Talk:Rate-determining step
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Problematic sentences edit
A reaction mechanism can be solved analytically using differential equations to yield a rate equation. However, the resulting equations often do not provide insight into the rate (or velocity) of the reaction.
The first sentence seems to me to be a little off-key. What does it mean to "solve" a reaction mechanism? Are we determining the reaction mechanism? Or are we given the reaction mechanism, and solving for the rate equation?
This then gives me difficulty in understanding the second sentence. If we have managed to derive a rate equation, wouldn't this tell us the rate? Is it that the ODEs are too hard to crack, so we don't really get a rate equation? Or do we get an incorrect rate? Or do we predict the correct rate, but can't grasp intuitively where it came from? In other words, what is meant by "insight"?
178.38.90.162 (talk) 20:10, 3 April 2015 (UTC)
RDS as the step with the highest barrier edit
The article says "the rate determining step can be determined by taking the largest energy difference between any starting material or intermediate on the diagram and any transition state that comes after it. That transition state will then be the rate-determining step of a given reaction" which is plainly not true. The rate determining step might arise from low concentration of the crucial reagent even if the subsequent energy barrier is slow (see Atkins, Physical Chemistry, 10th Edition, p846). — Preceding unsigned comment added by 131.111.184.8 (talk) 23:20, 7 May 2017 (UTC)
- I have moved this new comment to the end of the Talk page.
- The relation between the rate-determining step and the potential energy surface is a complex question which seems to be avoided completely in many kinetics texts, e.g. Steinfeld et al. (2nd ed.) and also Espenson (1st ed.). Atkins and de Paula do make the comment which you have cited, but their discussion is so brief that it is not clear, at least in the 8th edition (p.814) which I have. There is a more extensive discussion in Laidler (Chemical Kinetics (3rd ed. 1987, p.283-5) which does agree with the current text of the article for first-order reactions. The article should explain the point more clearly however, and point out that the transition state defined above can differ from the highest (Gibbs) energy state when there is an intermediate of lower energy than the initial reactant. Also for non-first-order reactions, Laidler notes that concentration terms should be included as Atkins and de Paula point out.
- In a few days I will try to make improvements to the article based on Laidler's discussion. Dirac66 (talk) 22:43, 11 May 2017 (UTC)
- Done. I have cited both Laidler and Atkins-de Paula, as well as an article by Murdoch which Laidler mentions. I have also moved this section further down in the article, since the question is complex and not really an appropriate introduction to the subject of the rate-determining step. Dirac66 (talk) 23:48, 17 May 2017 (UTC)