Rate of Reactions
Rate of Reactions hub for G3 Pure / O-Level Chemistry K324 / 6092: measurement, graphs, collision theory, rate factors, catalysts and investigations.
Learning goals
- 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.
Rate of Reactions connects measurement, graph interpretation and particle explanations. Study the four K324 / 6092 lessons below in order.
Jump to: Prerequisites · Quick Reference · Core knowledge · Sub-topics · Quiz
The six core Rate of Reactions outcomes are to interpret rate data, suggest an investigation method, explain rate using collision theory, describe and explain the effects of concentration, gas pressure, particle size, temperature and catalysts, state that catalysts lower activation energy, and name industrial and biological catalyst examples.
Prerequisites
- Particle motion in solids, liquids and gases
- Measurement of time, temperature, mass and volume
- Collection and measurement of gas volume
- Calculate a change, divide values and retain units.
This hub contains only Rate of Reactions topic 10. Redox chemistry and reversible ammonia manufacture are covered in their own topics.
Quick Reference
| Item | Exam-ready rule |
|---|---|
| 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 pressure | particles closer together → more frequent collisions |
| Particle size | smaller particles → larger total surface area → more collisions at the surface |
| Temperature | particles move faster; for K324/6092, also explain that a larger proportion can overcome Eₐ |
| 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; at K324/6092 depth, sufficient energy and suitable orientation are required.
- Industrial catalysts and enzymes are examples of catalysts; enzymes are biological catalysts.
- A catalyst increases rate but does not change the reactant and product energy levels or the final amount determined by starting quantities.
Sub-topics
Core Rate of Reactions
Speed of Reaction
Define rate, choose a measurable change and calculate average rate with units.
Determining Speed of a Chemical Reaction
Use fixed end-points, gas volume, mass loss, gradients and tangents.
Collision Theory
Explain effective collisions, activation energy and how rate factors change collision behaviour.
Factors Affecting Rate of Reaction
Apply concentration, pressure, particle size, temperature and catalyst explanations to data and fair tests.
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.
- Pressure: use it as a rate factor only when reacting particles are gases.
- 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.
Quiz
Check your understanding, continue to guided practice, practise with more questions, then try the structured questions.