AS/ A-level Chemistry: Catalysts

A catalyst is a substance which increases the rate of a reaction without being used up itself. They do chemically change during the reaction, but not permanently. They can either lower the activation energy of the reaction or change the mechanism of the reaction completely. Catalysts can either be a transition metal, a transition metal oxide, or an enzyme.

Adv: Small amount needed to catalyse a lot of reactions, also they are remade, thus reusable.

Disadv: High specificity to the reactions they catalyse.

Homogeneous catalysts exist in the same phase as the reactants, usually in a solution. For example, a liquid catalyst used in a reaction where the reactants are also liquids, the catalysts will be homogenous – you can remember this by knowing that ‘homo’ means ‘same’.

Under the AS/A-Level Chemistry, catalysis in this form is due to temporary changes in the oxidation state (and ligands) of a transition metal – this forms what is known as a catalytic cycle. Put more simply, the reactions happen through an intermediate species and the catalyst is reformed in the process.

Homogeneous catalysts can ‘mix in’ with the reactants which means the degree of interaction between the catalysts and the reactant molecules will be very high. However, this also means that the catalysts usually cannot be recovered after the reaction is complete.

Heterogeneous catalysts are catalysts which exist in a different phase to the reactants. For example, a solid catalyst used in a reaction with liquids would be a heterogeneous catalyst – you can remember this by knowing that ‘hetero’ means ‘different’.

When it comes to AS/A-Level Chemistry, Heterogenous catalysts work mainly through the same mechanism. One (or more) of the reactants is absorbed onto the surface of the catalyst at its active sites. Adsorption is not the same as absorption – adsorption is where the reactant molecule sticks to the surface of the catalysts, as opposed to absorption where the molecule is taken up.

After adsorption has occurred, the reactants molecules and the catalyst interact, making the reactant molecules more reactive. The main reaction then occurs to form the product, which is then desorbed from the surface of the catalyst. This leaves the active site of the catalyst empty, which means more molecules can adsorb onto it and react. The below image shows an example of a heterogeneous catalyst reaction.

For something to make a good heterogeneous catalyst, it must absorb the reactant molecules strongly enough for them to be able to react, but not too strongly so that the product can’t desorb. For example, platinum makes a good heterogenous catalyst because the reactants can adsorb easily, but the product can break away easily.

In AS/A-Level Chemistry, catalysts of this type are often in a fine powder form. This helps to maximise the surface area, and thus give the most efficient rate of reaction. This type of catalyst can also be separated very easily from the reaction mixture, which means expensive catalysts can be recovered and reused.

Congratulations! Now you understand catalysts in AS/A-Level Chemistry.