Page 143 - Introduction to chemical reaction engineering and kinetics

6.3 Energy in Molecules 125

Here, the CH,-H bond is formed as the C,H,-H bond is broken. For this system, the
other bond lengths and angles also affect the potential energy, and the potential energy
surface therefore depends on all other coordinates (3N - 6 or 30 in all). This system,
however, is similar to the triatomic case above, where A = C,HS, B = H’, and C =
CHJ. Again note that the transition state for the reverse reaction is the same.
The notion of the transition state is central to both theories discussed in this chapter.
The transition state is the atomic configuration that must be reached for reaction to
occur, and the bonding dictates the energy required for the reaction. The configuration
or shape of the transition state indicates how probable it is for the reactants to “line
up” properly or have the correct orientation to react. The rate of a reaction is the rate
at which these requirements are achieved. A quantitative interpretation of both these
issues, as treated by the two theories, is the subject of Sections 6.4 and 6.5.
In reactions which occur on solid surfaces, it is acceptable to think of the surface as a
large molecule capable of forming bonds with molecules or fragments. Because of the
large number of atoms involved, this is theoretically complicated. However, the bind-
ing usually occurs at specific sites on the surface, and very few surface atoms have their
bonding coordination changed. Therefore, the same general concepts are useful in the
discussion of surface reactions. For example, the nondissociated adsorption of CO on
a metal surface (Section 6.2.1.4) can be thought of as equivalent to bimolecular associ-
ation reactions, which generally have no barrier. Desorption is similar to unimolecular
dissociation reactions, and the barrier equals the bond strength to the surface. Some
reactions involving bond breakage, such as the dissociative adsorption of HZ on copper
surfaces, have energy barriers.

6.3.1.5 Other Electronic States

If the electrons occupy orbitals different from the most stable (ground) electronic state,
the bonding between the atoms also changes. Therefore, an entirely different potential
energy surface is produced for each new electronic configuration. This is illustrated in
Figure 6.6 for a diatomic molecule.
The most stable (ground state) potential energy curve is shown (for AB) along with
one for an electronically excited state (AB*) and also for a positive molecular ion
(AB+, with one electron ejected from the neutral molecule). Both light absorption and
electron-transfer reactions produce a change in the electronic structure. Since electrons
move so much faster than the nuclei in molecules, the change in electronic state is com-
plete before the nuclei have a chance to move, which in turn means that the initial
geometry of the final electronic state in these processes must be the same as in the ini-
tial state. This is shown by the arrow symbolizing the absorption of light to produce
an electronically excited molecule. The r,, distance is the same after the transition as
before, although this is not the most stable configuration of the excited-state molecule.
This has the practical implication that the absorption of light to promote a molecule
from its stable bonding configuration to an excited state often requires more energy