I think my question is one of semantics. You said in Bio 2401 lecture the other night that catalysts do
not influence the direction of the reaction, simply its RATE. By "rate" I assume you mean that as a greater
proportion of the molecules now have the required activation energy, the reaction is driven to completion
more quickly than if it were on its own, and NOT that the catalyst somehow acts to break and reform the bonds
at a faster rate of speed -- or do I have that backward?
A catalyst can facilitate the breakage of bonds, as in the case where the bonds between H's are weakened
upon binding to Pt. But once the bonds are broken, the catalyst cannot influence the probability that they
will reform to make products vs. reforming to remake the reactants. That ratio is governed by the relative
strengths (stabilities) of the bonds in the two alternative structures (reactants and products), which
ultimately are in equilibrium with each other. The direction of the
reaction is determined by this equilibrium ratio, as well as the relative concentrations of the reactants
and products that you are starting from. We will discuss how energy changes determine directions in a
reaction in more quantitative detail when we get to energy metabolism, right after enzymes.
Can catalysts can change equilibrium amounts of reactants and
products. I would guess they could because if the activation energy is lowered
sufficiently (so as to speed up the reaction) it should be easy for the
products and the reactants to interconvert. If the activation energy
is high (without catalyst) once a molecule makes it to the product state
it stays there because the chances of it mustering up enough energy to
make it over the super high energy barrier is very slim.
No, catalysts do not change the equilibrium. The equilibrium condition does
not depend on time, and so it depends only on the relative energy levels of
the reactants and the products, no matter what the route between them. If it
takes ninety years to reach equilibrium, so be it.