(1)

then the system can be assigned a kinetic order. An order is associated with a rate constant and is simply related to a concentration term. As pointed out by Shaw and Toby some time ago (2) there are anomalies associated with the characterization of eq 1 in terms of a rate constant. These are resolved by writing for the rate of reaction 1

where I_a is the absorbed intensity. I_a represents an intensity averaged over a photolysis cell of length L with units of quanta (or einsteins) per volume–time. Using the Beer–Lambert law and taking the absorption coefficient (molar or particle) as ε gives the instantaneous rate as

As Logan has pointed out (3), when the light absorption is high the rate is proportional to I₀ which corresponds to zero order in reactant concentration. At low light absorption the rate is proportional to [A] which is first order. Since the order depends on the magnitude of [A], the concept of reaction order for a photochemical reaction is of little use.

If we include the secondary reactions

(2)

(3)

then assuming a steady state in [A*] we have

where Φ is the overall quantum yield. The overall rate, like the primary rate, still lacks a meaningful order that is independent of [A].

Further, Hippler writes for the overall reaction of steps 1, 2, and 3

The equals sign implies that every quantum absorbed results in a molecule of product, which is clearly not true if reaction 3 is appreciable. As has been pointed out (4), overall reactions cannot generally be obtained by replacing arrows with equals signs. If we want the overall stoichiometry of the species in the system under consideration, it is

assuming no product is initially present.

Literature Cited

1. Hippler, M. J. Chem. Educ. 2003, 80, 1074.
2. Shaw, H.; Toby, S. J. Chem. Educ. 1966, 43, 408.
3. Logan, S. R. J. Chem. Educ. 1997, 74, 1303.
4. Toby, S. J. Chem. Educ. 2000, 77, 188.

See the author's reply.