4.1 Kinetic Fractionation
For physical, chemical and exchange reactions, the dissociation energy of molecular bonds controls the reaction rate. A bond involving lighter isotopes has a lesser dissociation energy and can break more easily than the same bond involving a heavier isotope. One can think of this as requiring more energy to break the bond with a heavier than a lighter isotope. For example, when a body of liquid water evaporates into air, the resultant water vapor (H2O gas mixed in air) will be relatively enriched in the lighter isotopes and therefore have lower values for the ratios 2H/1H and 18O/16O than the source body. This isotopic fractionation will be exaggerated if the vapor is being removed rapidly, as happens in windy situations or at low humidity when water evaporates from natural water bodies, and is known as kinetic fractionation. When the air is drier or the wind is stronger, removal of 1H and 16O bearing water molecules will increase, because there is less opportunity for the evaporated water to react backwards with the liquid water body. Bear in mind, that on the other hand, a lack of mixing in the source water body will result in a lesser fractionation effect, because the surface water layer will become depleted in 16O with time.
For diffusion, the diffusive velocity is inversely proportional to the mass of the molecule and therefore molecules with lighter isotopes will diffuse faster. For the above example, if there is no wind, but diffusion is high because humidity is low, then again, the increased diffusion of water molecules containing 1H and 16O will increase the kinetic effect.
For kinetic fractionation there is no fixed difference in isotope ratios between the source and receptor reservoirs, as this is dependent upon factors such as time, degree of removal of one reservoir and degree of mixing of the reservoirs (i.e., wind or diffusivity), as well as the type of reaction, or change of state, that is taking place.