Sub-module 3A
Partitioning.
Here's a little experiment. A 4 L closed jar holds air, a plant, and some water. If we inject some MEK into the system, where does it go? At first, if its a solid and heavier than water, it will fall to the bottom of the jar. If it is a liquid that is lighter than water, it will float on the surface of the water. If it is gas, it will diffuse into the air in the jar. But where is the MEK if you test the material in jar 24 hours later? Or a week later?
Although an exact answer to that depends on the chemical nature of MEK, it is a safe bet that the MEK will be distributed: some in the air, some in the plant, and some in the water. You learned earlier about unsteady state, and that is what this system is initially, when you first put the MEK into the jar. At some later time the system comes to equilibrium. We say MEK is in steady state, its concentrations in the media (air, plant, and water) do not change with time. What controls where the MEK is? Why does it move? The answer lies in the concept of chemical potential, Gibbs free energy, and the second law of thermodynamics, all places we will not go. But briefly the MEK distributes itself so that its chemical potential is equal in all the media or phases. Donald Mackay, in Chapter 5 of his book, goes into the concept of fugacity. Fugacity is a thermodynamic concept that is particularly handy in sorting out this type of equilibrium problem where there are many different media. (I sometimes refer to Mackay as the patron saint of fugacity.)
For the simple case of two media or phases, we can describe the partitioning of the chemical between the two media with familiar concepts. For example, if we place some pure chemical in the bottom of a closed flask initially filled with air that is sealed, the partitioning is described by the vapor pressure of the chemical. If we place a pure liquid chemical that is relatively insoluble in water into a flask with water, some of the chemical will dissolve in to the water, some will stay in a separate layer or phase. (Which may be on the top or bottom of the water, depending on the density of the chemical.) This partitioning is described by the solubility of the chemical in water.
Fortunately for us, Dr. Mackay, who is at Trent University, has placed a number of models on a web site, for free download. Next