Update: I found out that the coefficients of the chemical reaction are not the exponents in the rate law. It is actually the order of the species during the reaction.

It is known that in the reaction, first order in 2A + 3B → products, A was found to be a first order and B is second order. So, does it mean that the rate law for this reaction is R = k[A][B]2?

I have been taught that R = k[A]x[B]y

Where: R = Rate of the reaction; k = rate constant; A = Concentration of species A; B = Concentration of species B; x = order of reaction with respect to A; and y = order of reaction with respect to B

First question: So, does that mean for the particular reaction: 2A + 3B → products, will have a rate law of R = k[A]2[B]3?

Second Question: How do I determine the x and y? Like what does order of reaction mean?

  • 5
    $\begingroup$ R = k[A]^2[B]^3 would be true if the reaction was the result of a simultaneous collision of 5 molecules. We do not know such reactions. $\endgroup$
    – Poutnik
    Jan 11 at 11:05

If such a reaction "$\ce{2A + 3B -> }$ products" occurs, it is the sum of several more elementary reactions, which are taking place successively. Some may be fast, some slower. The rate of the global reaction is the rate of the slowest of these elementary reactions. It can be first or second order, or it can even be more complicated, depending on any catalytic effect. –

  • 2
    $\begingroup$ The rate determining step approximation is often not a very good approximation. For the kinetics of a reaction all steps just be considered. It is really only a useful approximation of there is exactly one step that is much, much slower than all of the other steps. In complex reactions, e.g. catalysis, this is hardly ever the case. The energy span model provides a more practical approach. $\endgroup$ Jan 11 at 17:57

If $\ce{2A + 3B -> \text{product(s)}}$ describes the stoichiometry of a reaction, than you describe the balance of starting materials against products; this however does not state much about the kinetics, elemental (perhaps rate determining) steps, and overall reaction order described by kinetics.

Uni- and bimolecular elemental steps are the most frequently encountered. The simultaneous collision of five atoms/molecules to yield in one step a product you might suggest however is of very low probability.

  • 3
    $\begingroup$ I remember that trimolecular elementary steps are already incredibly unlikely, so 'very low probability' might be an understatement. $\endgroup$ Jan 11 at 17:59
  • $\begingroup$ @Martin-マーチン Newer research suggest that they're aren't that rare, especially if there's for ex. hydrogen bonding involved, afaik. $\endgroup$
    – Mithoron
    Jan 12 at 13:34

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