How do we measure the rate of a reaction, and how do catalysts speed reactions up?
Measuring rates and catalysts: the core practicals measuring rate by gas volume and by a colour change, calculating rate from a graph, and how catalysts work by lowering the activation energy.
A focused answer to Edexcel GCSE Chemistry topic 7, covering the core practicals that measure the rate of reaction by the volume of gas produced and by the time for a precipitate to obscure a cross, how to calculate rate from a graph including the gradient of a tangent, and how catalysts increase rate by providing a pathway with a lower activation energy.
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What this dot point is asking
Edexcel wants you to describe the core practicals that measure the rate of reaction by the volume of gas produced and by the time for a precipitate to hide a cross, calculate the rate from a graph (including using a tangent), and explain how catalysts increase rate by lowering the activation energy. The rate calculation and the catalyst explanation are reliable marks.
Measuring rate by gas volume (core practical)
When a reaction produces a gas, you can measure the rate by collecting the gas and recording its volume over time.
- React, for example, marble chips with hydrochloric acid in a flask connected to a gas syringe (or measure mass loss on a balance).
- Record the volume of gas every few seconds.
- Plot volume against time: the curve is steepest at the start and levels off when the reaction stops.
A steeper curve means a faster reaction; the line becomes horizontal when a reactant runs out.
Measuring rate by a colour change (core practical)
The reaction of sodium thiosulfate with hydrochloric acid produces a cloudy yellow precipitate of sulfur. The rate is found by timing how long a cross drawn under the flask takes to disappear from view.
- Mix the solutions over a paper cross and start a timer.
- Stop the timer when the cross can no longer be seen through the cloudiness.
- A shorter time means a faster rate. Repeat at different temperatures or concentrations to investigate a factor.
Calculating rate from a graph
The mean rate over a period is the change in the measured quantity divided by the time:
The rate at a particular instant is the gradient of the curve at that point, found by drawing a tangent to the curve and calculating its gradient (change in y divided by change in x). The gradient is steepest at the start, confirming the rate is fastest then because the concentration is highest.
How catalysts work
By lowering the activation energy, a catalyst means a greater proportion of collisions have enough energy to be successful, so the rate increases. Because the catalyst is not consumed, only a small amount is needed and it can be reused. Different reactions need different catalysts; for example, iron is the catalyst in the Haber process.
Try this
Q1. State two ways to measure the rate of a reaction that produces a gas. [2 marks]
- Cue. Measure the volume of gas with a gas syringe, or measure the loss of mass on a balance.
Q2. A reaction produces cm of gas in s. Calculate the mean rate in cm/s. [2 marks]
- Cue. cm/s.
Q3. Explain how a catalyst increases the rate of a reaction. [2 marks]
- Cue. It provides an alternative pathway with a lower activation energy, so more collisions are successful.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksIn a reaction that produces a gas, cm of gas is collected in the first seconds, after which no more gas is produced. Calculate the mean rate of reaction over the first seconds in cm/s, and explain why the rate is fastest at the start of the reaction.Show worked answer →
A 4-mark rate-calculation and reasoning question.
Mean rate change in volume time cm/s (1 mark for the method, 1 mark for the value with units). The rate is fastest at the start because the concentration of the reactants is highest then (1 mark), so there are the most frequent collisions per second; as the reactants are used up, the rate decreases (1 mark).
Markers reward the correct rate with units and linking the fastest initial rate to the highest starting concentration.
Edexcel 20213 marksExplain how a catalyst increases the rate of a reaction, and state why only a small amount of catalyst is needed.Show worked answer →
A 3-mark catalyst-explanation question.
A catalyst provides an alternative reaction pathway with a lower activation energy (1 mark), so a greater proportion of collisions have enough energy to be successful, and the rate increases (1 mark). Only a small amount is needed because the catalyst is not used up in the reaction, so it can be used over and over again (1 mark).
Markers reward "lower activation energy" and "not used up", which are the two essential catalyst facts.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)