(a) What is a catalyst? (b) What is the difference between a homogeneous and a heterogeneous catalyst? (c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?

A catalyst is a substance that speeds up the rate of a chemical reaction without being consumed in the reaction. It does so by providing an alternative reaction pathway or mechanism with a lower activation energy.

Homogeneous catalysts are those that are in the same phase (solid, liquid, or gas) as the reactants, and they usually form intermediate species with the reactants during the reaction. Heterogeneous catalysts, on the other hand, are in a different phase than the reactants and typically work by adsorbing the reactants onto their surface, where the reaction takes place.

Catalysts do not affect the overall enthalpy change for a reaction, as they only provide an alternative pathway for the reaction to proceed. The overall enthalpy change (∆H) is a state function, which means it depends only on the initial and final states of the reactants and products, and not on the path taken to reach those states. However, catalysts do affect the activation energy of a reaction, as this is the minimum energy required for reactants to transform into products through a particular pathway. By providing an alternative pathway with a lower activation energy, catalysts allow the reaction to proceed faster, as more reactant molecules have enough energy to overcome the lowered activation energy barrier.