Four- and five-body simultaneous collisions
are very improbable events. A reaction in which four or five molecules
interact is much more likely to occur in a series of steps, in which
two reactant molecules first collide and form an intermediate substance,
this substance collides with the next reactant
molecule, and so on.
The observed rate
law is a summary of all of these steps, and may depend on reactant
concentrations in a complicated way. The form of the experimental
rate law is the first step on the way toward unscrambling the actual
mechanism of reaction, but it does not give us the entire story.
Only for simple, one-step reactions will
the order of the rate law necessarily agree with the coefficients
of the balanced equation. This occurs almost exclusively with first-order
decompositions and with those relatively rare second-order
collisions that are uncomplicated by further reactions.
What we have just said applies to rate expressions,
but not to equilibrium-constant
expressions. The exponential coefficients in the equilibrium-constant
expression do match the coefficients in the balanced chemical
equation. The equilibrium-constant expression for the HI reaction