Palladium acts as a heterogeneous catalyst in the reaction between an alkene with hydrogen by providing an alternative reaction route. Describe the stages of this reaction route. (3 marks)

The detail required for many A-level questions is indicated in the number of marks available. As this question awards 3 marks, 3 separate points must be made. For example, the first stage of a heterogeneous reaction usually involves mass transfer and adsorption of the reactant on surface active sites. Secondly, the bonds in the reactant molecule are weakened, allowing a surface reaction to take place. Lastly, the product is desorbed from the surface, and the active sites are free to adsorb another reactant molecule.