Why does a catalyst increase the rate of a reaction? What is the difference between a homogeneous and heterogeneous catalyst? Would a given reaction have the same rate law for both a catalyzed and an uncatalyzed pathway? Explain fully.
The Correct Answer and Explanation is:
A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. Activation energy is the energy required to initiate a chemical reaction. By lowering the activation energy, the catalyst allows more reactant molecules to have sufficient energy to react, thus speeding up the reaction. However, the catalyst is not consumed during the reaction; it remains unchanged and can be used repeatedly.
There are two types of catalysts:
- Homogeneous catalysts: These are in the same phase (state) as the reactants. For example, if the reactants are gases or liquids, the catalyst will also be in that same phase. An example of a homogeneous catalyst is sulfuric acid in the esterification of alcohols.
- Heterogeneous catalysts: These are in a different phase from the reactants. In this case, the reactants are typically gases or liquids, and the catalyst is a solid. An example of a heterogeneous catalyst is platinum used in catalytic converters for the reduction of harmful gases in car exhaust.
When it comes to the rate law, the rate law expresses the relationship between the rate of a reaction and the concentration of reactants. If a reaction occurs via a catalyzed pathway, the rate law will often be different compared to the uncatalyzed pathway, even though the overall reaction may be the same. This is because the catalyst changes the mechanism of the reaction. For example, the intermediate steps in a catalyzed reaction might involve different species or occur via different collision mechanisms. Therefore, the rate law is often specific to the pathway, and the presence of a catalyst alters how the concentrations of reactants and products influence the rate. Consequently, the rate law for a catalyzed reaction is usually modified from that of an uncatalyzed reaction.
