Dynamic Equilibrium

Although we have learned that the kinetics of a reaction may override predictions based purely on
thermodynamics, the two aspects are not totally separate. This is because, as mentioned in the
chapter on chemical equilibrium, one of the things that characterizes equilibrium is that the
forward and reverse reactions are occurring at the same rate: The relative rates of reactions set up a
thermodynamic equilibrium in the system.

The ratio of the forward rate constant to the reverse rate constant is the equilibrium constant for
that one-step process, i.e.:

where kf is the rate constant of the forward reaction and kr is the rate constant for the reverse
reaction. The larger the value of kf, the faster the forward reaction occurs (relative to the reverse
reaction); that is, products are formed more rapidly than they revert back to reactants, and so there
will tend to be more products around at equilibrium, which is reflected in a large equilibrium
constant.

It may now be more obvious why a catalyst does not affect the position of equilibrium: Both kf and
kr are increased by the same proportion, such that their ratio remains unchanged.

THINGS TO REMEMBER

Definition  of  Rate

Rate    Law

Zero-Order  Reactions

First-Order Reactions