Question

Is a bimolecular reaction necessarily second-order? Is a second-order reaction necessarily bimolecular? Answer, with explanations and clarifications.

Answer 1

No a bimolecular reaction may or may not be second order. For example in an SN2 reaction, we have a bimolecular reaction wherein both substrate and nucleophile participate in the reaction mechanism and part of rate determining step, so in this case the reaction becomes a second order. However, in case of an SN1 reaction, the reaction is bimolecular but only the substrate with leaving group participitate in the rate determining step and thus, the reaction becomes a first order in kinetics.

Yes, a second order reaction is always bimolecular in nature. Two molecules participate in the rate determining step and that makes the reaction as in SN2 or E2 reaction mechanisms a second order reaction.