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So we are told that a unimolecular elementary reaction has a rate law of $k[\text{A}]$ where a termolecular reaction with three unique reagents, $A$, $B$ and $C$ has a rate law of $k[\text{A}][\text{B}][\text{C}]$. Now, other things being equal, and assuming for the sake of argument that the initial concentrations are all greater than 1M, this means that the termolecular elementary reaction is more likely at the outset. But that doesn't make much sense. There may be some statistical mechanics principle that's lost on me but it seems more likely that A bumps into A than, A, B and C come together all at once. So I don't quite follow this reasoning. Is it the case that $k$ is generally quite smaller in these termolecular elementary reactions?

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I second J M’s answer: the rate does not depend only on concentrations, but the rate constant is key. However, I will add two more comments:

  • First, you cannot compare values of rate constants for reactions of different orders… they don't have the same dimension (unit). So, you cannot say things like “rate constants of third order reactions are typically smaller than those of first-order reactions”.
  • However, you can say (and it is true) that reactions rates of high-order elementary reactions tend to be small, because of the low probability of the molecules crossing path.
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  • $\begingroup$ Re: units. Good point. $\endgroup$ – readyready15728 Sep 3 '12 at 16:49
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That is exactly the case. My professor is famous (well within our group at least) for often saying "A rate is a rate constant times a concentration" You can flood your reaction with a high concentration of reactants, but if the rate constant is 0, nothing happens.

In the case of a termolecular reaction (which usually unlikely), the rate constant is going to be very small.

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