8.2 Temperature, rate and activation energy

Activation energy, Eₐ, is the minimum energy particles must have for a reaction to occur when they collide in a suitable orientation.

A catalyst provides an alternative pathway with a lower Eₐ. It does not change the reactants’ or products’ enthalpy, nor does it alter the equilibrium constant.

If an uncatalysed pathway has Eₐ=80 kJ mol⁻¹ and a catalyst pathway has Eₐ=45 kJ mol⁻¹, more particles exceed the threshold at the same temperature, so the rate increases.

Activation energy is not the overall ΔH of the reaction. An exothermic reaction can still have a large activation barrier.

A Boltzmann distribution plots the number or fraction of particles against their kinetic energy at a particular temperature. The area represents the total population.

The curve is broad rather than a single value. The fraction with energy above Eₐ is the area to the right of the Eₐ line, representing potentially successful collisions.

Raising temperature broadens and flattens the curve, shifts the most probable energy higher and increases the area beyond Eₐ, so rate rises.

The peak height is not the reaction rate. Compare the area beyond Eₐ and remember the total area remains tied to the particle population.

When temperature rises, the particles’ energy distribution changes so a larger fraction has energy at least Eₐ. This increases the number of successful collisions per second.

Temperature affects both collision frequency and the fraction energetic enough. It does not lower Eₐ unless a catalyst changes the pathway.

Two identical mixtures at 300 K and 320 K have the same Eₐ, but the 320 K distribution has a larger area beyond Eₐ and therefore a faster initial rate.

Do not draw a catalyst as shifting the whole distribution. A catalyst lowers Eₐ, leaving the temperature distribution itself unchanged.