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I was given a hypothetical reaction $\ce{A + 2B + 4C -> 2D + E}$ for which I have to propose the possible mechanism with a requirement, that the third step is rate determining.

From the other given inputs I determined that the rate law for this reaction will be $\ce{B + 2C}$.

I came up with a number of "possible" reaction steps, all of which seem to me equally unrealistic. Examples:

A +  B -> X
C +  X -> Y 
B + 2C -> 2D + Z
C +  Z +  Y -> E

This does not make sense to me as $\ce{C}$ reacts with $\ce{X}$, $\ce{B}$ and with the product of its reaction with $\ce{B}$.

A +  B -> X
C +  X -> 2D + Y 
B + 2C -> Z
C +  Z +  Y -> E

Same problem here, $\ce{C}$ reacts with just about everything.

A +  B -> 2D + X
C +  X -> Y 
B + 2C -> Z
C +  Z +  Y -> E

Needless to say I am skipping the variation where the last step uses 3 reactants and produces 2 products, as it's obviously introduces another multi-step reaction:

A +  B -> X
C +  X -> Y 
B + 2C -> Z
C +  Z +  Y -> 2D + E

Of course there can be 4 variants of each of the above approaches, achieved by simply combining the variables in a different order.

The above considerations do not seem to be bothering the author of the course. For the hypothetical reaction $\ce{2A + 3B + 5C -> 2D}$ they consider the following the correct answer:

2A + B -> W
2C + W -> D + X
B + 2C -> Y
C +  X -> Z
B +  Y + Z -> D

Here $\ce{C}$ reacts with $\ce{B}$ and the product of its own reaction with $\ce{W}$. I do not understand how it is possible to come up with a "realistic" mechanism of a reaction where the specific substances are unknown.

I tries google and found a number of videos explaining reaction mechanisms on the example of $\ce{NO2 + CO -> NO + CO2}$. In those videos they propose a mechanism, where $\ce{NO2}$ reacts with itself to form $\ce{NO}$ and $\ce{NO3}$. (Are they kidding? If this was possible, production of nitric acid would be a joke.) If I was to follow this approach, there could be a number of variations of the mechanism above where each of the reactants and intermediate products reacted with itself. I estimate that would introduce several hundreds of variants.

How is an average grade 12 student expected to solve this task?

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    $\begingroup$ The third step being rate-determining is hardly a constraint, as any step can be rate-determining, especially since the structures of your reactants and products are arbitrary to begin with. What's stopping you from just writing the first mechanism that you can conjure up? It's not like anybody can say it's wrong. Personally, I think this is a pointless exercise. $\endgroup$ – orthocresol May 19 '17 at 0:11
  • $\begingroup$ Let's not debate what is given. The 3d step as rate determining is a requirement of the assignment. $\endgroup$ – aiag May 19 '17 at 0:15
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    $\begingroup$ Does the problem state that $A \neq E$? One simple answer that comes to mind is a three body reaction. 2C and B form one 2CB. The A collides with the 2CB and forms D. The A loses energy/gains energy. elte.prompt.hu/sites/default/files/tananyagok/… $\endgroup$ – CoffeeIsLife May 19 '17 at 1:36
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    $\begingroup$ In other words, A would be acting sort of like a catalyst. $\endgroup$ – CoffeeIsLife May 19 '17 at 1:38
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    $\begingroup$ Honestly, this is a bad question (not the OP's post, but the question presented to him). $\endgroup$ – Pritt Balagopal May 19 '17 at 14:50

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