What is the rate of a reaction, and how does collision theory explain it?
The rate of reaction as how fast reactants are used or products form, and collision theory in terms of frequency and energy of collisions.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on reaction rates, covering what the rate of reaction means and how collision theory explains rate in terms of the frequency and energy of collisions.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to explain what the rate of a reaction means and use collision theory to explain how reactions happen.
What the rate of reaction means
A fast reaction (such as an explosion) finishes in moments; a slow one (such as rusting) takes years. The rate can change during a reaction: it is usually fastest at the start, when there are most reactant particles, and slows as they are used up.
Collision theory
Two things therefore control the rate:
- the frequency of collisions (how often particles collide), and
- the energy of the collisions (whether they have the activation energy).
Anything that increases either of these increases the rate. This single idea explains all the factors that affect rate.
Applying collision theory
- More frequent collisions result from higher concentration, higher pressure (for gases), or a larger surface area, because particles meet more often.
- More energetic collisions result from a higher temperature, because the particles move faster and collide harder, so more collisions reach the activation energy.
- A catalyst provides a different pathway with a lower activation energy, so more collisions are successful.
Why reactions slow down over time
As a reaction proceeds, the reactant particles are gradually used up, so their concentration falls. With fewer particles in the same volume, collisions become less frequent, so the rate decreases. This is why a graph of product formed against time is steepest at the start and gradually levels off as the reaction finishes.
Successful and unsuccessful collisions
Not every collision leads to a reaction. A collision is only successful if the particles hit each other with at least the activation energy; collisions with too little energy simply bounce apart unchanged. At any moment, only a fraction of the particles have enough energy, which is why most collisions are unsuccessful and reactions take time. Anything that increases the fraction of particles with the activation energy (such as heating) or the number of collisions (such as higher concentration) increases the number of successful collisions per second, and so the rate. Thinking in terms of successful collisions per second is the clearest way to answer rate questions.
Calculating an average rate
The rate can be worked out from measurements as a change divided by time. For example, if of gas is collected in , the average rate is . The units depend on what is measured (cm3 of gas per second, or grams of mass lost per second). Because the rate falls over time, this gives the average rate over the period; the rate at a single moment is found from the steepness of the graph at that point. Being able to calculate an average rate from data is a common exam skill.
Try this
Q1. State the minimum energy needed for a collision to cause a reaction. [1 mark]
- Cue. The activation energy.
Q2. Why is the rate of a reaction usually fastest at the start? [1 mark]
- Cue. The concentration of reactants is highest, so collisions are most frequent.
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 marksUsing collision theory, explain why increasing the concentration of a reactant increases the rate of reaction.Show worked answer →
A Unit 2 explain question worth 4 marks. Reward: for a reaction to happen, particles must collide (1) with enough energy (1); increasing the concentration means there are more particles in the same volume (1), so there are more frequent collisions, increasing the rate (1). Markers credit the need for collisions, the energy requirement, more particles per volume, and more frequent collisions. A common error is to say the particles "move faster" - that is the effect of temperature, not concentration.
WJEC style3 marksState what is meant by the rate of a reaction and give two ways it could be measured.Show worked answer →
A Unit 2 recall question. The rate of reaction is how fast the reactants are used up or the products are formed (1). It could be measured by recording the volume of gas given off over time (1), or the change in mass as a gas escapes, or the time for a colour change or for a cross to disappear (1). Markers credit the definition and any two valid measurement methods. A common error is to give "how much product is made" without the idea of how fast (per unit time).
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