(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) A catalyst is a substance that speeds up a chemical reaction by lowering its activation energy, without being consumed or permanently altered in the process. It provides an alternative reaction pathway with a lower activation energy. (b) A homogeneous catalyst is in the same phase as the reactants, while a heterogeneous catalyst is in a different phase than the reactants. For example, a homogeneous catalyst could be a liquid dissolved in a liquid, while a heterogeneous catalyst could be a solid interacting with a gas or liquid. (c) Catalysts do not affect the overall enthalpy change (\(\Delta H\)) for a reaction but lower the activation energy (\(\Delta E_a\)) for the reaction. This makes it easier for the reactants to approach the transition state and form products, resulting in a faster overall reaction.