Rate of reactions
Measuring reaction rate, reading rate graphs, collision theory, the factors that change rate, and catalysts for G3 Pure and O-Level Chemistry.
Offline pages
Saved on this device only. Connect at least once every 30 days to confirm Premium. Sign-out or a change of account removes saved copies.
Before you begin
This topic is about how fast reactions go and why. The first lessons define rate, show how to measure it and how to read it from a graph. The next ones use collision theory to explain how concentration, pressure, particle size and temperature change the rate, and finish with catalysts and enzymes.
Be comfortable with: particle motion in solids, liquids and gases, measuring time, temperature, mass and volume, collecting and measuring a gas volume, and calculating a change or a quotient with units.
Learning goals
- Describe how concentration, pressure, particle size and temperature affect reaction rate, and explain each effect using collisions between particles
- Define a catalyst and describe how catalysts, including enzymes, affect reaction rate
- Explain how a pathway with lower activation energy increases reaction rate
- State that some compounds act as catalysts in industrial processes and that enzymes are biological catalysts
- Suggest a method to investigate how a given variable affects reaction rate
- Interpret data from rate of reaction experiments
Syllabus statements covered
- describe the effect of concentration, pressure, particle size and temperature on the rates of reactions and explain these effects in terms of collisions between reacting particles
- define the term catalyst and describe the effect of catalysts (including enzymes) on the rates of reactions
- explain how pathways with lower activation energies account for the increase in rates of reactions (see also 9(b))
- state that some compounds act as catalysts in a range of industrial processes and that enzymes are biological catalysts (see also 8.3(b), 9(b), 10(c) and 12(d))
- suggest a suitable method for investigating the effect of a given variable on the rate of a reaction
- interpret data obtained from experiments concerned with rate of reaction.
Lessons
Work through them in order.
Measuring rate
- Rate of reaction and average rateDefine rate, choose a measurable change and calculate average rate with units.
- Measuring reaction rateChoose fixed end-points, gas volume or mass loss, and plan reliable rate comparisons.
- Reading reaction-rate graphsRead the height and gradient of gas-volume and mass-loss graphs, calculate an average rate over an interval, and find the rate at one time from a tangent.
Collision theory and factors
- Collision theoryExplain effective collisions, activation energy and how rate factors change collision behaviour.
- Factors affecting rate of reactionExplain concentration, gas compression, solid particle size and temperature; distinguish rate from final amount.
- Catalysts and enzymesExplain how a catalyst speeds up a reaction by providing a route with a lower activation energy, name catalysts used in industry, and describe enzymes as biological catalysts.
Practise and check
Or choose
Topic reference
Quick reference
| Idea | What to remember |
|---|---|
| Reaction rate | change in amount of reactant used up or product formed per unit time |
| Average rate | (change in measured quantity)/(time taken) |
| Instantaneous rate | gradient of a tangent to a curved graph at the required time |
| Concentration | more particles per unit volume → more frequent collisions |
| Gas compression | at fixed temperature, the same reacting particles in a smaller volume → more frequent collisions |
| Particle size | smaller particles → larger total surface area → more collisions at the surface |
| Temperature | particles move faster and a larger fraction of collisions can overcome the barrier |
| Catalyst | chemically unchanged at the end; provides an alternative pathway with lower Eₐ |
| Final graph value | shows final amount, not reaction rate |
Core knowledge to remember
- Rate must be linked to a measurable change and a time unit.
- For the same fixed change, a shorter time means a higher average rate.
- On a mass-loss graph, use the magnitude of the negative gradient.
- An effective collision produces products. In the collision model, sufficient energy and a suitable orientation are required.
- Industrial catalysts and enzymes are examples of catalysts; enzymes are biological catalysts.
- For the same overall reaction, a catalyst lowers the barrier without changing reactant and product energy levels. Distinguish product at an early time from the final amount allowed by the starting quantities.
Common mistakes
- Rate versus time: time taken is not itself the definition of rate. Use a fixed change divided by time.
- Rate versus final amount: a steeper graph can reach the same final value.
- Volume versus concentration: more solution does not automatically mean more particles per unit volume.
- Gas pressure: check whether reacting-particle concentration increased. Adding inert gas at fixed volume does not give the same explanation as compressing the reactants.
- Solid particle size: write “larger total surface area” and “more collisions at the surface per unit time”.
- Temperature: it does not lower activation energy. It changes particle kinetic energy and the proportion able to overcome the barrier.
- Catalyst: it lowers activation energy through an alternative pathway; it does not give particles energy or become used up.
- Curved graphs: use a tangent for an instantaneous rate and a wide triangle on the tangent to reduce percentage uncertainty.
- Mass-loss graphs: the gradient is negative, but the reported rate is the positive magnitude of mass lost per unit time.
- Fair tests: change one factor, control the others, repeat measurements and justify how anomalies are treated.