Calculating rates of reactions — AQA GCSE Chemistry
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Calculating rates of reactions explained
Rate of reaction measures how quickly a reactant is used up or a product is formed.
Read the full explanation
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.
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.
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.
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.
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.