Hydrolysis of Salt

Salts are compounds formed by the neutralization reaction between acids and bases. They generally ionize in water, producing cations and anions. Cations or anions formed during salt ionization either exist as hydrated ions in aqueous solution or interact with water to regenerate acids and bases. The process of salt interaction between cations or anions and water is known as salt hydrolysis. Based on hydrolysis, salts fall into three categories.

• Acidic salts

•  Basic salts

• Neutral salts

Types of Salt based on their Hydrolysis

• Salts of strong acid and strong base: The salt formed by the neutralization of strong acids and strong bases is essentially neutral because the bonds are not broken in the salt solution. They are generally hydrated but do not hydrolyze. Therefore, such salts are commonly known as neutral salts. For Example: NaCl

NaCl dissociates in water to form Na + and Cl– ions. 

 When these ions react with water, the product becomes NaOH and HCl. These are strong electrolytes and are completely ionized. The H + and 

OH-ions then recombine to form undissociated water molecules. It is neutral.

• Salts of weak acid and strong base: Salts formed by neutralization of weak acids and strong bases are basically basic. For Example, CH3COONa

CH3COONa → CH3COO– + Na+

CH3COO– + H2O ↔ CH3COOH + OH–

∴ [OH–] > [H+] hence the solution is basic in nature

Acetic acid is a weak acid and does not remain bound in solution. This increases the concentration of OH- ions and makes the solution alkaline. The pH of the solution is greater than 7.

• Salts of strong acid and weak base: Salts formed by neutralization of strong acids and weak bases are inherently acidic. For example: NH4Cl

Hydrolysis of NH4Cl may be represented as follows:

NH4Cl + H2O ↔ NH4OH + HCl

Or NH4+ + Cl– + H2O ↔ NH4OH + H+ + Cl–

As it produces H+ ions, so the solution of such a salt is acidic in character.

Ammonia hydroxide is a weak base and stays uniformly in solution. This increases the concentration of H+ ions and makes the solution acidic. The pH of such a solution is less than 7.

• Salts of weak acid and weak base: The salts formed by the neutralization of weak acids and bases are acidic, basic, or neutral, depending on the nature of the acids and bases involved.  For example: CH3COONH4

It is neutral because

CH3COONH4 → CH3COO– + NH4+

CH3COO– + H2O ↔ CH3COOH + OH–

NH4+ + H2O ↔ NH4OH + H+

Both reactions take place at the same speed so, solution is neutral in nature.

Hydrolysis of Salt Equation

Salts are ionic compounds formed when acids and bases neutralize each other. Saline always looks neutral, but is often either acidic or basic.

Consider the salt formed when weakly acidic hydrofluoric acid is neutralized by the strong base sodium hydroxide. The molecular formula and net ionic formula are shown below.

HF (aq) + NaOH(aq) → NaF(aq) +H2O(l)

HF (aq) + OH–(aq) → F–(aq) +H2O(l)

Sodium fluoride is a spectator ion for the neutralization reaction because it is soluble. Fluoride ions can react slightly with water by accepting protons.

F–(aq) + H2O(l) ↔ HF (aq) + OH–(aq)

Fluoride ions act as weak Bronsted-Lowry bases. The hydroxide ions produced as a result of the above reaction make the solution slightly basic. Salt hydrolysis is a reaction in which one of the salt’s ions reacts with water to form an acidic or basic solution.

Salt that forms Basic Solution 

When solid sodium fluoride is dissolved in water, it completely dissociates into sodium and fluoride ions. Sodium ions cannot be hydrolyzed, but fluoride ions hydrolyze to produce small amounts of hydrofluoric acid and hydroxide ions.

F–(aq) + H2O(l) ↔ HF (aq) + OH–(aq)

The salt resulting from neutralizing the weak acid (HF) with the strong base (NaOH) always produces a basic salt solution.

Salt that forms Acidic Solution

Ammonium chloride (NH4Cl) is a salt formed when the strong acid HCl is neutralized by the weak base NH3. Ammonium chloride is water soluble. The chloride ion produced is a conjugate base of the strong acid HCl and cannot be hydrolyzed. In other words, the Cl- ion cannot receive protons from water to form HCl and OH-, as did the fluoride ion in the previous section. However, ammonium ions act as acids because they can easily react with water to donate protons.

NH4+(aq) + H2O(l) ↔ H3O+ + NH3(aq)

Salt that forms Natural Solution

The salts resulting from the reaction of strong acids with strong bases form a solution at pH 7. An example is sodium chloride resulting from the neutralization of HCl with NaOH.  NaCl An aqueous solution is neither acidic nor basic because both ions cannot be hydrolyzed. Other salts that form a neutral solution include potassium nitrate (KNO3) and lithium bromide (LiBr).

Salts formed from the reaction of weak acids and weak bases are more difficult to analyze because the hydrolysis reaction competes between cations and anions.

Hydrolysis of Benzene diazonium chloride

When the benzenediazonium chloride is hydrolyzed, nitrogen molecules escape as nitrogen gas. The OH groups in water attack the vacancies that remain after the removal of nitrogen gas. This leads to the formation of phenol. 

Conclusion

Salts are ionic compounds composed of cations and anions, each of which can cause an acid or base ionization reaction with water. Thus, the aqueous salt solution can be acidic, basic, or neutral, depending on the relative acid-base strength of the constituent ions of the salt. Pure water is a weak electrolyte. does not completely dissociate into hydrogen and hydroxide, but it is weak because it exists in equilibrium with these two ions. It’s essentially neutral.  H+ ion concentration is exactly equal to OH ion concentration. When this condition is disturbed by reducing the concentration of either ion, the neutral nature changes to acidic or basic. If [H+]> [OH], the water will be acidic, if [H+] & lt; [OH], the water will be basic. This is exactly the change that occurs during a phenomenon known as salt hydrolysis. It is defined as a reaction in which the cations and / or anions of a salt react with water to produce acid or alkalinity.