Every chemical process is reversible on the molecular level. If A molecules can change into B molecules, then B must be able to change into A, although perhaps at a different rate. If and are the rate constants for the forward and reverse reactions, then as in the crabapple analogy,

forward rate = forward rate constant x concentration of reactants
= [A]

reverse rate = reverse rate constant x concentration of products
= [B]

[A] and [B] are the concentrations of molecules of A and B. Instead of units of apples per square foot, we measure molecular concentrations in molecules per cubic centimeter, or more conveniently, moles of molecules per liter (1 mole = 6.022 X 10 molecules and 1 liter = 1000 cm). If overall reaction rates are expressed in moles of substance reacting per second, then the rate constants, and have units of liters per second:



These rate constants again describe how fast each of the forward and reverse reactions "cover the territory" that the molecules occupy.