Cationic and Anionic Hydrolysis- Salts, Anionic Hydrolysis, Cationic Hydrolysis, Practice Problems and FAQs

You might have studied that acids and bases react to form a salt. You must expect,t all salts like sodium chloride made from the reaction of sodium hydroxide and hydrochloric acid to be neutral in nature.

Have you heard of ammonium nitrate? It is used as a fertiliser to improve plant growth and productivity.

It is one of the largest industrial manufactured chemicals.

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This, ammonium nitrate is formed by the reaction of the nitric acid and ammonium hydroxide base, This ammonium nitrate is not neutral, but an acidic salt.

But how is Ammonium Nitrate acidic in nature?

And do we have salt that can behave like a base? Yes, some salts exhibit basic properties in aqueous solutions too.

How can we identify the salt nature as acidic, neutral or basic from the reaction of an acid and a base?

Let us find out answers to all these questions.

In this article, we'll go over how these salts are made as well as some fascinating properties, particularly of salts that go through a process called hydrolysis. Let's take a closer look at them.

Table of Contents

Salts
Anionic Hydrolysis
Cationic Hydrolysis
Practice Problems
Frequently Asked Questions

Salts

There are four major categories of salt that exist in nature or that we use in our daily lives. Baking soda, washing soda, and ammonium nitrate in fertilisers are some examples of these salts.

These four categories are

Acid	Base	Salt	Examples	Nature of the salt
Strong Acid (SA)	Strong Base (SB)	Salt of SASB	(HCl+NaOH) NaCl	Neutral
Strong Acid (SA)	Weak Base (WB)	Salt of SAWB	(HCl+NH4OH) NH₄Cl	Acidic
Weak Acid (WA)	Strong Base (SB)	Salt of WASB	(CH3COOH+NaOH) CH₃COONa	Basic
Weak Acid (WA)	Weak Base (WB)	Salt of WAWB	(CH3COOH+NH4OH) CH₃COONH₄	$K_{a} = K_{b}$ , Neutral $K_{a} > K_{b}$ , Acidic $K_{a} < K_{b}$ , Basic

In this topic, we will discuss how these salts formed and some interesting aspects of salts that undergo cationic and anionic hydrolysis.

First and foremost, we will go over the fundamental terms that will be used on this concept page.

Hydrolysis: Hydrolysis is the process of breaking down compounds with water. It's fascinating that an ion from a weak electrolyte (a weak acid or a weak base) undergoes hydrolysis.

Hydration: Hydration is the electrophilic addition reaction in which the original molecule is not cleaved. Water molecules are added to the substance during hydration. It’s amusing that an ion from a strong electrolyte (a strong acid or a strong base) undergoes hydration rather than hydrolysis.

Let us understand this concept more deeply by taking examples of four different categorised salts:

(i) Salt of a Strong acid and Strong Base (SASB): Let us take one example of Salt of SASB; NaCl.

It is a combination of strong acid (HCl) and strong base (NaOH). So, the ion Na⁺ comes from a strong base (NaOH) and Cl- comes from strong acid (HCl). As per the concept, these ions get hydrated rather than hydrolysed.

Salt	Examples	Hydration of	Hydrolysis of
Salt of SASB	NaCl(HCl+NaOH)	Na⁺ & Cl^-	Nil

(ii) Salt of a Strong Acid and Weak Base (SAWB): Salt of SAWB; NH₄Cl is a combination of strong acid (HCl) and weak base (NH₄OH). So, the ion $N H_{4}^{+}$ comes from a weak base (NH₄OH) and Cl^- comes from strong acid (HCl). As per the concept, $N H_{4}^{+}$ ions get hydrolysed and Cl^- get hydrated.

Salt	Examples	Hydration of	Hydrolysis of
Salt of SAWB	NH₄Cl(HCl+NH₄OH)	Cl^-	$N H_{4}^{+}$

(iii) Salt of a Weak Acid and Strong Base (WASB): Salt of WASB; CH₃COONa is a combination of weak acid (CH₃COOH) and strong base (NaOH). So, the ion Na⁺ comes from a strong base (NaOH) and CH₃COO^- comes from weak acid (HCl). Hence, Na⁺ ions get hydrated and CH₃COO^- get hydrolysed.

Salt	Examples	Hydration of	Hydrolysis of
Salt of WASB	CH₃COONa(CH₃COOH+NaOH)	Na⁺	CH₃COO^-

(iv) Salt of a Weak Acid and Weak Base(WAWB): CH₃COONH₄ (WAWB salt) is a combination of weak acid (CH₃COOH) and weak base (NH₄OH). So, the ion $N H_{4}^{+}$ comes from a weak base (NH₄OH) and Cl^- comes from weak acid (CH₃COOH). As per the concept, these ions get hydrolysed rather than hydrated.

Salt	Examples	Hydration of	Hydrolysis of
Salt of WAWB	CH₃COONH₄(CH₃COOH+NH₄OH)	Nil	CH₃COO^-& $N H_{4}^{+}$

Anionic Hydrolysis:

Anionic Hydrolysis occurs in the salt of WASB and WAWB when the anion of a weak acid is hydrolysed. Let us discuss some important points regarding Anionic Hydrolysis in the salt of WASB.

Let's take the example of salt ${C H}_{3} C O O N a ({C H}_{3} C O O H (W A) + N a O H (S B))$

As an ion CH₃COO^- comes from a weak acid, it is hydrolysed and ion Na⁺ comes from a strong base, it is hydrated.

When an anion ${C H}_{3} C O O^{-}$ from weak acid CH₃COOH is considered

${C H}_{3} C O O^{-} (a q) + H_{2} O (l) ⇌ {C H}_{3} C O O H (a q) + {O H}^{-} (a q)$

The anion from a weak acid will react with water to produce OH^- ions.

In nature, the solution becomes basic. As a result, the pH of the solution rises due to an increase in OH^- ions.

Some more examples

Salt	Acid	Nature of acid	Anion	Hydrolysis
NaCN (WASB)	HCN	Weak acid	CN^-	yes
KNO₂(WASB)	HNO₂	Weak acid	${N O}_{2}^{-}$	yes
CH₃COONa(WASB)	CH₃COOH	Weak acid	${C H}_{3} C O O^{-}$	yes
CaCO₃(WASB)	H₂CO₃	Weak acid	${C O}_{3}^{2 -}$	yes

Anionic Hydrolysis also occurs in the salt of WAWB when the anion of a weak acid is hydrolysed.Some examples are

Salt	Acid	Nature of acid	Anion	Hydrolysis
FeS(WAWB)	H₂S	Weak acid	$S^{2 -}$	yes
Zn(HCOO)₂(WAWB)	HCOOH	Weak acid	${H C O O}^{-}$	yes
(C₅H₅NH)₂CO₃(WAWB)	H₂CO₃	Weak acid	${C O}_{3}^{2 -}$	yes

Cationic Hydrolysis:

Cationic Hydrolysis occurs in the salt of SAWB and WAWB when the cation of a weak base is hydrolysed. Let us discuss some important points regarding Cationic Hydrolysis in the salt of SAWB.

Let's take the example of salt ${N H}_{4} C l (H C l (S A) + {N H}_{4} O H (W B))$

As an ion ${N H}_{4}^{+}$ comes from a weak base, it is hydrolysed and ion Cl^- comes from a strong acid, it is hydrated.

When a cation ${N H}_{4}^{+}$ from a weak base NH₄OH is considered

${N H}_{4}^{+} (a q) + 2 H_{2} O (l) ⇌ {N H}_{4} O H (a q) + H_{3}^{+} O (a q)$ .

The cation from a weak base will react with water to produce H⁺ or $H_{3}^{+} O$ ions.

In nature, the solution becomes acidic. As a result, the pH of the solution decreases due to an increase in H⁺ or $H_{3}^{+} O$ ions.

Some more examples

Salt	Base	Nature of base	Cation	Hydrolysis
ZnSO₄(SAWB)	Zn(OH)₂	Weak base	${Z n}^{2 +}$	yes
NH₄Cl (SAWB)	NH₄OH	Weak base	${N H}_{4}^{+}$	yes
FeCl₂(SAWB)	Fe(OH)₂	Weak base	${F e}^{2 +}$	yes
(C₅H₅NH)₂Cl(SAWB)	C₅H₅NHOH	Weak base	$C_{5} H_{5} N H^{-}$	yes

Cationic Hydrolysis also occurs in the salt of WAWB when the cation of a weak base is hydrolysed.Some examples are

Salt	Base	Nature of base	Cation	Hydrolysis
FeS(WAWB)	Fe(OH)₂	Weak base	${F e}^{2 +}$	yes
Zn(HCOO)₂(WAWB)	Zn(OH)₂	Weak base	${Z n}^{2 +}$	yes
(C₅H₅NH)₂CO₃(WAWB)	C₅H₅NHOH	Weak base	$C_{5} H_{5} N H^{-}$	yes

Practice Problems

Q1. Comment on the type of hydrolysis and nature of Na₂S salt when added to water.

A. Cationic Hydrolysis, Acidic salt
B. Cationic Hydrolysis, Basic salt
C. Anionic Hydrolysis, Acidic salt
D. Anionic Hydrolysis, Basic salt

Answer: (D)

Solution: Na₂S is a combination of the weak acid and strong base.

${N a}_{2} S (a q) \to {N a}^{+} (a q) + S^{2 -} (a q)$

Ions

Origin

Hydrolysis

S^2-

$H_{2} S -$ Weak acid

Yes,

$S^{2 -} (a q) + 2 H_{2} O (l) ⇌ H_{2} S (a q) + {2 O H}^{-} (a q)$

Na⁺

NaOH - Strong base

In this salt, upon reaction with water produce OH- ions. This will increase the concentration of OH^- in in the solution and hence pH of solution increases. Being a mixture of a weak acid and a strong base, it is basic in nature.

There is the hydrolysis of anion from weak acid. Hence it is an Anionic Hydrolysis.

Hence, the correct answer is the option (D).

Q2. Predict Zn(CN)₂ salt undergoes which type of hydrolysis ?

A. Cationic Hydrolysis
B. Anionic Hydrolysis
C. Both (A) and (B)
D. None of these

Answer: (C)

Solution: Zn(CN)₂ salt is a combination of a weak acid and weak base. $Z n (C N)_{2} (a q) \to {Z n}^{2 +} (a q) + 2 C N^{-} (a q)$

Ions	Origin	Hydrolysis
CN^-	HCN - Weak acid	yes
Zn²⁺	Zn(OH)₂ - Weak base	yes

Being a mixture of a weak acid and weak base, both ions undergoes hydrolysis. There is the hydrolysis of anion from weak acid HCN and hydrolysis of cation from a weak base Zn(OH)₂. Hence it undergoes Anionic as well as Cationic Hydrolysis.

Hence, the correct answer is the option (C).

Q3. Comment on the nature of table salt when added to a beaker full of water.

A. Acidic Salt
B. Basic Salt
C. Neutral Salt
D. None of these

Answer: (C)

Solution: The chemical formula for Table salt is NaCl.

$N a C l (a q) \to {N a}^{+} (a q) + {C l}^{-} (a q)$

Ions	Origin	Hydrolysis
Cl^-	HCl- Strong acid	No
Na⁺	NaOH - Strong base	No

NaCl is a strong acid and strong base compound. The respective ions are from strong electrolytes, hence they should be hydrated rather than hydrolysed. There should be no excess of H⁺ or OH^- present in hte solution. As a result, the pH of the solution should be close to 7 and the salt is neutral in nature.

Hence, the correct answer is the option (C).

Q4. Arrange the solution of salt NaCN, MgCl₂ and FeSO₄ in order of decreasing pH.

A. NaCN< MgCl₂ <FeSO₄
B. NaCN> MgCl₂ >FeSO₄
C. NaCN< MgCl₂ >FeSO₄
D. NaCN= MgCl₂ <FeSO₄

Answer: (B)

Solution: FeSO₄ is a combination of the strong acid and weak base.

Ions

Origin

Hydrolysis

Fe²⁺

Fe(OH)₂ - Weak base

Yes,

${F e}^{2 +} (a q) + 4 H_{2} O (l) ⇌ F e (O H)_{2} (a q) + 2 H_{3}^{+} O (a q)$

${S O}_{4}^{2 -}$

H₂SO₄- Strong acid

In this salt, upon reaction with water produce H+ ions. This will increase the concentration of H+ in the solution and hence pH of solution decreases. Being a mixture of strong acid and weak base, it is acidic in nature.

NaCN is a combination of a weak acid and strong base.

Ions

Origin

Hydrolysis

CN^-

HCN - Weak acid

Yes,

${C N}^{-} (a q) + H_{2} O (l) ⇌ H C N (a q) + {O H}^{-} (a q)$

Na⁺

NaOH - Strong base

In this salt, upon reaction with water produce OH^- ions. This will increase the concentration of OH^- in in the solution and hence pH of solution increases. Being a mixture of a weak acid and a strong base, it is basic in nature.

$M g C l_{2} (a q) \to {M g}^{2 +} (a q) + 2 C l^{-} (a q)$

Ions	Origin	Hydrolysis
Cl^-	HCl - Strong acid	No
Mg²⁺	Mg(OH)₂ - Strong base	No

MgCl₂ is a strong acid and strong base compound. The respective ions are from strong electrolytes, hence they should be hydrated rather than hydrolysed. As a result, the pH of the solution should be close to 7 and the salt is neutral in nature.

Acidic salt - FeSO₄ (lowest pH), Neutral Salt - MgCl₂, Basic Salt - NaCN (Highest pH)

The correct order of pH is: NaCN> MgCl₂ >FeSO₄

Hence, the correct answer is the option (B).

Frequently Asked Questions

Q1. What is the nature of salt used to puff edible cakes?
Answer: We all love to eat yummy puffed cakes. But do you know, to puff the edible cakes, baking soda is used which contains salt NaHCO₃. It is a mixture of a strong base (NaOH)and a weak acid (H₂CO₃). As a result, it is a basic salt.

Q2. Is washing soda an acid, a base, or a salt?
Answer: Washing soda (Na₂CO₃) which is used in laundry, is formed by the reaction of a strong base (NaOH) and a weak acid (H₂CO₃). As a result, it is a basic salt.

Q3. Why are strong acid and strong base ions hydrated rather than hydrolyzed?
Answer: Because strong acid and strong base ions are stable in ionic form and do not want to react further, they are hydrated rather than hydrolyzed.

Q4. What exactly are neutral salts?
Answer: Neutral salts are salts formed by the reaction of a strong acid and a strong base. An example: NaCl is a common example of a neutral salt.

Q5. How Ammonium nitrate salt is acidic in nature?
Answer: Ammonium Nitrate (NH₄NO₃) is a combination of a weak base (ammonium hydroxide) and strong acid (nitric acid). Hence it is an acidic salt.