forgetting to convert mass to moles before dividing by time; correct this by calculating moles first using moles = mass ÷ relative formula mass.
- Dividing time by the change in moles; correct this by always dividing the change in moles by the time taken.