Question

Why do most chemical reactions require multiple steps (reaction mechanism) and can't complete itself in one collision?

For ex:
2 NO2 (g) → NO3(g) + NO(g)
NO3(g) + CO(g) → CO2 (g) + NO2(g)

instead of just:
CO(g) +NO2 (g) → CO2 (g) + NO(g)

Answer 1

The one-step reaction would be acceptable if it agreed with the rate law data for the reaction. If it does not, a reaction mechanism is proposed that does agree.

Explanation:

For example, in the above process, we might find that the rate of the reaction is not affected by changes in the concentration of the CO gas.

A single-step process would be difficult to suggest as we would find difficulty in explaining why a reaction that seems to depend on a single collision between two molecules would be affected if the concentration of one molecule is altered, but not if the other molecule's concentration changes.

The two-step mechanism (with a rate-determining step in step 1) would agree much better with these observations.

Beside that, if the number of molecules in the reaction is more than about three, or if the changes in the molecules are extensive, it is difficult to justify that all these changes occur in a single collision event, or the numerous molecules can all collide at one place and at one time.

So, mechanisms are suggested to give better agreement with what we know about the reaction (especially the rate law).