Rate of chemical change
Measuring rates of reaction
The rate of reaction is a measure of how quickly reactants are converted to products.
rate = change in amount (or concentration) of reactant or product ÷ time
Units commonly used in WJEC: cm³/s (gas volume), g/s (mass loss), mol/dm³/s (concentration change).
Practical methods:
- Gas collection: measure volume of gas evolved vs time with a gas syringe or upturned burette over water.
- Loss of mass: place reaction on a balance; CO₂ escapes and mass decreases — record mass vs time.
- Colorimetry: monitor colour change (useful for reactions that change colour).
- Titrimetry: withdraw samples and quench, then titrate to find concentration at each time point.
Graphs: plot product formed (or reactant remaining) vs time. The gradient at any point = rate at that instant. The initial gradient = initial rate. The curve flattens when the reaction stops.
Collision theory
For a reaction to occur, reactant particles must collide with:
- Sufficient energy — at or above the activation energy (Eₐ) threshold.
- Correct orientation — the right parts of the molecules must meet.
Only a small fraction of all collisions are "successful" (lead to reaction).
Activation energy is the minimum energy needed for a reaction to occur. It is the "energy barrier" shown on reaction profile (energy level) diagrams.
Factors affecting rate
| Factor | Effect on rate | Explanation (collision theory) |
|---|---|---|
| Concentration ↑ | Rate ↑ | More particles per unit volume → more frequent collisions |
| Pressure ↑ (gases) | Rate ↑ | Equivalent to increasing concentration of gas |
| Temperature ↑ | Rate ↑↑ | Particles move faster → more frequent collisions AND a greater proportion exceed Eₐ |
| Surface area ↑ | Rate ↑ | More exposed reactant surface → more frequent collisions |
| Catalyst | Rate ↑ | Provides an alternative reaction pathway with lower Eₐ |
Temperature has a particularly large effect because it simultaneously increases collision frequency AND increases the proportion of particles with sufficient energy — often a 10 °C rise approximately doubles the rate.
Catalysts
A catalyst increases the rate of reaction without being used up (it is regenerated). It works by providing an alternative reaction pathway with a lower activation energy.
- The catalyst appears in the mechanism but is recovered unchanged at the end.
- On an energy profile diagram, a catalyst lowers the "hump" (Eₐ) — both forward and reverse reaction are speeded up equally.
- Catalysts do NOT change ΔH (the overall energy change).
Types:
- Homogeneous catalyst: same phase as reactants (e.g. H⁺(aq) catalysing ester hydrolysis).
- Heterogeneous catalyst: different phase from reactants (e.g. Fe(s) catalyst in Haber process, Pt(s) in catalytic converters).
Industrial importance: catalysts reduce energy costs by allowing reactions to proceed at lower temperatures. WJEC examiners expect you to state economic and environmental benefits.
WJEC required practicals for U2.2
- Reaction of marble chips (CaCO₃) with hydrochloric acid — measuring gas volume or mass loss: vary concentration/surface area/temperature; plot volume of CO₂ vs time; compare initial gradients.
- Iodine clock reaction (sodium thiosulfate + HCl, or H₂O₂ + KI): measure time for cross to disappear through sulfur precipitate; 1/time as measure of rate; investigate effect of concentration.
Common examiner traps
- "Particles have more energy at higher temperature" — partially true, but the key mark-scoring point is that a greater proportion of particles exceed the activation energy at higher temperature.
- Catalyst is not consumed — students write "catalyst lowers activation energy and is used up." Never — catalysts are regenerated.
- Surface area vs particle size: increasing surface area means using smaller/powdered particles, not larger chunks.
- Flattening of the rate graph: the reaction does not stop because of "low energy" — it stops because a reactant has been used up (or concentration too low for further reaction under those conditions).
AI-generated · claude-opus-4-7 · v3-wjec-chemistry