It is obvious that the frequency of collision between molecules will depend on several factors, including:

Diffusion coefficient (and hence h , r, T, etc)

Diffusion distance (concentration)

Target size (also r)

When reactions between molecules occur at every collision, the reaction is said to proceed at the diffusion limit. Such reactions have NO ENERGY OF ACTIVATION, and are called diffusion-controlled reactions.

from the Arrhenius equation

k = A exp[-Eact/RT]
(where k is the rate constant, R is gas constant).

When Eact = 0, k = A.

For a diffusion-controlled reaction, A is given (approximately) by:

Adiff = 4p(rm + rn) (Dm + Dn) No / 1000
(units M-1 s-1)

here rm, rn, Dm, and Dn are the radii (in cm) and diffusion coefficients (in cm2 s-1) of reaction species m and n, and No is Avagadro's number. Division by 1000 cm3 gives k in units M-1s-1.

I trust this will settle the Hash of Rapp for the time being. I am equally certain that the Hash of Rapp will not remain settled for any significant period of time.