CHAPTER 4  >  READINGS  >  A BRIEF TEXT 5

5. Chemical Potential of Dissolved Substances


Dependence of the chemical potential upon concentration

In the case of solutions that are not too strong, the (partial) pressure of the dissolved substance in the solution obeys the equation of state of the ideal gas. 
p_g · V = n·R·T
p_s = R·c·T
4.23
The pressure of the solvent (such as water) therefore is
p_w = p_air – p_s
4.23b
Therefore, Equ. 4.17 holds as well for the pressure ps of a solute. Since the concentration of the solute is proportional to the pressure, the same form must hold for the concentration dependence of the dissolved substance (Fig. 1):
   4.24














Figure 1

Osmosis

The pressure of the solvent (such as water) decreases according to Equ. 4.23b if the concentration of a solute increases. This effect is at the root of the phenomenon of osmosis (water or some other solute will flow from places where the concentration of a solute is lower to one where the concentration is higher).


Chemical equilibrium of dissolved substances

Chemical equilibrium is attained if
   4.25
If the substances A and B in a simple reaction A <–> B are dissolved in solvents, or if they are gases, their chemical potentials depend logarithmically upon concentration. Therefore, the equilibrium condition leads to:
   
The index 0 refers to standard conditions, eq stands for equilibrium. The equilibrium constant K of this reaction is defined according to:
   4.26
which leads to
   4.27



Mass action

Traditionally, Equ. 4.27 is called the principle of mass action.