How do we measure the rate of a reaction and interpret a rate graph?
Methods of measuring rate by gas volume, mass loss or a colour change, the required practical, and interpreting rate graphs.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on measuring reaction rate, covering the methods using gas volume, mass loss or a colour change, the required practical, and how to read and compare rate graphs.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to describe how to measure the rate of a reaction, carry out the required practical, and interpret rate graphs.
Methods of measuring rate
The method chosen depends on the reaction: a reaction that makes a gas suits the gas syringe or balance, while a reaction that turns cloudy suits the disappearing-cross method.
The required practical
A standard practical uses sodium thiosulfate and hydrochloric acid, which react to form a cloudy yellow precipitate of sulfur. The flask is placed over a cross drawn on paper, and the time for the cross to disappear is measured. Repeating with different concentrations (keeping volume and temperature constant) shows how concentration affects the rate: the higher the concentration, the shorter the time, so the faster the rate. The rate can be taken as .
Interpreting rate graphs
A graph of product (or gas volume) against time has a characteristic shape:
- It is steepest at the start, where the rate is fastest because the reactant concentration is highest.
- It gradually becomes less steep as the reactants are used up.
- It levels off (flattens) when the reaction is complete and no more product forms.
A steeper line means a faster reaction. If two reactions use the same amounts of reactant, their graphs level off at the same height, because they make the same total amount of product, even if one is faster.
Keeping the experiment fair
When investigating one factor, every other variable must be kept the same so the test is fair. For the thiosulfate practical, if you are changing the concentration you must keep the volume of solution, the temperature, and the size of the cross constant, and always judge the cross disappearing in the same way. Controlling these variables means any change in the time can be put down to the concentration alone. Repeating each measurement and taking a mean improves reliability. Examiners reward naming the control variables and explaining why they matter.
Finding the rate from a graph
The rate at any moment equals the steepness (gradient) of the line at that point. At the start, the line is steep, so the rate is high; later it flattens, so the rate is low. To estimate the rate over a period, divide the change in the measured quantity (such as gas volume) by the time taken, giving units such as cm3 per second. Being able to describe how the rate changes by referring to the changing steepness of the curve, and to read values off the axes, is a key data-handling skill in this topic.
Try this
Q1. Name a piece of apparatus used to measure the volume of gas given off. [1 mark]
- Cue. A gas syringe.
Q2. On a graph of gas volume against time, what does a steeper line show? [1 mark]
- Cue. A faster rate of reaction.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how you could investigate the effect of concentration on the rate of the reaction between sodium thiosulfate and hydrochloric acid.Show worked answer →
A Unit 2 practical question worth 4 marks. Reward: mix the two solutions in a flask placed over a cross drawn on paper (1); time how long the cross takes to disappear as the cloudy sulfur forms (1); repeat with different concentrations of thiosulfate, keeping the volume and temperature the same (a fair test) (1); a shorter time means a faster rate, so plotting rate against concentration shows the effect (1). Markers credit the cross method, timing, varying concentration fairly and linking time to rate. A common error is to change more than one variable.
WJEC style3 marksA graph shows the volume of gas given off against time for two reactions, A and B. A is steeper at the start and levels off sooner. Compare the two reactions.Show worked answer →
A Unit 2 graph question. Reward: reaction A has a faster rate because its line is steeper at the start (1); A finishes sooner (levels off earlier) (1); both produce the same final volume of gas if the same amounts of reactant are used, shown by the lines reaching the same height (1). Markers credit the steeper line meaning faster rate, A finishing first, and the same final amount. A common error is to say A makes more gas overall.
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