Final answer:
The incorrect statement among the options provided for a valid reaction mechanism is that each elementary reaction must be bimolecular. Elementrary reactions can vary in molecularity and are not restricted to being bimolecular.
Step-by-step explanation:
The statement that is not a requirement for a valid reaction mechanism is 'each elementary reaction must be bimolecular.'
In reaction mechanisms, the elementary reactions can have different molecularities, meaning they can be unimolecular, bimolecular, or even termolecular, with bimolecular reactions being common but not exclusive. It is also not required for a mechanism to have only one rate-determining step, though often one step will be substantially slower.
However, it is necessary that there must not be net production of any intermediates, the elementary reactions must add up to the net chemical reaction, and the rate law predicted by a mechanism must agree with the experimental rate law to consider a mechanism valid.