Salt hydrolysis is the process of breaking down salt into its component ions in interaction with water. This is also responsible for the acidity or basicity of the solution. Considering that, this process is also named salt dissolution.
The reaction is usually represented by the equation given below:
The interaction of cations is known as cationic hydrolysis whereas the interaction of the anions is known as anionic hydrolysis. However, hydrolysis is the reverse of the neutralization reaction. Therefore, it is an endothermic process.
The acid and base produced depend on the salt that is being hydrolyzed. In addition to it, the pH of this solution also depends upon the nature of the salt dissolved. For example, the hydrolysis of sodium chloride (NaCl) would produce hydrochloric acid (HCl) and sodium hydroxide (NaOH).
Types of salt hydrolysis
There are the following types of hydrolysis of salt:
Hydrolysis of salt of strong acid and weak bases
The salt which is formed by the neutralization of strong acids and weak bases are the acidic salts. They have a pH of less than 7. When these salts are dissolved in water, they undergo partial hydrolysis.
For example, Ferric chloride (FeCl3) is formed by the reaction of strong acids such as hydrochloric acid and weak base ferric hydroxide. When it is dissolved in water, it will dissociate into ions. such as Fe3+ and Cl–.
FeCl3 → Fe3+ + Cl–
In an aqueous solution, only Fe3+ ions react with water to form ferric hydroxide. On the other hand, chloride ions do not react with water. This is because the ferric chloride hydrolysis results in an acidic solution.
Fe3+ + 3H2O → Fe(OH)3 + 3H+
hydrolysis of salt of the weak acid and strong base
The salt which is formed by the reaction of weak acids and strong bases are basic salts. They have a pH greater than 7.
For example, potassium cyanide is formed by the reaction between potassium hydroxide (strong base) and hydrogen cyanide (weak acid. Hence, when this salt is dissolved in water it will give K+ and CN–.
KCN → K+ + CN–
However, cyanide ion reacts with water to form hydrogen cyanide and hydroxide ion, while potassium ion stays in their ionic form. Thus the hydrolysis of potassium cyanide salt results in a basic solution.
CN– + H2O → HCN + OH–
Hydrolysis of salt of the weak acid and weak base
The solution of the salt of a weak acid and weak base can be acidic, basic, and neutral. However, pH depends upon the strength of the weak acids and weak bases.
For example, Ammonium acetate (CH3COONH4) is a salt of weak acid and weak acid and therefore neutral. It undergoes cationic as well as anionic hydrolysis.
CH3COONH4 → CH3COO– + NH4+
Hydrolysis of salt of a strong base and strong acid
The solution of the strong acid and strong base is also neutral. Ideally, they have a pH equal to 7.
For example, K2SO4 is a salt of strong acid and a strong base. It dissociates during salt hydrolyzed completely into its ions.
Degree of hydrolysis
The degree of hydrolysis is the amount of salt which undergoes hydrolysis. For example, if any salt solution undergoes 50% hydrolysis. Then the degree of hydrolysis is 50%. It is represented by ‘h’.
h = Number of moles of salt hydrolyzed / Total moles of the salt taken
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What is salt hydrolysis?
The interaction between ions of the salt and water which cause the acidity and basicity of the solution is known as salt hydrolysis.
What is the basic difference between salt hydrolysis and hydration?
Salt hydrolysis is a decomposition reaction. This is because it involves the splitting of the water molecule. Whereas hydration is the process in which a water molecule is added to another substance. It does not undergo splitting.
What are the types of salt hydrolysis?
There are four types:
- Hydrolysis of salt of strong acid and strong base
- Hydrolysis of the salt of the weak acid and weak base
- dissolution of salt of the weak acid and strong base
- Hydrolysis of salt of strong acid and weak base
What are some examples of neutral salts?
The neutral salts are formed by the neutralization reaction between strong acids and strong bases, such as NaCl and KCl.
What is the difference between basic salt and acidic salt?
The salts which are formed from strong bases and weak acids are known as basic salts. Whereas the salts which are formed from strong acids and weak bases are known as acidic salts.
NaHSO4 is a salt, however, it is slightly acidic. Why?
When NaHSO4 dissociates into the water it gives Na+ and HSO4– ions. The Na+ does not hydrolyze while HSO4– ions are hydrolyzed to give hydronium ions. Hence it makes the solution acidic.
- Physical Chemistry by R. L. Madan
- Hydrolysis of salts (openstax.org)