Question

How do unimolecular and first order relate to each other? (Are all first order reactions unimolecular? Or the other way round? What is the difference?)

Answer 1

Mostly all of the unimolecular reactions will be first order. but all first order reactions need not be unimolecular

unimolecular reaction is reaction in which only one molecule will undergo reaction to give one or more products

ex: decomposition reaction, fission reaction, isomerisation

PCl5 ---> PCl3 + Cl2

Order of reaction can also be defined as number of molecules of the reactant(s) whose concentration changes during the chemical change is called order of reaction.

Order of a bimolecular or more can also be a first order. so first order need not be unimolecular

ex:

2 N2O5 ----> 4 NO2 + O2

bimolecular first order.

but all unimolecular reaction will be mostly first order reactions.except in very few cases it might be zero Order or fractional order reaction