I have mentioned several times in the past few chapters that complex reactions can have either simple or complex rate laws. We will now see how complex reactions can give rise to complex rate laws, but also how these rate laws often simplify to simple forms.

You may notice as you read this chapter that it seems to focus a lot on gas-phase chemistry, and that there are few biological examples and problems. This is a deliberate choice. First of all, gas-phase chemistry is of interest to biologists since the atmosphere ties us all together. My second reason for focusing on gas-phase chemistry here has to do with pedagogy: examples of complex mechanisms deriving from gas-phase chemistry are often simpler than those arising in biochemistry.We will get a good grounding in the basic methods for treating complex mechanisms in this chapter before moving on to enzyme kinetics in the next chapter.

Two-step mechanisms

In order to get our feet wet with complex reactions, we will try to resolve a puzzle: on the surface, unimolecular gas-phase reactions, i.e. reactions with the stoichiometry A → product(s), should be the simplest reactions around. However, these reactions always have complex rate laws: at low pressures, they display second-order kinetics, while at high pressures, they follow first-order kinetics with respect to the reactant. (At intermediate pressures, the rate can't be described by either a first- or a second-order rate law.)