AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to carry out rate calculations for chemical…
Full specification wording
The human digestive system: Students should be able to carry out rate calculations for chemical reactions.
Rate calculations quantify how quickly a reaction proceeds.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 4 of 25
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Rate calculations quantify how quickly a reaction proceeds. In enzyme practicals, rate can be found by measuring the amount of product formed or reactant used over a measured time. A common method mixes enzyme and substrate, then records the volume of gas produced or the loss in mass at intervals. Rate is calculated by dividing the change in amount by the time taken, for example rate = volume of gas ÷ time, giving units such as cm³ s⁻¹. Alternatively, rate can be found from the gradient of a tangent drawn to a curve of amount against time. Choosing the initial, steepest part of the curve gives the initial rate.
What earns the marks
- Identify the change in amount of reactant used or product formed from the measurements.
- Identify the time interval over which that change occurred.
- Calculate rate using rate = change in amount ÷ time, including correct units such as cm³ s⁻¹ or g min⁻¹.
- Determine rate from the gradient of a tangent drawn to a graph of amount against time.
- Use the initial, steepest part of the curve to find the initial rate of reaction.
- Control variables such as temperature, pH and substrate concentration so rate comparisons are valid.
- Repeat measurements and calculate a mean to improve reliability of the calculated rate.
Common mistakes and how to put them right
Dividing time by amount instead of amount by time.
Put it right: Rate = change in amount ÷ time.
Using the total time rather than the time interval for the chosen change.
Put it right: Use the time over which that specific change occurred.
Omitting units or mixing units.
Put it right: State units consistently, for example cm³ s⁻¹ when volume is in cm³ and time in seconds.
Exam tips
- Write the equation rate = change in amount ÷ time before substituting numbers to reduce errors.
- Check that units are consistent, converting minutes to seconds or grams to kilograms where needed.
- When reading a graph, draw the tangent at the steepest point and show the triangle used for the gradient.
Try the lesson
The start of the The human digestive system: Students should be able to carry out rate calculations for chemical reactions. lesson, made from checked teaching notes for this specification point.
Have a go first
During an investigation into enzyme activity, 48 cm³ of oxygen gas is collected over a period of 12 seconds. What is the mean rate of this reaction?
Listen
Rate calculations quantify how quickly a reaction proceeds. In enzyme practicals, rate can be found by measuring the amount of product formed or reactant used over a measured time. A common method mixes enzyme and substrate, then records the volume of gas produced or the loss in mass at intervals. Rate is calculated by dividing the change in amount by the time taken, for example rate = volume of gas ÷ time, giving units such as cm³ s⁻¹. Alternatively, rate can be found from the gradient of a tangent drawn to a curve of amount against time. Choosing the initial, steepest part of the curve gives the initial rate. A strong answer earns marks for these points. Identify the change in amount of reactant used or product formed from the measurements. Identify the time interval over which that change occurred. Calculate rate using rate = change in amount ÷ time, including correct units such as cm³ s⁻¹ or g min⁻¹. Determine rate from the gradient of a tangent drawn to a graph of amount against time. A common slip: Dividing time by amount instead of amount by time. Instead, rate = change in amount ÷ time. Exam tip: Write the equation rate = change in amount ÷ time before substituting numbers to reduce errors.
Key words
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Rate of reaction
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Gradient of a tangent
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Control variables
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Units
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See every definition
- Rate of reaction
- Calculated by dividing the change in amount by the time taken, for example rate = volume of gas ÷ time.
- Gradient of a tangent
- A way to find rate from a curve of amount against time, using the initial, steepest part of the curve for the initial rate.
- Control variables
- Variables such as temperature and pH that are controlled so rate comparisons are valid.
- Units
- Stated consistently, such as cm³ s⁻¹ or g min⁻¹, when calculating rate.
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