A reaction involving two different reactants can never be
- unimolecular reaction
- bimolecular reaction
- second order reaction
- first order reaction
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 be 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.
The correct option is A.