What controls how fast a chemical reaction goes, and how do we measure it?
Rates of reaction: following the course of a reaction, calculating average rate, and explaining the effects of concentration, particle size, temperature and catalysts using the idea of collisions.
An SQA National 5 Chemistry answer on rates of reaction, covering how a reaction is followed, calculating average rate from data, reading rate graphs, and explaining the effects of concentration, particle size, temperature and catalysts in terms of collisions.
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What this key area is asking
The SQA wants you to follow the course of a reaction using a measurable quantity, calculate the average rate from data, read and interpret a rate graph, and explain the effects of concentration, particle size, temperature and catalysts using the idea of particle collisions. This is an early key area that turns up again whenever you handle experimental data, so it rewards precise wording over vague statements.
Following the course of a reaction
To measure a rate you need something that changes as the reaction goes, and a clock. Common methods are:
- Volume of gas collected, using a gas syringe or an upturned measuring cylinder over water.
- Loss of mass as a gas escapes, read from a balance.
- Change in concentration or colour over time.
- Time for a precipitate to hide a cross drawn under a flask.
Calculating average rate
The average rate is the change in a measured quantity divided by the time taken:
Always state the unit. A rate without a unit usually loses a mark.
Worked example: average rate over an interval
The factors that change rate
A reaction can only happen when reacting particles collide. Anything that makes collisions more frequent, or a greater proportion of them successful, speeds the reaction.
A catalyst is not used up
A catalyst speeds a reaction without being used up. It can be filtered off and weighed at the end and its mass is unchanged. Catalysts are important in industry because a faster reaction at a lower temperature saves energy and money. The catalytic converter in a car uses platinum and rhodium to speed the conversion of harmful exhaust gases into less harmful ones, and the enzymes in living cells are biological catalysts.
Examples in context
In a school lab, the decomposition of hydrogen peroxide is far faster when a little manganese dioxide is added as a catalyst: the gas-collection graph rises much more steeply, yet the total volume of oxygen is unchanged because the catalyst only changes the rate, not the amount of product. The same collision logic explains why flour or custard powder, which is a very fine solid with a huge surface area, can burn explosively when dispersed in air, while a solid lump of the same material simply chars.
Try this
Q1. A reaction produces of gas in . Calculate the average rate. [1 mark]
- Cue. .
Q2. Explain, using collisions, why increasing the concentration of an acid increases the rate of its reaction with a metal. [2 marks]
- Cue. There are more acid particles in a given volume, so collisions are more frequent and more successful collisions happen each second.
Q3. State two pieces of evidence that show a catalyst has not been used up. [2 marks]
- Cue. It can be filtered off at the end, and its mass is unchanged.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2019 style3 marksExcess marble chips were added to dilute hydrochloric acid and the volume of carbon dioxide collected was measured. In the first 20 s, 36 cm cubed of gas was produced. Calculate the average rate of reaction over this time, in cm cubed per second, and state what happens to the rate as the reaction continues.Show worked answer →
Markers reward the formula, the substitution, a value with the correct unit, and the qualitative comment.
Average rate is the change in quantity divided by the change in time:
As the reaction continues the rate decreases. The acid is used up, so its concentration falls, there are fewer acid particles in a given volume, fewer collisions happen each second, and the rate drops until the acid runs out and the graph levels off.
A mark is commonly lost by quoting the rate with no unit, or by saying the rate stays constant.
SQA N5 2017 style3 marksExplain, in terms of collisions, why powdered calcium carbonate reacts faster with dilute acid than the same mass of large lumps, and why warming the acid speeds the reaction further.Show worked answer →
A 3 mark answer needs the surface-area point, the collision-frequency point, and the temperature point.
Powder is broken into many small pieces, so it has a much larger total surface area than large lumps of the same mass. More of the calcium carbonate is exposed to the acid, so more collisions between acid particles and the solid happen each second, and the rate is faster.
Warming the acid gives the particles more kinetic energy, so they move faster and collide more often, and a greater proportion of collisions now have enough energy to react. Both effects increase the number of successful collisions per second, so the rate rises.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)