Hydrolysis of salts

How is a salt formed? When an acid and a base react, H+ of the acid combines with OH- of the base and they neutralise each other, while remaining anion of the acid and cation of the base combine to form the salt. So we can form different categories of salts on the basis of parent acid-base:

• Salt of strong acid and strong base
• Salt of strong acid and weak base
• Salt of weak acid and strong base
• Salt of weak acid and weak base

When we put a salt in water, it dissolves in water. Polar force of water weakens the electrostatic attraction which binds the anion and cation of the salt and separates them into ions. Water molecules hydrate these ions and keep them separated. Sometimes these ions react with water molecules, anions try to polarise water molecule and if they succeed to create enough polarization it may cause the breakage of O-H bond of water. This process is called the hydrolysis.

Hydrolysis of salt

In this post we will discuss different categories of salts and see what happens when they are dissolved in water.

Let’s discuss the first category: Salt of strong acid and strong base, like NaCl. NaCl is made by the reaction of HCl and NaOH. When it is dissoved in water it dissociates to form Na⁺ and Cl^-which then react with water molecule and reproduce their parent acid and base. Since both parent acid and base are equally strong, they neutralise each other and the pH of water remains unchanged. So it may seem that such salts don’t cause hydrolysis.

NaOH + HCl ⟶ NaCl + H2O
Na⁺_(aq) + H2O ⇌ NaOH + H⁺
Cl^-_(aq)  + H⁺ ⇌ HCl

Salt of strong acid and weak base: NH4Cl is made from acid HCl and base NH₄OH. In water NH₄Cl dissociates completely in NH4⁺ and Cl^- ions. NH₄⁺ ions successfully polarize water molecule and cause hydrolysis.

NH₄OH + HCl ⟶ NH₄Cl + H₂O
NH₄⁺_(aq)  + H₂O ⟶ NH₄OH + H⁺

Ammonium ion (NH₄⁺) reacts with water and form ammonium hydroxide (NH₄OH) and H⁺. NH₄OH is a weak base which dissociates a little. Thus OH^- ions of water are consumed by NH₄⁺ and H⁺ reacts with Cl^-ions.

Cl^- _(aq) + H⁺ ⇌ HCl

HCl is a strong acid and it dissociates completely. That means H⁺ions produced by hydrolysis of NH₄Cl salt remain in solution while OH^- ions get trapped by NH₄⁺. This higher concentration of H⁺ ions makes the solution acidic.

Salt of weak acid and strong base: CH₃COONa is a salt of weak acid CH₃COOH and strong base NaOH. When it is dissolved in water it dissociates completely.  

CH₃COONa + H₂O ⟶ CH₃COO^-+ Na⁺
Na⁺ + H₂O ⇌ NaOH + H⁺
CH₃COO^- + H⁺ ⟶ CH₃COOH

Na⁺ ions cause hydrolysis and produce NaOH and H⁺ions. NaOH is a strong base and it dissociates completely thus OH^-ions of water remain in the solution. And H⁺ ions react with acetate ion (CH3COO^-) and form acetic acid, which is a weak acid. It dissociates partially so H⁺ ions of water get trapped and the pH of solution increases.

Salt of weak acid and weak base: CH₃COONH₄is a salt of weak acid CH₃COOH and weak base NH₄OH. When it is dissolved in water, it dissociates completely.  

CH₃COOH + NH₄OH ⟶ CH₃COONH₄+ H₂O
CH₃COONH₄ + H₂O ⟶ CH₃COO^-+ NH₄⁺

NH₄⁺ ions cause hydrolysis and produce weak base NH₄OH and H⁺ ions. And when H⁺ ions react with CH₃COO^- ions, they produce weak acid CH₃COOH. The resulting acid and base both are weak and dissociate partially. It means, the concentration of both H⁺ and OH^- gets affected and to calculate the pH of such solution we have to know the values of pK_aand pK_b.

pH = 7+ ½ (pK_a - pK_b)

And by using the above equation we can calculate the pH of such solutions. We have learnt how salts affect the pH of water. But not everything is completely soluble in water, there are a number of substances which are slightly soluble or insoluble in water. So what are the factors which decide the solubility of any substance? In the next post we will learn about solubility and see if it is possible to increase or decrease the solubility of any substance. 

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