Catalyst role in a reation

[VarshiniC]

If a catalyst speeds up the rate of reaction, how can it have no effect on the overall rxn?

[yushensun 3F]

I assume you mean the equation of the overall reaction?
It's because catalysts are consumed then produced again, making catalysts show up on both the reactant and product sides of equations, so they can cancel each other out
For example:

H₂O is on the reactant side of the first step and product side of the last step (it was consumed then produced again) so it is a catalyst, it's not in the overall equation because when something is on both sides of an equation, they can cancel each other out

[Levy Shaked]

Agreed. Catalysts provide a different reaction mechanism with a lower activation energy (as the transition state between reactants and products, with weakened reactant bonds and unformed product bonds, is more stable). However, even though a catalyst can be consumed in a particular elementary step or produced in a particular elementary step, the net reaction involves no change in catalyst concentration such that it is not considered to participate in the overall reaction.