Rates of Reaction
What is rate of reaction?
Rate of reaction measures how quickly reactants are converted to products. It can be expressed as:
Rate = change in amount of reactant or product ÷ time
In practice, rate can be followed by measuring:
- Volume of gas produced over time (e.g. CO₂ from CaCO₃ + HCl — syringe or inverted burette).
- Change in mass (e.g. loss of CO₂ from an open flask on a balance).
- Change in colour/turbidity (e.g. thiosulfate + HCl — sodium thiosulfate + HCl → S precipitate, time for cross to disappear).
- Change in pH or conductance.
A rate-time graph shows rate on the y-axis and time on x-axis. A steeper gradient = faster rate.
Collision theory
For a reaction to occur, reactant particles must:
- Collide with each other.
- Collide with sufficient energy (at least the activation energy, Eₐ) — this is an effective collision.
- Collide with the correct orientation (important for complex molecules).
Rate depends on the frequency of effective collisions.
Factors affecting rate
1. Concentration (solutions)
Higher concentration → more particles per unit volume → more frequent collisions → faster rate.
2. Pressure (gases)
Higher pressure → gas molecules closer together → more frequent collisions → faster rate.
3. Temperature
Higher temperature → particles have more kinetic energy → move faster → collide more frequently AND with more energy (more collisions exceed Eₐ) → faster rate.
4. Surface area
Smaller particle size (powder vs lump) → larger surface area exposed → more collisions possible with reactant particles → faster rate.
5. Catalyst
A catalyst provides an alternative reaction pathway with a lower activation energy. More particles now have enough energy to react → faster rate. The catalyst is not consumed overall.
CCEA practical: effect of surface area (marble chips + HCl) Use large chips, small chips, and powdered CaCO₃ with excess HCl. Monitor CO₂ volume collected every 30 s. Powder gives steepest gradient; same total volume when reaction is complete (same moles of reactant).
Catalysts
- Homogeneous catalyst: same phase as reactants (e.g. H⁺ ions in ester hydrolysis).
- Heterogeneous catalyst: different phase (usually solid catalyst, liquid/gas reactants — e.g. Fe in Haber process, Pt in contact process).
- Enzymes: biological catalysts; proteins. Very specific; work best at optimum temperature and pH.
Interpreting rate graphs
On a volume-of-gas vs time graph:
- Steeper initial gradient = faster initial rate.
- Horizontal line = reaction finished (all reactant used up).
- Changing concentration or surface area changes the gradient but the final volume is the same (if limiting reactant is the same amount).
- Changing temperature also changes gradient and total time, but same final volume.
- Adding a catalyst → steeper curve, same final volume.
⚠Common mistakes— Common mistakes (CCEA)
- Saying "particles move faster" for concentration — concentration does NOT make particles move faster; it increases the frequency of collisions.
- Confusing "more collisions" with "harder collisions" — temperature increases BOTH frequency and energy; concentration increases only frequency.
- Stating a catalyst is "used up" — it is not; it reforms at the end of the catalytic cycle.
AI-generated · claude-opus-4-7 · v3-ccea-chemistry