5.2.4Additional lessons
Factors Affecting Rates of Reaction — IB Chemistry SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 69 of 98
Part 1 of 5Concentration and Pressure
Each factor changes either how often particles collide or how many have enough energy.
Concentration
- A higher concentration of a reactant in solution means more particles per unit volume,
- so collisions are more frequent and the rate rises.
Gas pressure
- Increasing the pressure of a gas squeezes the molecules closer together, so concentration and collision frequency increase.
No change in energy
- Neither change alters the fraction of particles with enough energy; only the collision frequency is affected.
Limit
- As reactants are used, concentration decreases and the rate slows over time.
Part 2 of 5Surface Area and Temperature
Surface Area and Temperature
Surface area
- Crushing a solid exposes more particles to the other reactant, which increases collision frequency and the rate.
Examples
- Powdered calcium carbonate reacts faster with acid than marble chips of the same mass.
Temperature
- Particles move faster and have more kinetic energy, so there are more frequent collisions.
Greater effect
- The main effect is that a much larger fraction of particles has energy equal to or greater than the activation energy.
Part 3 of 5Catalysts
1
A catalyst is a substance which speeds up a reaction without being permanently used up.
2
It provides an alternative reaction pathway with a lower activation energy.
3
A greater proportion of collisions then has sufficient energy, so the rate increases.
4
The catalyst is regenerated at the end of the reaction, and it does not change the enthalpy change or the amount of product.
Part 4 of 5Summary and Explaining Answers
Summary and Explaining Answers
Structure of answers
- State the change, say how it alters collision frequency or energy, and conclude that the rate increases.
Command words
- "Explain" needs the cause, so refer to collisions and activation energy rather than only saying particles move faster.
Light
- Light can supply energy to start some reactions, such as chlorine with alkanes, by breaking bonds.
Control variables
- To test one factor in an experiment, keep all other factors the same.
| Factor | Effect on collisions | Rate |
|---|---|---|
| ↑ Concentration | More frequent | Increases |
| ↑ Pressure (gas) | More frequent | Increases |
| ↑ Surface area | More frequent | Increases |
| ↑ Temperature | More frequent and more energetic | Increases |
| Catalyst | More collisions have E ≥ Eₐ | Increases |
Part 5 of 5Exam Checklist & Tips
5.2.4 – Key Points for the Exam
- Catalyst A substance which increases the rate of a reaction by providing an alternative pathway of lower activation energy, and is not used up.
- Surface area The total area of solid exposed and available for reaction.
- Concentration The amount of substance per unit volume.
- Activation energy The minimum energy needed for a successful collision.
Common Mistakes
- Saying that a catalyst is not involved in the reaction; it takes part but is regenerated.
- Explaining a temperature effect only through more frequent collisions.
- Stating that a catalyst increases the energy of the particles.
- Saying that more surface area increases the activation energy or the yield.
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