In chemistry, we may use chemical equations in various ways. In some instances we regard it as very important that our equation denotes the process that actually occurs, and this would usually apply in kinetic studies. On the other hand, we may sometimes wish to evaluate the standard enthalpy change of a reaction process that could not conceivably be achieved in the laboratory. So for simplicity, let me confine my remarks to the area of kinetics, which is the one to which Toby (1) has referred.

Some reactions are known to be elementary processes, occurring in a single step involving precisely those species written on the left-hand side of the balanced equation, and some are known not to be. For the latter, an important and necessary distinction is that between the overall reaction process and the individual reactions steps by which this process is achieved. This was recognized, for example, by the IUPAC Sub-Committee on Chemical Kinetics, chaired by K. J. Laidler. However, the solution proposed in their report (3) was that the two should be denoted by slightly different types of arrow, one with a simple arrowhead (→) and the other with a filled arrowhead (). At best, this method of distinction would seem (no pun intended) a rather blunt weapon and in certain fonts the difference might not be clearly apparent.

A recent chemical kinetics text (4) has adopted a different solution, using an equals sign for the overall reaction:

H₂ + Br₂ = 2HBr

whereas an arrow, →, is used in each of the reaction steps that are proposed to constitute the mechanism, and for any elementary process. This would seem to be in accord with the policy advocated by Toby (1).

Literature Cited

1. Toby, S. J. Chem. Educ. 2000, 77, 188–190.
2. Diemente, K. J. Chem. Educ. 1998, 75, 319–321.
3. Laidler, K. J. Pure and Applied Chem. 1981, 53, 753–771.
4. Logan, S. R. Fundamentals of Chemical Kinetics; Longman: Harlow, England, 1996.