Reaction rates, collision theory and catalysts

Collision theory

Particles react only when they collide with energy at least equal to the activation energy, Ea (the minimum energy needed for a reaction), and in the correct orientation.

• Higher concentration or pressure: more particles per unit volume, so more frequent collisions.
• Larger surface area: more particles exposed, so more frequent collisions.
• Higher temperature: particles move faster, and more importantly, a larger proportion have energy at least Ea.

Measuring rate

Rate is the change in amount or concentration per unit time. Follow a reaction by gas volume, mass loss, colour change or a precipitate.

• For a fixed end point (like a cross disappearing), rate is proportional to 1 / time.
• From a concentration-time graph, the rate at any moment is the gradient of the tangent at that time.

Maxwell-Boltzmann distribution

The curve shows the number of molecules with each energy. It starts at the origin, has no maximum energy and never touches the axis at high energy. The area under the curve is the total number of molecules. At a higher temperature the peak moves right and lower, and the area beyond Ea is much bigger, so many more collisions are successful.

Catalysts

A catalyst increases the rate by providing an alternative reaction route with a lower activation energy, and is chemically unchanged at the end. On a reaction profile the catalysed hump is lower; on a Maxwell-Boltzmann curve Ea moves left so more molecules exceed it. Catalysts cut energy use and costs in industry.

Worked example

A cross disappears after 40 s at 20 °C and after 10 s at 40 °C. Compare the rates.

1. Rate is proportional to 1/time.
2. 1/40 = 0.025 /s; 1/10 = 0.10 /s

Answer: The rate at 40 °C is four times the rate at 20 °C.

Check yourself

1. A reaction reaches a fixed end point in 25 s. Calculate 1/time.
   Show answer

   0.040 /s. 1/25 = 0.040 /s.

2. The concentration of a reactant falls from 0.100 to 0.040 mol/dm³ in 30 s. Find the mean rate.
   Show answer

   0.0020 mol/dm³/s. 0.060 / 30 = 0.0020 mol/dm³/s.

3. What is the main reason a temperature rise increases the rate?
   • Particles get larger
   • A much larger proportion of particles have energy at least equal to the activation energy
   • The activation energy decreases
   • There are more particles
   Show answer

   A much larger proportion of particles have energy at least equal to the activation energy. Collision frequency rises a little, but the fraction of successful collisions rises far more.

4. How does a catalyst increase the rate?
   • It increases the energy of the particles
   • It shifts the equilibrium to the right
   • It provides an alternative route with a lower activation energy
   • It increases the concentration
   Show answer

   It provides an alternative route with a lower activation energy. More molecules can now react on collision.

5. On a Maxwell-Boltzmann distribution at a higher temperature, the peak is
   • lower and further to the right
   • higher and further to the left
   • the same height but to the right
   • higher and further to the right
   Show answer

   lower and further to the right. The total area (number of molecules) is the same, so a wider spread means a lower peak.

6. What does the total area under a Maxwell-Boltzmann curve represent?
   • The activation energy
   • The rate of reaction
   • The total number of molecules
   • The enthalpy change
   Show answer

   The total number of molecules. Each molecule appears somewhere under the curve.

7. Why does powdered marble react faster with acid than a lump of the same mass?
   • It has a larger surface area, so collisions are more frequent
   • It has a lower activation energy
   • Its particles have more energy
   • It is a catalyst
   Show answer

   It has a larger surface area, so collisions are more frequent. More particles are exposed to the acid.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.