ABSTRACT
A convenient and effective technique for following the course of a chemical or physical process is by means of a two-dimensional diagram depicting the change in the energy of the system as it proceeds from its initial to final state. This is commonly called a potential energy plot, even though it is the total energy that is being shown, because the latter is typically varying in accordance with the relative positions of the atoms that comprise the system. (In general, potential energy is associated with position, kinetic energy with motion.)
The other axis in such a plot is the ‘‘reaction coordinate,’’ which is simply some variable in terms of which the progress of the process can be measured. For a bond dissociation, XY ! XþY, an obvious choice for the reaction coordinate is the X-Y separation. But in many processes, there maybe several possibilities, some better than others. For example, in the addition of Cl2 to ethylene, one option
might be the C-C distance, as the bond changes from double to single. Another might be one of the H-C-C-H dihedral angles, reflecting the transition of the carbons from planar to tetrahedral configurations.
