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Thus far we have established that the rates of chemical reactions depend on the concentrations of reactant species, and that this dependence often has a complicated form that cannot be predicted directly from the coefficients of the balanced chemical equation (as the equilibrium-constant expression can). The reason for these complications is that the actual mechanism of reaction may involve a series of small, one or two-molecule reactions, with intermediate complexes of atoms that are used up in subsequent steps. The rate equation only summarizes the overall process and does not tell us what is happening in the individual steps. Nevertheless, if we can come up with a series of hypothetical reaction steps that faithfully reproduce the observed rate expression, then we feel confident that our hypothetical mechanism must be close to the actual mechanism. Molecules react when they collide, provided that the collision generates enough energy to tear the atoms apart from one another and rearrange them into new molecules. Bimolecular collisions (between two molecules) are common in gases, but simultaneous trimolecular collisions are a thousand times rarer, and four-molecule simultaneous impacts are so infrequent as to be eliminated from consideration. Then how does the following multimolecular smog reaction take place?
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