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Introduction:
Some salts when dissolved in water they will appear some colours, for example, K2CO3, NH4Cl, NaHCO3, Na3PO4, Na2HPO4, NaH2PO4 these salts look white in colour, but appear in different colours when dissolved in melted water. This is due to the hydrolysis of salt.
Salt: A compound obtained from reaction between acid and base. The salt gets cation from base and anion from acid. The process in which salts are formed is known as Neutralisation.
Acid + Base → Salt + Water
pH: The potential of hydronium ion which measures acidic or basic nature of substance, is known as pH. The pH usually ranges from 0 to 14. If pH value is 7 indicates neutral solution, below 7 indicates acidic solution and above 7 indicates basic solution.
Equilibrium constant (k): The ratio of product of concentration of products to the product of concentration of reactants in a reaction.
Salt hydrolysis: The process of salt reacting with water to produce acidic, basic or neutral solution is called hydrolysis.
There are three types of salt hydrolysis: Complete hydrolysis, No hydrolysis, Limited hydrolysis.
Complete hydrolysis: The complete hydrolysis consists of two types. They are acidic salt complete hydrolysis and basic salt complete hydrolysis.
Acidic salt complete hydrolysis: If the anion is more acidic than their conjugate base and water is more basic than their conjugate base then the solution is acidic. Here H+ is released so it is an acidic solution.
Basic salt complete hydrolysis: If the cation is more basic than their conjugate acid and water are more acidic than their conjugate base then the solution is acidic. Here OH– is released so it is a basic solution.
No hydrolysis: No hydrolysis consists of two types. They are Acidic salt hydrolysis and Basic salt hydrolysis.
Acidic salt hydrolysis: If anion is less basic than its conjugate pairs and water is less acidic than its conjugate pairs.
Basic salt hydrolysis: If cation is less acidic than its conjugate pairs and water is less basic than its conjugate pairs.
Limited hydrolysis: When cations or anions are less strong when compared to their conjugate pairs, hydrolysis takes place relatively.
Types of salts:
Type 1 – Salts of strong acids with strong bases
Strong acids with a strong base, such as HCl (aq) and NaOH (aq) will react to form a neutral solution as the conjugate-produced partners have undeniable strength.
Cations from strong bases are Cs+, Li+, Na+, etc.
Anions from strong acids are Cl-, Br–, I– etc.
NaCl + H2O ⇢ Na+ + H+ + OH– + Cl–
Type – 2 Salts of strong acids and weak bases
A strong acid with a weak base produces a weak solution, not because of the strong acid involved, but because of the conjugate acid of a weak base.
Cations from weak bases are Cu2+, Zn2+, Fe3+.
Anions from strong acids are Br–, Cl–, I–.
B(aq) + H₃O⁺(aq) → HB⁺(aq) + H₂O(l)
Type – 3 Salts of weak acids and strong bases
Weak acid and strong base produce a weak basic solution. Weak acid solution responds with a strong base solution to form a weak acid conjugate base and strong base conjugate acid. Strong conjugate acid is a weaker acid than water and has no effect on the acid of the resulting solution. However, the conjugate base of the weak acid is a weak base and slightly ionises in water. This increases the amount of hydroxide ion in the solution produced in the reaction and makes it less stable.
Cations from strong bases are Li+, Na+, k+ etc.
Anions from weak acids are F–, CN–, S-2 etc.
HA (aq) + OH⁻(aq) → A⁻(aq) + H₂O(l)
Type 4 – Salts of weak acids and weak bases
Weak acid and weak base can produce a solution that is acidic, basic, or neutral. This is the most complex of the four types of reactions. When conjugate acid and conjugate base have unequal power, the solution may be acidic or basic, depending on the relative strength of the two conjugates. Sometimes a weak acid and a weak base will have the same strength, so their conjugate and acid base will have the same strength, and the solution will be neutral.
Cations from weak bases are NH4+, Fe+3, Cu+2, etc.
Anions from weak acids are CN–, S-2, CH3COO– etc.
HA (aq) + B(aq) ⇌ A⁻(aq) + HB⁺(aq)
Conclusion:
The characteristic features of aqueous solutions of Bronsted-Lowry acids are due to the presence of hydronium ions; those of aqueous solutions in Bronsted-Lowry bases are due to the presence of hydroxide ions. Neutrality occurs when aqueous solutions of acids and bases are combined with the effect of hydronium reaction with hydroxide ions to form water. Some salts formed by a neutral reaction may make product solutions less acidic or less basic.
Solutions containing salts or hydrated ions of iron with a pH determined by the magnitude of the hydrolysis of ions in the solution. The pH of the solutions may be calculated using standard measurement techniques, or it may be determined by quality to be acidic, basic, or neutral depending on the Ka and Kb associated ions involved.
