Rate of Reaction (C6.1)
Calculating rate
rate of reaction = amount of reactant used (or product formed) ÷ time
Units depend on what is measured: mol/s, g/s, cm³/s.
From a graph: rate at any instant = gradient of the tangent to the curve at that point.
- Initial rate = steepest gradient at t = 0.
- Rate decreases over time (reactant concentration falls).
Collision theory
For a reaction to occur:
- Reactant particles must collide.
- Collisions must have sufficient energy ≥ Eₐ (activation energy).
A successful collision = one with enough energy + correct orientation.
Rate of reaction increases when:
- Collision frequency increases, OR
- Proportion of successful collisions increases.
Factors affecting rate
| Factor | Effect | Explanation (collision theory) |
|---|---|---|
| Concentration ↑ | Rate ↑ | More particles per volume → more frequent collisions |
| Temperature ↑ | Rate ↑ | Particles have more kinetic energy → more frequent AND more energetic collisions → more exceed Eₐ |
| Surface area ↑ | Rate ↑ | More exposed particles → more frequent collisions |
| Catalyst | Rate ↑ | Lowers Eₐ → greater proportion of collisions have enough energy to react |
| Pressure ↑ (gases) | Rate ↑ | Particles closer together → more frequent collisions |
Catalysts
A catalyst increases rate of reaction without being used up (remains unchanged at end). It provides an alternative reaction pathway with lower activation energy.
Heterogeneous catalyst: different phase from reactants (e.g. iron catalyst in Haber process — solid in gas phase). Reactants adsorb to surface → reaction occurs → products desorb.
Homogeneous catalyst: same phase as reactants (e.g. acid catalyst in esterification).
Enzymes are biological catalysts — protein molecules; very specific (lock-and-key model); optimum temperature (~37°C in humans); denatured above ~40°C; affected by pH.
Required practical: investigating rate
Method 1 — loss of mass (CO₂ gas evolved):
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Place marble chips + acid on balance; measure mass every 30 s.
Method 2 — gas volume collected:
Collect CO₂ in an inverted measuring cylinder or gas syringe; record volume vs time.
Method 3 — cross/disappearing method:
Na₂S₂O₃ + H₂SO₄ → S precipitate (makes solution cloudy). Time how long it takes for a cross to disappear under the flask.
Common exam errors
- Saying a catalyst "provides energy" — it lowers activation energy, not provides energy.
- Saying increasing temperature just "gives particles more energy" — also increases frequency of collisions AND proportion with enough energy.
- Forgetting that rate decreases over time in a closed system (reactants consumed).
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