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⁺

K_h = [NH₄OH]·[H⁺]/[NH₄⁺] [H₂O] = cte

Multiplying by [OH^–]

K_h = [NH₄OH]·[H⁺]·[OH^–]/[NH₄⁺]·[OH^–] = [NH₄OH]·K_w/[NH₄⁺]·[OH^–] = K_w/K_b

K_h = K_w/K_b the stronger the base the less it hydrolyzes

NH₄OH <–> NH₄⁺ + OH^–

K_b = [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^–

K_h = [CNH]·[OH^–]/[CN^–] [H₂O] = cte

Multiplying by [H⁺]

K_h = [CNH]·[OH^–]·[H⁺]/[CN^–]·[H⁺] = [CNH]·K_w/[CN^–]·[H⁺] = K_w/K_a

K_h = K_w/K_a the stronger the acid the less it will hydrolyze

CNH <–> CN^– + H⁺

K_a = [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

K_h = [NH₄OH]·[ CH₃COOH]/ [CH₃COO^–] [NH₄⁺] [H₂O] = cte

Multiplying by [OH^–] y [H⁺]

K_h = [CH₃COOH]·[NH₄OH]·[H⁺]·[OH^–]/[ CH₃COO^–][NH₄⁺]·[H⁺]·[OH^–]

= K_w/K_a·K_b

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

K_h = 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.