Salt Hydrolysis definition chemistry is Neutralization reaction between an acid and a base results in the formation of salt. Most salts ionize in water to form acids and bases. These salts dissociate into their respective ions, which exist as hydrated ions in aqueous solutions or can interact with the hydrogen or hydroxyl ion of the solvent to form the resulting bases and acids. The mechanism of interaction between the ions of the salts with that of water is called hydrolysis of salts. As per the salt hydrolysis definition and the extent of hydrolysis, salts can be categorized as:
Basic salt
Acidic Salt
Neutral or amphoteric salts.
The formation of these salts depends on the type of salt hydrolysis. They are:
Salts of a Strong Base and a Strong acid
Salts that are produced by the reaction between a strong base and a strong acid are neutral in nature. The bonds formed between the anion and the cation are strong, and they do not break apart in solution. The electronic distribution of both the cation and the anion after the formation of the bond between them is such that both the ions complete their most stable electronic configuration. These salts are formed by ionic or electrostatic bonds, which cannot be broken in solution. These salts do not get hydrolyzed, but they can get hydrated. Since these salts do not carry any charge, they are called neutral or amphoteric salts. Sodium chloride (NaCl) is a classic example of neutral salt.
Salts of a Weak Acid and a Strong Base
Salts that are formed by the neutralization of weak acid and strong base have a positive charge. These salts are called a basic salt. These salts hydrolyze easily in water. For example, Sodium acetate (CH3COONa). The salt hydrolysis formula of Sodium acetate is as follows:
CH3COONa (aq) → CH3COO- (aq) + Na+ (aq)
The acetate ion, thus formed, interacts with the hydrogen ion in water to form acetic acid and hydroxyl ion.
CH3COO- (aq) + HOH → CH3COOH+ OH- (aq)
Since acetic acid is a weak acid, it does not get ionized in water. However, the increase in the OH- ion results in the increased basicity of the solution. Therefore these salts are called basic salts. The pH of the aqueous solution of these salts is always more than 7.
Salts of a Weak Base and a Strong Acid
Salts formed from neutralization reaction between weak bases and strong acids are found to be acidic in nature. Ammonium chloride is an example of acidic salt.
NH4Cl(aq) → Cl−(aq) + NH4+(aq)
Ammonium ions thus formed then interacts with the hydroxyl ions in the water molecule to form ammonium hydroxide. The hydrogen ions thus formed results in the acidic nature of the solution. Therefore an aqueous solution of these salts is acidic and has a pH of less than 7.
NH+4(aq) + H2O ⇋ NH4OH(aq) + H+(aq)
Salts of Weak Base and Weak Acid
Salts resulting from the neutralization reaction between a weak acid and weak base can be mildly basic, mildly acidic, or neutral depending on the nature of the bases and acids involved. The degree of hydrolysis and ionization of water is independent of the concentration of the solution. An example of such salts is CH3COONH4. The mechanism of formation of such ions are as follows:
CH3COO− + NH+4 + H2O ⇋ CH3COOH + NH4OH
The pH of the aqueous solution of such salts are denoted as
pH= 7 + 12 (pKa – pKb)
That is, the pH depends on the pKa and pKb of the acid and the base, respectively.
FAQs on Hydrolysis of Salts
1. What is meant by the hydrolysis of salts in chemistry?
Salt hydrolysis is a chemical reaction in which the cation, anion, or both ions of a salt react with water molecules. This interaction alters the concentration of hydrogen (H⁺) or hydroxide (OH⁻) ions in the solution, causing it to become acidic, basic, or remain neutral. It is essentially the reverse process of a neutralisation reaction.
2. What are the main types of salt hydrolysis?
Salt hydrolysis is primarily classified into four types based on the strength of the parent acid and base from which the salt is formed:
- Salt of a strong acid and a weak base: Results in an acidic solution (e.g., Ammonium Chloride, NH₄Cl).
- Salt of a weak acid and a strong base: Results in a basic solution (e.g., Sodium Acetate, CH₃COONa).
- Salt of a weak acid and a weak base: The solution can be acidic, basic, or neutral depending on the relative strengths of the acid and base (e.g., Ammonium Acetate, CH₃COONH₄).
- Salt of a strong acid and a strong base: These salts do not undergo hydrolysis, and the solution remains neutral (e.g., Sodium Chloride, NaCl).
3. Why does a solution of ammonium chloride (NH₄Cl) turn acidic?
Ammonium chloride (NH₄Cl) is a salt formed from a strong acid (HCl) and a weak base (NH₄OH). When dissolved in water, the ammonium ion (NH₄⁺) hydrolyses by reacting with water to produce hydronium ions (H₃O⁺) and unionised ammonium hydroxide. This increase in H₃O⁺ concentration makes the solution acidic, with a pH less than 7.
4. How does the hydrolysis of a salt like sodium acetate (CH₃COONa) lead to a basic solution?
Sodium acetate (CH₃COONa) is a salt of a weak acid (CH₃COOH) and a strong base (NaOH). In an aqueous solution, the acetate ion (CH₃COO⁻) reacts with water (hydrolyses) to form acetic acid and hydroxide ions (OH⁻). The accumulation of OH⁻ ions increases the solution's alkalinity, resulting in a pH greater than 7.
5. Why do salts of strong acids and strong bases, like NaCl or KNO₃, not undergo hydrolysis?
Salts like Sodium Chloride (NaCl) are formed from a strong acid (HCl) and a strong base (NaOH). The resulting ions, Na⁺ and Cl⁻, are the very weak conjugate base and acid, respectively. They have no tendency to react with water molecules to alter the H⁺ or OH⁻ concentration. Therefore, the solution remains neutral with a pH of exactly 7, and no hydrolysis occurs.
6. What is the fundamental difference between the dissolution and hydrolysis of a salt?
Dissolution is a physical process where the ions of a salt simply separate and become surrounded by water molecules (hydration). In contrast, hydrolysis is a chemical reaction where these separated ions actually react with water molecules, breaking H-OH bonds to form a weak acid or a weak base and changing the solution's pH. All salts that hydrolyse must first dissolve, but not all salts that dissolve will undergo hydrolysis.
7. What is the hydrolysis constant (Kh) and how is it related to Ka and Kb?
The hydrolysis constant (Kh) is the equilibrium constant for the hydrolysis reaction, indicating the extent of hydrolysis. Its relationship with the ionic product of water (Kw) and the ionisation constants of the weak acid (Ka) or weak base (Kb) is crucial:
- For a salt of a weak acid and strong base: Kh = Kw / Ka
- For a salt of a strong acid and weak base: Kh = Kw / Kb
- For a salt of a weak acid and weak base: Kh = Kw / (Ka × Kb)
A higher Kh value indicates a greater degree of hydrolysis.
8. How can you predict if a salt solution will be acidic, basic, or neutral without calculations?
You can predict the nature of a salt solution by identifying its parent acid and base:
- Strong Acid + Strong Base = Neutral Salt (e.g., KCl)
- Strong Acid + Weak Base = Acidic Salt (e.g., CuSO₄)
- Weak Acid + Strong Base = Basic Salt (e.g., KCN)
- Weak Acid + Weak Base = The outcome depends on the relative values of Ka and Kb. If Ka > Kb, the solution is acidic. If Kb > Ka, it is basic. If they are nearly equal, the solution is almost neutral.
