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8873 / 2027
Reaction Kinetics

Lesson 2 of 3

Why rates change

Compare collisions, temperature and catalytic pathways.

A-Level 8873, revised syllabus (2026-2027)

  • I can explain concentration effects.Syllabus

    Syllabus 8873, 7(d). Explain concentration effects.

  • I can explain activation energy using a distribution.Syllabus

    Syllabus 8873, 7(e). Explain activation energy using a distribution.

  • I can explain temperature effects on k and rate.Syllabus

    Syllabus 8873, 7(f). Explain temperature effects on k and rate.

  • I can explain the catalytic effect.Syllabus

    Syllabus 8873, 7(g). Explain the catalytic effect.

  • I can explain heterogeneous catalytic action.Syllabus

    Syllabus 8873, 7(h). Explain heterogeneous catalytic action.

Separate more collisions from a larger successful fraction

Concentration and temperature affect different parts of the explanation.

Make a guess. You are not marked.

A reaction mixture is warmed by 10 K. What is the main reason the rate rises?

  1. More collisions have energy at or above Ea
  2. The activation energy becomes smaller when warmed
  3. The particles collide much more often than before
Show the answer

More collisions have energy at or above Ea

The energy distribution shifts to higher energies, so a much larger fraction of collisions can react. Collision frequency rises only a little.

In the collision model, particles must collide with enough energy and in a suitable orientation. A higher concentration puts more particles in each unit of volume, so reactant particles collide more often. At the same temperature, the energy distribution and k do not change. The rate changes as the rate equation predicts for that concentration.

Temperature changes the energy distribution

For the same total number of particles, the hotter distribution is broader and has a lower peak. Its area beyond the fixed activation energy is larger. The curves share the same total area over the full energy range.

Energy is in relative units in this qualitative model. The vertical quantity is number per energy interval; areas, not curve heights alone, represent particle populations.

A Boltzmann distribution shows that particles have a range of energies. The area to the right of Ea is the fraction of particles with enough energy to react. A higher temperature spreads the curve out and makes this fraction much larger. Particles also move faster, so they collide a little more often. More effective collisions each second mean a larger k and a faster reaction at the same concentrations.

Watch out for this

A student says: "Heating speeds up a reaction because it lowers the activation energy." What is wrong with this?

Show the answer

Heating does not lower the activation energy. The energy needed stays the same; what changes is how many particles have that energy. The rule that "the rate doubles every ten degrees" is only a rough guide, not a law.

Check your understanding

Why is "particles collide more often" an incomplete answer to why heating speeds up a reaction?

  1. It misses the larger fraction of collisions with energy at least Ea, which matters most.
  2. It misses that heating raises the concentration of the reactants in the flask.
  3. It misses that heating lowers the activation energy, which matters most here.
Show the answer

It misses the larger fraction of collisions with energy at least Ea, which matters most.

Heating raises collision frequency a little but greatly increases the fraction of particles with energy at or above Ea. Both lead to more effective collisions per second and a larger rate constant.

A catalyst changes the route, not the starting energy distribution

Connect a lowered barrier with a surface process.

A lower threshold at the same temperature

There is one distribution because temperature is unchanged. The catalytic threshold lies to the left of the original threshold, so more particles have sufficient energy. The catalyst does not supply a hotter population.

The shaded region illustrates the fraction above the lower threshold. The same-temperature distribution is unchanged.

A catalyst gives a different pathway with a lower activation energy, so the rate constant increases. The catalyst takes part in the steps but is made again by the end. It does not change the overall enthalpy change, the equilibrium constant or the final equilibrium composition. It only helps the system reach equilibrium faster.

Heterogeneous catalysis at a solid surface
  1. Adsorb reactants

    Gaseous reactants attach to active surface sites. Adsorption is attachment to the surface, not absorption into the bulk.

  2. React by an easier route

    Surface interactions weaken relevant bonds and bring reactants into suitable positions. The pathway has lower activation energy.

  3. Desorb products

    Products leave, freeing sites for another catalytic cycle.

A car catalytic converter uses a solid catalyst, while the exhaust gases react. The catalyst is in a different phase from the reactants, so this is heterogeneous catalysis. A typical reaction that removes nitrogen oxides is 2NO(g) + 2CO(g) → N2(g) + 2CO2(g). The gases adsorb on the surface, react and then desorb. NO is reduced to nitrogen while CO is oxidised. CO2 is less toxic than CO, but it is still a greenhouse gas, so do not write that all the products are harmless.

Check your understanding

Why can a contaminant that binds strongly to a solid catalyst reduce its activity?

  1. It occupies active sites, leaving fewer sites for reactants to adsorb and react.
  2. It raises the activation energy of the uncatalysed reaction in the gas phase.
  3. It reacts with the products, which moves the equilibrium back to the reactants.
Show the answer

It occupies active sites, leaving fewer sites for reactants to adsorb and react.

Heterogeneous catalysis happens on the surface. A strongly bound substance blocks active sites, so fewer reactant molecules can adsorb. This is called catalyst poisoning.