Factors affecting the rate of reaction
Five factors change the rate at which a reaction goes:
- Concentration of reactants in solution.
- Pressure of reactant gases.
- Temperature.
- Surface area of solid reactants.
- Presence of a catalyst.
All can be explained by collision theory: reactions happen when particles collide with enough energy (≥ activation energy) and the right orientation.
1. Concentration (solutions)
More concentrated → more particles per unit volume → more frequent collisions → faster rate.
Doubling concentration roughly doubles the rate.
2. Pressure (gases)
Increasing pressure squeezes gas particles closer together — more particles per unit volume → more frequent collisions → faster rate.
Pressure has the same effect on gas reactions as concentration has on solutions.
3. Temperature
Higher temperature → particles move faster AND a greater proportion have ≥ Ea (activation energy). Both effects increase rate dramatically.
A rule of thumb: a 10 °C rise roughly doubles the rate.
4. Surface area (solids)
Smaller pieces (e.g. powder vs lump) expose more atoms to attack → more collision sites → faster rate.
Compare marble chip + acid (slow) vs marble powder + acid (fast bubbles, sometimes dangerous).
5. Catalysts
A catalyst speeds up a reaction without being consumed. It provides an alternative pathway with lower activation energy, so a greater proportion of collisions are successful.
Catalysts are not in the equation — they're written above the arrow: 2H₂O₂ →(MnO₂) 2H₂O + O₂
Collision theory in one sentence
Increasing frequency of collisions OR increasing the proportion of collisions with ≥ Ea both raise the rate.
✦Worked example— Worked example: explaining concentration
A student doubles the concentration of HCl reacting with magnesium. The rate doubles. Why?
- Twice as many H⁺ ions per unit volume.
- Twice as many collisions per second between H⁺ and Mg.
- Same proportion successful, so successful collisions per second is doubled → rate doubles.
✦Worked example— Worked example: explaining temperature
When the temperature is raised by 10 °C, the rate of reaction approximately doubles. Why?
- Particles move faster (more kinetic energy).
- More frequent collisions.
- A greater proportion of particles have ≥ Ea, so a much higher proportion of collisions are successful.
- The second effect dominates.
⚠Common mistakes
- "Particles get bigger when hot." No — they move faster, but their size is unchanged.
- "More concentration = more energy" — no, just more particles per volume.
- Saying "catalysts speed up by giving extra energy" — they don't supply energy; they reduce the energy threshold.
- Forgetting orientation — even high-energy collisions can fail if particles approach the wrong way.
Links
Builds on C5.2 (activation energy and reaction profiles). Sets up C6.3 (catalysts), C6.4 (reversible reactions).
AI-generated · claude-opus-4-7 · v3-deep-chemistry