Hydrolysis Reactions

 

I. Introduction

     A. The term hydrolysis refers to dissolution reactions in which either (or both) of the O-H  

     bonds in water is broken resulting in the release of H+ and OH- into solution. (Faure, pp. 137-

     139)

 

     B. Some minerals do not react with the water molecule upon dissolution and, therefore, do

      not change the pH of the solution.

          1. These are the compounds and minerals that contain cations of strong bases and

           anions of strong acids so that these ions remain completely dissociated in solution

           and do not tie up H+ or OH- and, therefore, do not change pH.

 

     C. Determining the nature of the cations and anions in a compound allows you to make

      predictions about whether or not it causes hydrolysis reactions and allows you to pre-

      dict potential pH changes.

 

II. Predicting pH changes

     A. Salts of weak acids and strong bases:

 

     K2CO3 (s)  +  H2O (l)  =  2 K+ (aq)  +  HCO3- (aq)  +  OH- (aq)

 

      "K+" is the cation of the strong base, KOH and "CO3-" is the anion of the weak acid, H2CO3.

     Therefore, the K stays dissociated and does not absorb OH, while the carbonate ion attaches

      to H+ of the water molecule releasing OH- into solution to increase the pH.

 

    B. Salts of strong acids and weak bases:

 

        Fe2(SO4)3 (s)  +  2 H2O (l)  =  2 FeOH2+ (aq)  +  3 SO42- (aq)  +  2 H+ (aq)

 

   "Fe2+" is the cation of the weak base, FeOH and "SO42-" is the anion of the strong acid, H2SO4.

    Therefore, the "SO42-" stays dissociated and does not absorb H+, while the Fe2+ ion attaches

     to OH- of the water molecule releasing H+ into solution to decrease the pH.

 

     C. If only one ion of the pair hydrolyzes as in the above cases, the solution will be acidic or

     basic. In these cases we can visualize the reactions as a partitioning of H+ between OH- and  

     CO32- in the first case and a partitioning of OH- between Fe2+ and H+ in the second case.

 

     D. If both ions hydrolyze (FeCO3, for example) their effects may partly or completely cancel  

      each other and leave the solution nearly unchanged. Such compounds are composed of the

     cation of a weak base and the anion of a weak acid.

 

                     FeCO3 (s)  +  2 H2O (l)  =   FeOH2 (aq) +  H2CO3 (aq) 

 

E. If neither ion hydrolyzes (NaCl, of CaSO4, for example) the solution pH is unchanged.

     Such compounds are composed of the cation of a strong base and the anion of a strong acid.