Catalysts

[Andy Nguyen 1A]

How do we know if there is a catalyst in a reaction mechanism?

[Kaylin Krahn 1I]

if it is seen in the overall reactant and also in the products therefore "cancelling out" and not being consumed

[Julian Krzysiak 2K]

We know if there is a catalyst if it was present in the reactants in the first step, and is present as a product in the last step. It is never consumed.
This is opposed to an intermediate, which is formed in the first step and later consumed in a later step.

A+B->C
C+D->E+B
A+D->E

In this, B is the catalyst, and C is the intermediate. Notice how both are absent from the overall equation, but only C was used up, while B is still present at the end.

[Guangyu Li 2J]

First, you need to make sure that if a certain molecule speeds up the reaction. Second, you need to check that if the molecule is not consumed in the process of the reaction. If a given molecule satisfied both of the two standards, it is a catalyst.