What's the substance over or under the arrow in the chemical rxn?

[robert zellmer]

It's a catalyst.

This comes from a fellow student.  I actually addressed this in lecture.

Often times in chemical reaction equations the catalyst is written
over (maybe under) the reaction arrows.  The catalyst is a "reactant"
in one of the steps of the mechanism but it is not one of the actual
reactants in the overall reaction.  When you add the steps in a
mechanism the catalyst cancels out (remember it's a reactant in one
step and reproduced later as a product - regenerated to be used over
and over again).

While we're at it, in a mechanism when the steps are added both
intermediates and catalysts cancel and do not wind up in the balanced
eqn for the overall reaction of interest.  How do you tell the difference
between them?

An intermediate appears for the first time in a mechanism as a product
and is used in a subsequent step as a reactant to get rid of it.

A catalyst appears for the first time in a mechanism as a reactant and is
reproduced in a subsequent step as a product to get it back so it can be
reused.  That's why we say a catalyst speeds up a reaction w/o being
consumed.

Dr. Zellmer