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Reaction Kinetics

Topic 2 of 3

Why rates change

Compare collisions, temperature and catalytic pathways.

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

Separate more collisions from a larger successful fraction

Concentration and temperature affect different parts of the explanation.

For a reaction on the collision model, particles must collide with adequate energy and a suitable orientation. Increasing a reactant concentration gives more particles per unit volume, so collisions between reactants occur more frequently. At unchanged temperature, the energy distribution and k do not change; the rate changes according to the concentration dependence in the rate equation.

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 represents the fraction with enough energy for the pathway. Raising temperature broadens the distribution and substantially increases that fraction; faster particle motion also raises collision frequency. More effective collisions per second mean a larger k and a faster reaction at the same concentrations.

Check your understandingWhy is "particles collide more often" incomplete for a temperature question?Think it through, then reveal the answer
It omits the increase in the fraction with energy at least Ea, often the dominant effect. A complete explanation relates both the distribution and collision frequency to more effective collisions 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 offers a different pathway with lower activation energy, increasing the rate constant. It takes part in steps but is regenerated overall. It does not change the overall enthalpy change, the equilibrium constant or the final equilibrium composition; it helps the system reach that composition 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 exhaust reactants are gases: the catalyst is in a different phase, so the catalysis is heterogeneous. A representative nitrogen-oxide removal reaction is 2NO(g) + 2CO(g) → N2(g) + 2CO2(g). Adsorption, surface reaction and desorption convert NO to nitrogen while oxidising CO. CO2 is less acutely toxic than CO but remains a greenhouse gas; "all products are harmless" would be misleading.

Check your understandingWhy can a contaminant that binds strongly to the catalyst reduce its activity?Think it through, then reveal the answer
It can occupy active sites, leaving fewer sites for reactant adsorption and reaction. This is an explanation about available surface pathways, not a change in the balanced overall equation.