# Confusion regarding order of reaction

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?

• 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. Commented Jun 16, 2022 at 5:21
• @Poutnik Can you elaborate on that in an answer?
– user121185
Commented Jun 16, 2022 at 5:22
• No resources for that now, but see e.g. colby.edu/chemistry/PChem/notes/ChainMech.pdf Commented Jun 16, 2022 at 5:27
• 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. Commented Jun 16, 2022 at 8:48
• Well, earlier one was better. Your post is kinda "what is this kinetics all about", though. Commented Jun 16, 2022 at 16:21