Are there exceptions to the rate law for elementary reactions?

According to several authentic sources (for example, [ChemLibreTexts](https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/03%3A_Rate_Laws/3.02%3A_Reaction_Mechanisms/3.2.01%3A_Elementary_Reactions)), for elementary reactions, the order is equal to molecularity according to the rate law.




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**Example:** For an elementary reaction $$\ce{aA +bB -> cC +dD}$$ the rate of reaction is given by $$R = k [\ce{A}]^{a} [\ce{B}]^{b},$$ where $k$ is the rate constant and $[\cdot]$ represents concentration.




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I am aware that, in real solutions, concentrations are replaced by activities; however, the structure of the *real* rate law is still same as that obtained from the *ideal* rate law.


I wanted to know if there are any exceptions to this. Is there an elementary reaction for which the rate law is *not* obeyed, and the molecularity is *not* equal to order?




It is true that, for elementary reactions, the reaction rate order is equal to molecularity.


The point is, for $a + b \gt 3$ ( and often for $a + b = 3$ or even $a + b = 2$ ), the reaction is not elementary.


Typical case is $\ce{Br2 + H2 -> 2 HBr}$. It looks like it may be an elementary reaction, but it is not. It has the loopbacked chain reaction schema. Its reaction rate is [a rational function with some non-integer exponents](https://chemistry.stackexchange.com/q/81063/35806):



> 
> $$r= \frac{k[\ce{H2}][\ce{Br2}]^{1/2}}{1 + j[\ce{HBr}]/[\ce{Br2}]}$$
> 
> 
> 


( $k$ and $j$ are rate constants for particular reactions steps withing the reaction kinetic schema. )


Another exception are reactions of the zeroth order, with the rate not depending on the analyte concentration, typically photolytic reactions.


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