What is the rate of a reaction, and how do we measure it?
The rate of reaction; how to measure rate by following mass loss or gas volume; calculating mean rate; and finding the rate at a point from a tangent on a graph.
A focused answer to AQA GCSE Chemistry 4.6.1, covering what the rate of a reaction means, methods for measuring it by mass loss or gas volume, calculating the mean rate, and finding the rate at a particular time from the gradient of a graph.
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What this dot point is asking
AQA wants you to define the rate of a reaction, describe how to measure rate by following the loss of mass or the volume of gas produced, calculate the mean rate over a time, and find the rate at a particular moment from the gradient of a graph. The graph skills (mean rate, and tangents for the Higher tier instantaneous rate) are routinely examined on Paper 2.
What rate means
Rate is always a quantity divided by a time, so its units are an amount per second, for example cm of gas per second or grams lost per second.
Measuring rate
A third method follows how long a reaction takes to produce a fixed visible change, such as a cross drawn under a flask becoming hidden as sulfur precipitates (the sodium thiosulfate and acid reaction). Here a shorter time means a faster rate.
Calculating mean rate
For example, if cm of gas forms in s, the mean rate is cm/s. The mean rate over the whole reaction is always lower than the rate at the start, because the reaction slows down as it proceeds.
Rate at a point (Higher)
To find the rate at a particular moment, draw a tangent to the curve at that time and calculate its gradient (the change on the vertical axis divided by the change on the horizontal axis along the tangent). The steeper the curve, the faster the rate.
The rate is fastest at the start, when reactant concentration is highest, and slows as reactants are used up, becoming zero when the reaction finishes (the line becomes flat and horizontal).
Try this
Q1. State two methods for measuring the rate of a reaction that produces a gas. [2 marks]
- Cue. Loss of mass on a balance; volume of gas in a gas syringe.
Q2. A reaction produces cm of gas in s. Calculate the mean rate. [1 mark]
- Cue. cm/s.
Q3. Explain why the rate of reaction is fastest at the start. [2 marks]
- Cue. The reactant concentration is highest at the start, so collisions are most frequent; the rate falls as reactants are used up.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA student measures the volume of gas produced when calcium carbonate reacts with hydrochloric acid. After s, cm of gas has been collected; the reaction finishes after s having produced cm in total. Calculate the mean rate over the first s and the mean rate for the whole reaction, and state why the two values differ.Show worked answer →
A 4-mark Paper 2 question on mean rate.
Mean rate over first s cm/s (1 mark). Mean rate over the whole reaction cm/s (to 2 significant figures) (1 mark, allow the unrounded value). The first-30-seconds rate is higher (1 mark). This is because the reaction is fastest at the start when the reactant concentration is highest, and slows as the reactants are used up (1 mark).
Markers reward correct units (cm/s) and the explanation that rate falls as reactants are consumed.
AQA 20223 marksA student plots a graph of volume of gas (vertical axis) against time (horizontal axis) for a reaction. Describe how to use the graph to find the rate of reaction at exactly s, and state how the gradient of the curve changes as the reaction proceeds.Show worked answer →
A 3-mark Higher question on finding rate from a tangent.
Method (2 marks): draw a tangent to the curve at the point where time s, then calculate the gradient of that tangent (change in volume divided by change in time over a section of the tangent line). The gradient is the rate at that instant.
How the gradient changes (1 mark): the curve is steepest at the start (highest rate) and gets less steep over time, becoming flat (gradient zero) when the reaction finishes.
Markers want the tangent-and-gradient method explicitly, not reading a value straight off the axis.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)