For gas phase reaction the rate is often alternatively expressed by partial pressures.
The exponents n and m are called reaction orders and depend on the reaction mechanism.
Stoichiometry, molecularity (the actual number of molecules colliding), and reaction order coincide necessarily only in elementary reactions, that is, those reactions that take place in just one step. The reaction equation for elementary reactions coincides with the process taking place at the molecuar level, i.e. molecule A collides with molecule B. From collision theory follows that the likelihood of a collision of three molecules is highly unlikely. Therefore molecularity for elementary reactions is either one ore two. Empirically, other values can be assigned to allow mathematical description of the rate. Then, positive rational numbers are not uncommon but should not be assigned physical meaning.