What changes the speed of a chemical reaction, and how do we measure and explain the rate?
The factors that change the rate of a reaction (concentration, temperature, surface area and catalysts), how rate is measured by gas volume or mass loss, how to read a rate graph, and the collision theory that explains it all.
A focused CCEA GCSE Single Award Science answer on rates of reaction, covering the factors that change the rate, how rate is measured by gas volume or mass loss, how to read a rate graph, and the collision theory that explains why each factor works.
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What this dot point is asking
CCEA wants you to state the factors that change the rate of a reaction, describe how rate is measured, interpret a rate graph, and use collision theory to explain why each factor works.
The factors that affect rate
Measuring rate
Two common practical methods:
- Volume of gas produced: collect the gas in a gas syringe and record the volume at set times.
- Loss of mass: stand the flask on a balance and record the falling mass as gas escapes.
A third method times a colour or cloudiness change, for example how long a cross takes to disappear as sulfur forms.
Reading a rate graph
The final height (total gas) depends on the amount of reactant, not the rate.
Collision theory
Each factor works through collisions: more concentration or surface area gives more frequent collisions; higher temperature gives more frequent and more energetic collisions; a catalyst lowers the energy needed, so more collisions succeed.
Examples in context
Example 1. Why flour mills can explode. Flour is a fine powder with an enormous surface area, so it can react with oxygen in the air extremely fast. In a flour mill, a cloud of flour dust can ignite and burn so quickly that it causes an explosion. This dramatic example shows how surface area controls rate, and why such factories control dust so carefully.
Example 2. Why food keeps longer in a fridge. The reactions that spoil food, including the growth of microbes, go faster when warm. Cooling food in a fridge lowers the temperature, so the particles move more slowly and collide less often and less energetically, slowing the spoiling reactions. This is everyday collision theory: lower the temperature and you lower the rate.
Try this
Q1. Name two factors that increase the rate of a reaction. [2 marks]
- Cue. Any two of: higher concentration, higher temperature, larger surface area, a catalyst.
Q2. On a rate graph, what does a flat (horizontal) line show? [1 mark]
- Cue. The reaction has finished.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20204 marksExplain, using collision theory, why increasing the temperature increases the rate of a reaction.Show worked answer →
Four marks for the two effects of temperature on collisions.
At a higher temperature the particles have more kinetic energy and move faster.
So they collide more frequently, giving more chances to react.
They also collide with more energy, so a greater proportion of collisions have enough energy to react (succeed).
Both effects increase the number of successful collisions per second, so the rate increases. Markers reward more frequent collisions and a greater proportion of successful collisions.
CCEA SAS 20194 marksA student measures the volume of gas given off in a reaction every 30 seconds. Describe how the rate changes during the reaction and explain why, using the graph shape.Show worked answer →
Four marks for the trend and the reason.
The reaction is fastest at the start, so the graph is steepest at the start, because the reactants are at their highest concentration and collisions are most frequent.
As the reaction goes on, the reactants are used up, so collisions become less frequent and the rate slows; the graph becomes less steep.
When the reaction is complete the line goes flat (horizontal), because no more gas is produced.
Markers reward steepest at the start, slowing as reactants are used up, and flat when finished.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)