6.1.4Chemical Reactions
Explaining Rates Using Collision Theory — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 4 of 10 Unit quiz
Section 1 of 7The idea behind collision theory (HT only)
Reaction needs collisions
- Reaction needs collisions Particles can only react when they collide.
- The more frequent the collisions, the faster the reaction.
Collisions must be successful
- Collisions must be successful A successful collision has at least the activation energy, so the particles have enough energy to break bonds.
- Only a fraction of collisions are successful; the rest simply bounce apart.
Rate depends on two things
- Rate depends on two things The rate rises if the number of collisions per second increases,
- or if a larger proportion of collisions have enough energy.
Section 2 of 7Factors that change the number of collisions (HT only)
More particle collisions per second
Concentration
- Concentration A more concentrated solution has more reactant particles in each unit of volume, so collisions are more frequent.
- Doubling the particles doubles the collisions per second.
Pressure (gases)
- Pressure (gases) Raising the pressure squeezes the same number of gas particles into a smaller volume.
- They are closer together, so collisions happen more often.
Surface area
- Surface area Breaking a solid into smaller pieces exposes more particles to the other reactant, so more collisions take place each second.
- Powders react faster than lumps.
Section 3 of 7Temperature and catalysts (HT only)
| Factor | What happens to the particles | Effect on collisions |
|---|---|---|
| Higher temperature | They gain kinetic energy and move faster | More collisions per second, and a larger proportion have at least the activation energy |
| Catalyst | An alternative pathway with a lower activation energy is provided | A larger proportion of collisions are successful, though the collision frequency is unchanged |
Section 4 of 7Writing a good explanation
1
State what has changed, for example 'the concentration is doubled'.
2
Say what that does to the particles: 'there are more particles per unit volume'.
3
Link this to collisions: 'so there are more frequent collisions between reactant particles'.
4
Finish with the outcome: 'so the rate of reaction increases'.
5
For temperature always add both effects: the particles move faster and collide more often, and more of them have energy greater than or equal to the activation energy.
Section 5 of 7Further points (HT only)
Temperature is special
- Temperature is special Only temperature increases the energy of the collisions.
- Concentration, pressure and surface area change only how often collisions occur.
- A rough guide for solutions and gases is that the rate doubles for each 10 °C rise, so a small temperature change has a large effect.
Catalysts
- Catalysts A catalyst is not used up and only a small amount is needed.
- Iron is the catalyst in the Haber process, and enzymes are biological catalysts that work best within a limited temperature and pH range.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Collision theory | The idea that reactions occur when particles collide with enough energy. |
| Activation energy | The minimum energy colliding particles must have for a reaction to occur. |
| Catalyst | A substance that speeds up a reaction without being used up, by providing a pathway with lower activation energy. |
| Rate of reaction | How quickly reactants are used up or products are formed. |
| Surface area | The total area of solid exposed and available to react. |
Section 7 of 7Exam technique
1
Saying 'more collisions' without adding 'per second' or 'more frequent' collisions.
2
Claiming that a higher concentration makes the particles move faster; it only increases the number per unit volume.
3
Explaining temperature only by more collisions; the greater energy of the collisions is the more important effect.
4
Saying a catalyst increases the energy of the particles; it lowers the activation energy instead.
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