The equilibrium constants of the reactions involving isotope molecules may also be different:

AX 1 BYoAY 1 BX (3.34)

AX01 BYoAY 1 BX0 (3.35)

Both X and X0 mean the isotopes of the same element. The equilibrium con­stants are as follows:

The ratio of the two equilibrium constants is:

K = K = = [BX][AX0]

K0 [BX0][AX]

This ratio gives the equilibrium constant of the isotope exchange reaction:

AX 1 BX0 з AX01 BX

When the equilibrium constant of the isotope exchange is equal to 1, there is no isotope effect—the distribution of the isotopes is the same in both compounds.

The equilibrium constants of some isotope exchange reactions are listed in Table 3.4. The calculated values were obtained from the thermodynamic properties.

As seen in Table 3.4, the equilibrium constants of isotope exchange reactions are close to 1 but frequently not equal to 1. These small differences from 1 have, however, a great theoretical and practical importance because they provide a way to separate isotopes and give important geological information (discussed further in Section 3.4).