Special Chemical Equilibria

• Write the equilibrium constant expression when the solvent is one of the products or reactants.
  

• Write the equilibrium constant expression for heterogeneous equilibria.

Special Chemical Equilibria

The mass action law is valid for the case when the solvent is a reactant or product. However, due to the large amount of solvent present, the equilibrium constant expression can be simplified.

Special consideration also apply to heterogeneous equilibria, in which, solids or liquids are involved. Phases such as liquid and solid are not sensitive to pressure, and their "concentration" are constant as long as these phases are present. Their "concentrations" are not defined, and that is why we use a " " to mark the special meaning of these phases. Perhaps "activity" or "property" is a better term to use than "concentration" in these cases.

Equilibrium Constant for Reactions Involving Solvents

For example, when water is used as a solvent, the concentration of water is almost always a constant. Its concentration can be calculated to be 55.6 M. (1 L of water has 1000 g, and molecular weight 18 g/mol; and thus [H₂O] = 1000/18 = 55.6 mol/L)

For example, consider the following reaction,

       [CH3COOC2H5] [H2O]
 K' =  -------------------
       [CH3COOH] [C2H5OH]
or
       [CH3COOC2H5]
 K  =  -------------------
       [CH3COOH] [C2H5OH]

Heterogeneous Equilibria

In heterogeneous equilibria, the activities (or concentrations) of solid and liquid but not gas are always a constant. In these cases, their activities or concentrations are omitted in the expression of Q or K.

Let us look at these expressions for the equilibria:

Confidence Building Questions

• What is the concentration of water in pure water?
  

  Hint 55.6
  Consider...
  For most solutions, [H₂O] = 1000 / 18 = ?

• Give the K_p expression for the reaction HgO(s) = Hg(s) + O₂(g)
  

  Hint P(O₂)
  Consider...
  This reaction is used to produce O₂ from HgO.

• For the phase transition, H₂O(l) = H₂O(g), the vapor pressure of water depends on temperature. At 298 K, the saturated vapor pressure is 23.8 torr. What is the equilibrium constant based on pressure K_p in atm? (1 atm = 760 torr)
  

  Hintr 0.031 atm
  Consider...
  K_p = P(H₂O) in this case. Just want you to consider one of the common equilibrium state. The amount of water present does not affect the vapor pressure.

• At some low temperature, the reaction of ethyl acetate with water, CH₃COOC₂H₅(aq) + H₂O(l) = CH₃COOH(aq) + C₂H₅OH(aq) has an equilibrium constant of 0.10. If [CH₃COOC₂H₅] = 0.90 M in a system that is at equilibrium, what is [C₂H₅OH]?
  

  Answer 0.3
  Consider...
  

  CH3COOC2H5(aq) + H2O(l) = CH3COOH(aq) + C2H5OH(aq)
          0.9                 x             x
  

  x² / 0.90 = K = 0.1;       x = ?
  What was the original [CH₃COOC₂H₅] before any hydration reaction takes place? (Answer, 1.2)

• The value of K_p is 81 Pa at some temperature for the reaction, NH₄Cl(s) = NH₃(g) + HCl(g). What is the partial pressure (in Pa) of HCl in an enclosed system containing solid NH₄Cl and its vapor?
  

  Answer 9
  Consider...
  P(HCl) = square root (K_p)

     NH4Cl(s) = NH3(g) + HCl(g).
                 x        x
  

  K_p = x²
  The partial pressure of NH₃ is the same as that of HCl.