Homogenous Catalysts are in the same physical state as the reactants.Heterogenous Catalysts are in a different physical phase to the reactants. Reactants adsorb onto the active sites on the surface of the catalyst. Bonds within the reactants break. Bonds between the reactants form. The products desorb from the surface of the catalyst. Catalysts increase the rate of reaction by providing an alternative pathway with a lower activation energy, so catalysts increase the value of the rate constant, k.