Hydrolysis

What is hydrolysis?

Literally hydrolysis means breakage by water. We know that when an acid and a base react with each other a salt plus water is formed:

AH +BOH <–> AB + H2O

It is conceivable that if the process is reversible, i.e., that when a salt dissolves in water, an acid or base may appear in the medium, assigning it a certain pH.

AB + H2O <–> AH + BOH

The hydrolysis reaction occurs only with certain salts, and four different cases can occur:

  1. strong acid-strong base salt:

ClNa neither the cation nor the anion is hydrolyzed.

ClNa <–> Cl + Na+

Cl + H2O (no) <–> ClH + OH

Na+ + H2O (no) <–> NaOH + H+

As neither the cation nor the anion can be hydrolyzed, the pH will be neutral.

  1. strong acid-weak base salt:

ClNH4 hydrolyzable cation salt

ClNH4 <–> Cl + NH4+

Cl + H2O (no) <–> ClH + OH

NH4+ + H2O <–> NH4OH + H+

Kh = [NH4OH]·[H+]/[NH4+] [H2O] = cte

Multiplying by [OH]

Kh = [NH4OH]·[H+]·[OH]/[NH4+]·[OH] = [NH4OH]·Kw/[NH4+]·[OH] = Kw/Kb

Kh = Kw/Kb the stronger the base the less it hydrolyzes

NH4OH <–> NH4+ + OH

Kb = [NH4+]·[OH]/[NH4OH]

  1. weak acid-strong base salt:

CNK hydrolyzable anion salt

CNK <–> CN + K+

K+ + H2O (no) <–> KOH + H+

CN + H2O <–> CNH + OH

Kh = [CNH]·[OH]/[CN] [H2O] = cte

Multiplying by [H+]

Kh = [CNH]·[OH]·[H+]/[CN]·[H+] = [CNH]·Kw/[CN]·[H+] = Kw/Ka

Kh = Kw/Ka the stronger the acid the less it will hydrolyze

CNH <–> CN + H+

Ka = [CN]·[H+]/[CNH]

  1. weak acid-weak base salt:

CH3COONH4 sal de catión y anión hidrolizables

CH3COONH4 -> CH3COO + NH4+

CH3COO + H2O -> CH3COOH + OH

NH4+ + H2O -> NH4OH + H+

————

CH3COO + NH4+ + H2O -> CH3COOH + NH4OH

Kh = [NH4OH]·[ CH3COOH]/ [CH3COO] [NH4+] [H2O] = cte

Multiplying by [OH] y [H+]

Kh = [CH3COOH]·[NH4OH]·[H+]·[OH]/[ CH3COO][NH4+]·[H+]·[OH]

= Kw/Ka·Kb

Degree of hydrolysis

The degree of hydrolysis (α) is the percentage of salt that is hydrolyzed.

If α = 1 the salt is completely hydrolyzed.

If α = 0 there is no hydrolysis.

The relationship between the degree of hydrolysis and the hydrolysis constant is:

Ac + H2O <–> AcH + OH

Equilibrium C(1-α) Cα + Cα

Kh = cα2/(1-α)

The degree of hydrolysis and therefore the hydrolysis of a salt increases with temperature, with dilution and with the greater weakness of either the acid or the corresponding base.