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 k_f is the rate constant of the forward reaction and k_r is the rate constant for the reverse reaction. The larger the value of k_f, 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 k_f and k_r 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
• Collision Theory
• Transition States
• Energy Profiles
• Reaction Mechanisms
• Dynamic Equilibrium