Consider the reversible unimolecular reaction:
$A \overset{k_{1}}{\underset{k_{2}}{\rightleftharpoons}} B$$$\ce{A <=>[k_1][k_2] B}$$
We know that the forward reaction is often considerably more thermodynamically favourable than the reverse reaction, and therefore the relationship k1 >> k2$k_1 \gg k_2$ holds between the rate constants. The rate constants are in the same units, and so it is possible to write this relationship. A similar example would be a reaction that is bimolecular on both sides, $A + B \rightleftharpoons C + D$$\ce{A + B <=> C + D}$, etc.
However, consider this reversible reaction:
$A + B \overset{k_{1}}{\underset{k_{2}}{\rightleftharpoons}} C$$$\ce{A + B <=>[k_1][k_2] C}$$
Assume the reaction proceeds at rate $k1[A][B]$$k_1 [\ce{A}][\ce{B}]$ in the forward direction and $k2[C]$$k_2 [\ce{C}]$ in reverse. If the overall reaction rate has units $M s^{-1}$, then $k1$$k_1$ necessarily has units $M^{-1} s^{-1}$ and $k2$$k_2$ has units $s^{-1}$.
My question is: For this second case, we can no longer impose that $k1 >> k2$$k_1 \gg k_2$, due to difference in units, but can we say anything about their relationship? Does setting $k1$$k_1$ to some value constrain the choice of $k2$$k_2$ in any way?
