In summary, I have extended the adsorption rate constant of diffusive molecules at a fixed surface to free molecules in the bulk solution. A concentration-dependent rate constant is obtained as an alternative model to the classical Smoluchowski rate equation, giving a surprising extra 1/# dimension dependence on the concentration. Note that there are still missing parts in the new model. The physical meaning of such a rate is very different than that in the classical collision theory where a hard-sphere collision model can be used to approximate the interactions. The molecules collide in the solution from unpredictable trajectories and an unknown number of collisions within an observation time window. All we know in this new model is that on average, there has been at least one collision happening during the critical observation time window with the designated collision geometry. The average number of collisions during this period is missing. It is very challenging to define the duration of a diffusive collision as we know the number of collisions is dependent on the observation time. The average distance is inversely proportional to the 1/3 order of the summed concentration and should be updated for reaction systems that may not follow this relationship due to the dynamic evolution of concentration gradient within the molecular level. This model also ignores the complication of 3D fractal diffusion which should be examined. Nevertheless, with these limitations, I believe that the new model is compatible with the collision theory opening up a whole dimension to predicting the bimolecular reaction rate of diffusive molecules in diluted solutions where the steady-state Smoluchowski model has its limit. The concept of the effective reaction cross-section can be introduced as complementary to the steric effect that has been widely used in collision theory. Combined with the activation energy data, we may eventually gain a much better understanding of the collision frequency and reaction rate of molecules in a diluted gas or liquid solution.