Contents
1. What is Rate of Reaction?
The change in amount of reactant used up or product formed per unit time.
A reaction is fast if it is complete in a short time. Reactions can be monitored by measuring: loss in mass (if gas is produced), volume of gas collected, change in colour or turbidity, change in pH, or change in conductivity.
2. Collision Theory
Reactions occur when particles collide with sufficient energy (at least the activation energy) and with the correct orientation. The rate depends on:
- Collision frequency — how often particles collide per second.
- Activation energy — the minimum energy needed for a collision to result in a reaction.
The minimum energy that colliding particles must have for the collision to result in a chemical reaction.
Only a fraction of all collisions have sufficient energy to react. Increasing temperature or using a catalyst increases the proportion of successful collisions.
3. Factors Affecting Rate
| Factor | Change | Effect on rate | Explanation |
|---|---|---|---|
| Concentration | Increase | Increases | More particles per unit volume → higher collision frequency |
| Pressure (gases) | Increase | Increases | Gas particles closer together → higher collision frequency |
| Temperature | Increase | Increases | Particles move faster → more frequent AND more energetic collisions → greater proportion exceed activation energy |
| Surface area | Increase (smaller particles) | Increases | More surface exposed → more particle collisions per second |
| Catalyst | Add catalyst | Increases | Provides alternative pathway with lower activation energy |
Increasing temperature increases rate because (1) particles move faster so collision frequency increases, AND (2) a greater proportion of particles have energy ≥ activation energy. Stating only one point loses a mark.
4. Catalysts
A substance that increases the rate of a reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy.
- A catalyst is not consumed — it can be recovered chemically unchanged after the reaction.
- A catalyst does not change the products formed or the energy change of the reaction.
- It increases the proportion of collisions that are successful by lowering the activation energy threshold.
| Reaction | Catalyst used |
|---|---|
| Decomposition of H₂O₂ | Manganese(IV) oxide (MnO₂) |
| Haber process (N₂ + H₂ → NH₃) | Iron |
| Contact process (SO₂ → SO₃) | Vanadium(V) oxide (V₂O₅) |
| Catalytic converter (car exhaust) | Platinum / Palladium / Rhodium |
5. Measuring Rate
Gas collection method
Collect gas in a gas syringe or inverted measuring cylinder over water. Record volume every 30 seconds. Plot volume vs time — the gradient at any point gives the rate. Rate is fastest at the start (steepest gradient) and decreases as reactants are consumed.
Loss in mass method
Place the reaction flask on a balance. Record mass every 30 seconds. As gas escapes, mass decreases. Plot mass vs time. Rate = gradient of curve (negative, so take magnitude).
Draw a tangent to the curve at the point of interest. Rate = gradient of tangent = Δy ÷ Δx. The reaction is complete when the curve becomes horizontal (no more change in mass or volume).
- Four factors increasing rate: higher concentration, higher temperature, larger surface area (smaller particles), catalyst.
- Temperature is most powerful factor: doubles rate roughly every 10 degC rise (more collisions AND more exceed Ea).
- Catalyst: lowers activation energy. Not used up. Same products and yield, just faster.
- Measuring rate: (1) gas produced - measure volume vs time. (2) precipitate - measure turbidity. (3) mass loss - continuous weighing.
- Rate = 1 / time (for experiments using time to completion). Units: 1/s or s-1.
- Increasing concentration/temperature: more frequent collisions AND more successful collisions (exceed Ea).
6. Common Exam Traps
For concentration and surface area, explain "more collisions per unit time" (collision frequency). For temperature, also explain "greater proportion of particles have energy ≥ activation energy". Both are needed for full marks on temperature questions.
"The catalyst is used up during the reaction" is always wrong. A catalyst is chemically unchanged at the end — this is part of the definition.
Only a catalyst changes the activation energy (lowers it). Increasing concentration increases collision frequency — it does not change the activation energy or the proportion of particles above it (at constant temperature).
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.