Generally, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation. Thus, a reaction involving two different reactants can never be unimolecular.
But a reaction involving two different reactants can a first order reaction. For example, for the following reaction
RCl + H2O → ROH + HCl Expected rate law :
Rate = k[RCl][H2O] expected order = 1 + 1 = 2
But actual rate law : Rate = k'[RCl] actual order = 1
Here water is taken in excess, hence its concentration may be taken constant.
Here the molecularity of the reaction = 2 and the order of the reaction = 1.
