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The following points are given in my textbook about order and molecularity:-

(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non-integer. Its values are limited to 1,2 or 3.

(ii) Order is applicable to elementary as well as complex reactions whereas molecularity is applicable only for elementary reactions.For a complex reaction, molecularity has no meaning.

(iii) Order is given by the slowest step(also known as RDS) and molecularity of the slowest step is same as the order of the overall reaction.

(iv) Molecularity is equal to the sum of stoichiometric coefficients of reactants in the elementary reaction.

However, I have observed an inconsistency in the above points and would like a clarification.

Let us consider 2 cases:-

Case 1: Complex Reaction

By (iii)-order of overall reaction=molecularity of RDS. Since molecularity is 1,2 or 3, order must also be 1, 2 or 3.

Case 2:Elementary Reaction

By (iii)-order of overall reaction=molecularity of the reaction(since there is only 1 step so that must be the RDS). Since molecularity is 1,2 or 3, order must also be 1,2 or 3.

Now, combining these two cases, order =1,2 or 3. However, (i) mentions that the order can be fractional or zero. How is this possible?

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    $\begingroup$ Authors of the rules have not thought through all possible cases. E.g. nonlinear reaction schemes with a reaction feedback, regenerating reactants of some prior reaction steps. $\endgroup$
    – Poutnik
    Jun 16, 2022 at 5:21
  • $\begingroup$ @Poutnik Can you elaborate on that in an answer? $\endgroup$
    – user121185
    Jun 16, 2022 at 5:22
  • $\begingroup$ No resources for that now, but see e.g. colby.edu/chemistry/PChem/notes/ChainMech.pdf $\endgroup$
    – Poutnik
    Jun 16, 2022 at 5:27
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    $\begingroup$ Note that many processes have their pseudo-order lowered, if one component content is about invariant within the process. E.g. A + B -> C with [A] >> [B] is reaction of pseudo-order 1, with [A] implicitly involved in reaction rate constant. This applies to the above candle example too. $\endgroup$
    – Poutnik
    Jun 16, 2022 at 8:48
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    $\begingroup$ Well, earlier one was better. Your post is kinda "what is this kinetics all about", though. $\endgroup$
    – Mithoron
    Jun 16, 2022 at 16:21

1 Answer 1

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Here is some clarification.

  1. You are correct that order of reaction is an experimental quantity.
  2. Just like order of reaction is is an experimental quantity, molecularity is purely theoretical quantity and as of today, I have not found anyone who can observe it. And, it only work for elementary (1-step) reaction.
  3. Order of reactions do only measurable for RDS. But just by stating that there exist an RDS, you are also stating that the reaction is not elementary, thus the idea of molecularity simply does not apply for a reaction with more than one step.
  4. Molecularity have to be positive whole number the same reason you cannot have less than one atom, it is simply not make sense.

I hope this clear things up.

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