Why would the Eact decrease if a catalyst is added?
A catalyst by its very nature increases the rate of reaction by binding to a reactant, hence changing its shape and reactivity with other reactants. However, the catalyst is neither consumed nor will it change the chemical shape of the products.
Catalysts work by providing an (alternative) mechanism involving a different transition state and lower activation energy. The effect of this is that more molecular collisions have the energy needed to reach the transition state. Hence, catalysts can perform reactions that, albeit thermodynamically feasible, would not run without the presence of a catalyst, or perform them much faster, more specific, or at lower temperatures. This can be observed on a Boltzmann distribution and energy profile diagram. This means that catalysts reduce the amount of energy needed to start a chemical reaction.
