⏱️ Rates of Reaction
Secondary School Chemistry • MalawiHub
1. Introduction
The rate of a reaction describes how quickly reactants are changed into products.
Learning Objectives
- Define rate of reaction.
- Explain collision theory.
- Describe factors affecting reaction rate.
- Explain the action of catalysts.
- Interpret reaction-rate graphs.
- Design simple rate experiments.
2. Collision Theory
Particles must collide before a reaction can occur.
However, not every collision produces a reaction. Particles must collide with enough energy and in a suitable orientation.
Particles collide → sufficient energy → bonds break/form → products form.
3. Factors Affecting Rate
| Factor | Effect of increasing it | Reason |
|---|---|---|
| Temperature | Rate increases | Particles move faster and more collisions have enough energy. |
| Concentration | Rate increases | More particles are present in a given volume. |
| Pressure of gases | Rate increases | Gas particles are closer together. |
| Surface area | Rate increases | More particles are exposed for collision. |
| Catalyst | Rate increases | Provides a pathway with lower activation energy. |
4. Temperature
Increasing temperature increases the average kinetic energy of particles.
Particles move faster, collide more frequently and a greater proportion of collisions have enough energy to react.
5. Concentration
Increasing concentration increases the number of reacting particles in a given volume. This increases collision frequency and therefore increases reaction rate.
6. Surface Area
A powdered solid has a larger surface area than a large lump of the same mass.
Therefore, more particles are exposed and collisions occur more frequently.
7. Catalysts
A catalyst increases reaction rate by providing an alternative pathway with lower activation energy.
The catalyst is not permanently consumed in the reaction.
8. Measuring Reaction Rate
Rate can be determined by measuring how quickly a measurable quantity changes.
For example:
Rate = change in quantity ÷ time taken
Rate = 40 ÷ 20
Rate = 2 cm³ s⁻¹
9. Reaction Graphs
On a graph showing product formed against time:
- A steep slope means a fast reaction.
- A gentle slope means a slower reaction.
- A horizontal section means no further measurable change.
The initial rate is usually greatest because reactant concentration is highest at the beginning.
10. MANEB Examination Focus
- Explain collision theory.
- Explain the effect of temperature.
- Explain concentration effects.
- Explain surface-area effects.
- Explain catalysts using activation energy.
- Calculate simple reaction rates.
- Interpret graphs.
- Identify variables in experiments.
Explain why powdered calcium carbonate reacts faster with hydrochloric acid than large pieces of calcium carbonate of the same mass.
The powder has a larger surface area. More particles are exposed to the acid, causing more frequent successful collisions per second. Therefore the reaction is faster.
11. Practice Exercises
- What is meant by rate of reaction?
- State two conditions needed for a successful collision.
- Why does increasing temperature increase reaction rate?
- Why does increasing concentration increase reaction rate?
- Why does powdered solid react faster than a large lump?
- What is the role of a catalyst?
- A reaction produces 60 cm³ of gas in 30 s. Calculate the average rate.
Answers
- The speed at which reactants are converted into products.
- Sufficient energy and suitable orientation.
- Particles move faster and more collisions have enough energy.
- There are more reacting particles per unit volume.
- It has a larger surface area.
- It provides an alternative pathway with lower activation energy.
- 60 ÷ 30 = 2 cm³ s⁻¹.
12. Common Mistakes
- Do not say a catalyst increases the energy of particles.
- Do not confuse collision frequency with successful collisions.
- Remember that increasing temperature affects both collision frequency and collision energy.
- Always include units when calculating rate.
13. Exam Tip
When asked why a factor increases rate, always connect your answer to particle movement, collision frequency and successful collisions.