Bimolecular Elementary Reactions

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OpenStax Chemistry 2e

A bimolecular reaction involves two reactant species, for example:

A + B ⟶ products
2A ⟶ products

For the first type, in which the two reactant molecules are different, the rate law is first order in A and first order in B (second-order overall):

rate = k[A][B]

For the second type, in which two identical molecules collide and react, the rate law is second order in A:

rate = k[A][A] = k[A]2

Some chemical reactions occur by mechanisms that consist of a single bimolecular elementary reaction. One example is the reaction of nitrogen dioxide with carbon monoxide:

NO2(g) + CO(g) ⟶ NO(g) + CO2(g)

The probable mechanism for the reaction between NO2 and CO to yield NO and CO2. Source: OpenStax Chemistry 2e

Bimolecular elementary reactions may also be involved as steps in a multistep reaction mechanism. The reaction of atomic oxygen with ozone is the second step of the two-step ozone decomposition mechanism discussed earlier in this section:

O(g) + O3(g) ⟶ 2O2(g)