The Principles of Catalysis

Enthalpy profile for an non–catalysed reaction

The progress of a typical, non–catalysed reaction can be represented by means of a potential energy diagram. This diagram illustrates an exothermic reaction in which the products have a lower enthalpy than the reactants. For the reaction to occur, some of the existing bonds in the reactants must be broken. This involves an input of energy called the activation enthalpy.

Enthalpy diagram for uncatalysed reaction.

Energy Spread

Reactant molecules will posses a range of energies at any temperature, some high, some low, but most in between. Let's consider a gas: the distribution of energies is called the Maxwell-Boltzmann distribution.

Maxwell-Boltzmann distribution

Activation Energy

One of the requirements for a successful conversion of reactant to product is that the reactant molecules possess at least the activation enthalpy when they collide. In the collision, they lose KE as they do work to break their bonds. Let's look at the distribution of energies next to the enthalpy graph.

Distribution of energies next to the enthalpy graph.

Only those particles with enough kinetic energy (i.e more than EA) will react. If the activation enthalpy is low, a greater proportion of reactant molecules can overcome the activation energy. Therefore, many more collisions will lead to the conversion of reactant to product. The rate of conversion will therefore be rapid .

Reactant molecules have a range of energies. Some of the molecules can overcome the activation energy. More can overcome a smaller activation energy.

Rate of Reaction

If the activation enthalpy is high, only a few reactant molecules will have a big enough energy. Therefore, only a few collisions will result in a successful reaction and the rate of conversion of reactant to product will be slow.