right, the configuration of all the species participating in the reaction goes from being in the
reactant stage to resembling more and more that of the products, until the reaction has gone to
completion and the products are formed. The y-axis plots the potential energy of the varying
configuration of the atoms. The most important thermodynamic factor in these diagrams is the
relative energy of the products and reactants. The overall energy change of the reaction is the
difference between the potential energy of the products and the potential energy of the reactants. If
the energy level of the products is lower than that of the reactants, the products are more stable
thermodynamically and there is a net release of energy for the reaction. If the energy of the
reactants is lower than that of the products, the reverse is true: The reactants are more stable
thermodynamically and energy is absorbed in the process.
The activated complex exists at the top of the energy barrier. It has greater energy than either the
reactants or the products, and is denoted by the symbol ‡. The energy required to bring the
reactants up to this level is the activation energy for the reaction, Ea. Once an activated complex is
formed, it can either go on to form the products or revert to reactants without any additional energy
input. (The difference in energy between the activated complex and the products is the activation
energy for the reverse reaction: that of products going to reactants.)
It is important to note that even if the reaction results in a net release of energy, i.e., even if the
products are more stable thermodynamically than the reactants, the reactants need to have
sufficient energy initially to overcome this activation barrier, since the transition state is always at a
higher energy than either the reactants or the products. This is the reason why we do not observe
diamond turning into graphite: Under standard conditions, the activation energy for these
processes is so high that regardless of the thermodynamic favorability of the reactions, the
reactants simply cannot overcome the energy barrier. In plainer terms, the ultimate payoff may be
great for these reactions, but the effort called for is too much.
In terms of the rate law, the height of this barrier (the activation energy) is what determines the
value of the rate constant k: The higher the barrier, the slower the rate, i.e., the smaller the value of