Final answer:
The rate-determining step in the formation of carbonyl chloride is unimolecular, as suggested by the three-step mechanism. This means that only one molecule participates in this step.
Step-by-step explanation:
The molecularity of a reaction is the number of molecules reacting in an elementary step. A unimolecular reaction is one in which only one reacting molecule participates in the reaction. A bimolecular reaction involves two reactant molecules colliding with one another, while a termolecular reaction involves three reacting molecules in one elementary step.
In the case of the formation of carbonyl chloride (COCl2), the three-step mechanism suggests that the rate-determining step is unimolecular. This means that only one molecule participates in the rate-determining step. The mechanism is called elimination unimolecular (E1) and it competes with SN1 because they have a common first step.