Rate and extent of chemical change — section overview
Section C6 covers how fast chemical reactions occur and how far they go — two separate questions that are both important in industrial chemistry.
Factors affecting rate of reaction
Rate increases when you increase:
| Factor | Explanation | Particle collision theory |
|---|---|---|
| Temperature | Particles move faster | More frequent AND more energetic collisions; more exceed activation energy |
| Concentration (solution) | More particles per unit volume | More frequent collisions |
| Pressure (gas) | More particles per unit volume | More frequent collisions |
| Surface area (solid) | More particles exposed | More frequent collisions |
| Catalyst | Lowers activation energy | More particles have sufficient energy |
Collision theory
For a reaction to occur:
- Particles must collide
- With sufficient energy (≥ activation energy)
- In the correct orientation
Increasing temperature has a GREATER effect than just increasing collision frequency — the proportion of particles with energy ≥ Ea increases significantly.
Measuring reaction rate
Rate = change in amount of reactant or product ÷ time
Methods:
- Gas collection — measure volume of gas produced over time
- Mass loss — measure mass decrease as gas escapes
- Colorimetry — measure colour change for coloured reactions
- Turbidity — measure time for precipitate to form (cross disappears)
Reversible reactions and equilibrium
Some reactions are reversible: reactants → products AND products → reactants.
Written with ⇌ symbol: A + B ⇌ C + D
Dynamic equilibrium: rate of forward reaction = rate of reverse reaction. Concentrations remain constant but are not necessarily equal.
Le Chatelier's principle (HT)
If conditions change, the equilibrium shifts to oppose the change:
| Change | Equilibrium shift | Example |
|---|---|---|
| Increase temperature | Towards endothermic direction | Haber process: lower temp → more NH₃ but slower |
| Increase pressure | Towards side with fewer moles of gas | |
| Increase concentration of reactant | Towards products |
Common exam mistakes in C6
- Rate and equilibrium position are different things — a catalyst increases rate but does not shift equilibrium
- Le Chatelier's: temperature increase shifts towards endothermic direction — always; the examiners test this every year
- Measuring rate from a graph — rate = gradient of the curve at that point; early = steeper (faster rate)
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