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Salt hydrolysis
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It is defined as the process in which a salt reacts with water to give back the acid and the base. Hydrolysis is the reverse of neutralisation, Salt + Water ® Acid + Base
The general equation for the hydrolysis of a salt (BA) may be written as :
Applying the law of chemical equilibrium, we get ([HA][BOH])/(BA][H2O])=K the equilibrium constant. Since water is present in very large excess in the aqueous solution, its concentration [H2O] may be regarded as constant so that we have, ([HA][BOH])/([BA]) = K[H2O] = Kh where Kh is called the hydrolysis constant.
All salts are strong electrolytes and thus ionize completely in the aqueous solution. If the acid (HA) produced is strong and the base (BOH) produced is weak, we can write the above equation as
B+ + A- + H2O ® H+ + A- + BOH or B+ + H2O ® H+ + BOH
Thus, in this case the cation reacts with water to give an acidic solution. This is called cationic hydrolysis. Again, if the acid produced is weak and the base produced is strong, we can write.
B+ + A- + H2O ® HA + B+ + OH- or A- + H2O ® HA + BOH
Here the anion reacts with water to give the basic solution. This is called anionic hydrolysis. Hence salt hydrolysis may be defined as the reaction of the cation or the anion of the salt with water to produce acidic or basic solution.
Thus depending upon the relative strengths of the acid and the base produced, the resulting solution is acidic basic or neutral on this basis, the salts are divided into four categories :
(1) Salts of strong acids and strong bases : Example are : NaCl, NaNO3, Na2SO4, KCl, KNO3, K2SO4 as an illustration, let us discuss the hydrolysis of NaCl. We may write as follows :
NaCl + H2O ® NaOH + HCl or Na+ + Cl- + H2O ® Na+ + OH- + H+ + Cl- or H2O ® H+ + OH-
Thus it involves only ionization and no hydrolysis. Further in the resulting solution, [H+] = [OH-] So the solution is neutral. Hence it can be generalized that the salt of strong acids and strong bases do not undergo hydrolysis and the resulting solution is neutral.
(2) Salts of weak acids and strong bases :
For salts of weak acid and strong base hydrolysis constant, Kh = (Kw/Ka )
Degree of hydrolysis, h= Ö(Kh/C) = Ö(Kw/KaC)
pH = - 1/2 [log Kw + log Ka - log C](C = constant of solution in mol L-1)
Example are : CH3COONa, Na2CO3, K2CO3, Na3PO4 etc. As an illustration, the hydrolysis of sodium acetate (CH3COONa) may be represented as follows :
CH3COONa + H2O « CH3COOH + NaOH or
CH3COO- +Na+ + H2O « CH3COOH + Na+ + OH- or
CH3COO- + H2O ® CH3COOH + OH-
As it produces OH- ions, the solution of such a salt is alkaline in nature.
(3) Salts of strong acids and weak bases : For salts of strong acid and weak base hydrolysis constant,
Example are : NH4Cl, CuSO4, NH4NO3, AlCl3, CaCl2 etc. As an illustration the hydrolysis of NH4Cl may be represented as follows :
NH4Cl + H2O « NH4OH + HCl or NH+4 + Cl- + H2O « NH4OH + H+ + Cl- or NH+4 + H2O ® NH4OH + H+
As it produces ions, the solution of such a salt is acidic in character.
(4) Salts of weak acids and weak bases :
For salt of weak acid and weak base hydrolysis constant,
pH = - 1/2 [log Ka + log Kw -log C]
Example are : CH3COONH4, (NH4)2CO3, AlPO4 etc. As an illustration, the hydrolysis of ammonium acetate may be represented as follows :
CH3COONH4 + H2O « CH3COOH + NH4OH or
CH3COO- + NH+4 + H2O ® CH3COOH + NH4OH
Thus it involves both anionic and cationic hydrolysis. The resulting solution may be neutral or slightly acidic or basic depending upon the relative degrees of ionization of the weak acid and the weak base produced. In the present example, the acid (CH3COOH) and the base (NH4OH) formed are almost equally weak. Hence the resulting solution is almost neutral.
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