What is the rate of a reaction, and how does collision theory explain it?
Define the rate of reaction, describe how it is measured, and use collision theory to explain how reactions occur.
A focused answer to WJEC GCSE Chemistry topic 1.5, covering what the rate of a reaction means, how rate is measured by following mass or gas volume against time, how to read rate graphs, and how collision theory and activation energy explain reactions.
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What this topic is asking
WJEC topic 1.5 wants you to know what the rate of a reaction means, how it is measured (following a change such as gas volume or mass against time), how to read a rate graph, and how collision theory and activation energy explain why reactions happen and at what speed.
What "rate of reaction" means
Common ways to follow a reaction over time include:
- Volume of gas produced: collect the gas in a gas syringe or over water and record the volume at set times.
- Loss in mass: stand the reaction flask on a balance and record the mass as a gas escapes.
- Change in appearance: time how long a precipitate takes to hide a cross drawn under the flask (a "disappearing cross" experiment).
Reading a rate graph
If you plot the amount of product (or gas volume) against time, the shape tells you about the rate:
Collision theory
For a reaction to happen, particles of the reactants must meet and react.
Not every collision leads to a reaction. A collision is only successful if the particles have at least the activation energy; otherwise they simply bounce apart unchanged. The rate of reaction therefore depends on how often successful collisions happen.
What changes the rate
Anything that increases the frequency of collisions, or the proportion of collisions that have the activation energy, will increase the rate:
- more frequent collisions (higher concentration, pressure or surface area), or
- more energetic collisions (higher temperature), or
- a lower activation energy (a catalyst).
These are explored in detail in the next dot point, but they all come back to the same idea: more successful collisions per second means a faster rate.
Measuring rate accurately
To get reliable rate data you need to control the experiment carefully. You should start timing immediately when the reactants are mixed, take readings at regular time intervals, and keep everything except the variable you are studying constant (same volumes, same temperature, same concentrations). Repeating the experiment and taking a mean reduces the effect of random errors. When you plot the results, the gradient (slope) of the line at any point gives the rate at that moment: a steep gradient early on shows a fast rate, and the gradient gradually falls to zero as the reaction finishes. Comparing two curves on the same axes lets you say which reaction was faster overall and which produced more product.
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 sample3 marksA student follows a reaction that gives off a gas by measuring the volume of gas collected every 30 seconds. Describe how the rate changes during the reaction and explain why, using collision theory.Show worked answer →
A Unit 1.5 structured question. Reward: the rate is fastest at the start and slows down over time, eventually stopping. At the start the concentration of reactant is highest, so there are more frequent collisions per second and the rate is high. As reactants are used up, their concentration falls, collisions become less frequent, and the rate slows until a reactant runs out and the reaction stops. Markers credit fastest at the start, slowing over time, and the link to falling concentration causing fewer collisions per second. A common error is to say the reaction speeds up over time.
WJEC sample2 marksExplain what is meant by the activation energy of a reaction.Show worked answer →
A Unit 1.5 definition question. Reward: the activation energy is the minimum amount of energy that colliding particles must have in order to react (to break bonds and start the reaction). Only collisions with at least this energy are successful. Markers credit minimum energy needed for a collision to lead to reaction. A common slip is to describe it as the energy given out by the reaction, which confuses activation energy with the overall energy change.
Related dot points
- Describe and explain the effect of temperature, concentration or pressure, surface area and catalysts on rate using collision theory.
A focused answer to WJEC GCSE Chemistry topic 1.5, explaining how temperature, concentration or pressure, surface area and catalysts each affect the rate of reaction in terms of collision theory, including how catalysts provide a lower activation energy pathway.
- Describe exothermic and endothermic reactions, draw and interpret reaction profiles, and identify activation energy and overall energy change.
A focused answer to WJEC GCSE Chemistry topic 2.4, covering exothermic and endothermic reactions, drawing and interpreting reaction profile (energy level) diagrams, identifying activation energy and overall energy change, and everyday uses such as hand warmers and cold packs.
- Describe reversible reactions and dynamic equilibrium, and predict the effect of changing temperature, concentration and pressure using Le Chatelier's principle.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering reversible reactions and the symbol used, dynamic equilibrium in a closed system, and how changing temperature, concentration and pressure shifts the position of equilibrium using Le Chatelier's principle.
- Describe the Haber process and the Contact process, including their conditions, and explain why the conditions are a compromise between yield, rate and cost.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering the Haber process for ammonia and the Contact process for sulfuric acid, their reactions and conditions, the use of the products including fertilisers, and why industrial conditions are a compromise.
- Explain bond breaking and bond making in terms of energy and calculate the energy change of a reaction from bond energies.
A focused answer to WJEC GCSE Chemistry topic 2.4, explaining why bond breaking is endothermic and bond making is exothermic, and showing how to calculate the overall energy change of a reaction from given bond energies.
Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)