A bimolecular reaction involves two reactant species, for example:
$$A+B⟶products$$
$$and$$
$$2A⟶products$$
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:
$$NO_2(g)+CO(g)⟶NO(g)+CO_2(g)$$
(see [link])
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:
$$O(g)+O_3(g)⟶2O_2(g)$$