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Cationic Hydrolysis- Salts, Types, Concept of Hydration and Hydrolysis, Types, Practice Problems and FAQs

Cationic Hydrolysis- Salts, Types, Concept of Hydration and Hydrolysis, Types, Practice Problems and FAQs

Everybody knows about table salt and you know more that salts are formed by the reaction between an acid and a base.

Why study this concept? You may be thinking that all salts have similar properties, but not so.

image

For a surprise, there are four major categories in which thousands of salt that exists in our nature and being used by us in our daily life can be classified. In this topic, we are going to discuss how these salts are formed and some interesting aspects about them.

Table of content

  • What are salts?
  • Concept of hydration
  • Salt Hydrolysis
  • Cationic Hydrolysis
  • Practice Problems
  • Frequently Asked Questions -FAQ

What are salts?

Salts are made up of a positive ion of a base and a negative ion of an acid

image

Cl-in NaCl salt, is the negative ion (anion) of the acid HCl. 

Na+ in NaCl salt is the positive ion (cation) of the base NaOH.

There are four major categories of salts

Acid

Base

Salt

Examples

Strong Acid (SA)

Strong Base (SB)

Salt of SASB 

NaCl(HCl+NaOH)

Strong Acid (SA)

Weak Base (WB)

Salt of SAWB 

NH4Cl(HCl+NH4OH)

Weak Acid (WA)

Strong Base (SB)

Salt of WASB 

CH3COONa(CH3COOH+NaOH)

Weak Acid (WA)

Weak Base (WB)

Salt of WAWB 

CH3COONH4(CH3COOH+NH4OH)

Concept of hydration 

All the four categories of salt ionize when dissolved in water to release the cations and anions. Some of these cations and anions will be surrounded by the water molecules. Higher the charges and size higher will be the number of water molecules around them. This process of being surrounded by water molecules is called hydration 

The cations and anions of salts produced by the reaction of strong acids and strong bases are hydrated. For example Na+and Cl- in NaCl (HCl+NaOH) get hydrated because they are stable in ionic form and don't want to react further.

imageimage

But some other ions can react with water and this reaction is called as Hydrolysis. 

It's really interesting that ion from weak electrolyte (weak acid or weak base) undergoes hydrolysis and ion from strong electrolyte (strong acid and strong base) undergoes hydration..

Salt

Examples

Ion undergoing hydration

Ion undergoing hydrolysis

Salt of SASB

NaCl (HCl+NaOH)

Both Na+ & Cl-

nil

Salt of SAWB 

NH4Cl (HCl+NH4OH)

Cl- 

NH4+

Salt of WASB 

CH3COONa (CH3COOH+NaOH)

Na+

CH3COO- 

Salt of WAWB

CH3COONH4 (CH3COOH+NH4OH)

nil

CH3COO-, NH4+ 

Let's take example of NH4Cl (HCl+NH4OH), NH4+ - Hydrolysed, Cl- - Hydrated

image

Salt Hydrolysis

It is the chemical reaction of water with cations or anions or both types of ions present in salts.

 It can be categorized in two types of hydrolysis.

1. Cationic Hydrolysis
2. Anionic Hydrolysis

We will discuss Cationic hydrolysis here

Cationic Hydrolysis

The cation of salt always comes from the base and if it is from the weak base then it will react with water to give H+ions. The salt solution will be acidic in nature. Hence, pH of the solution decreases because of increasing H+ions. So, we can say that when the cation of a weak base is hydrolysed, this is known as Cationic Hydrolysis- example ammonium chloride.

If we consider cation B+ from weak base BOH, it reacts with water to form a less ionizable weak base BOH.

B+(aq)+2H2O(l)⇌BOH(aq)+H3+O(aq)

Let's take example of NH4Cl (HCl+NH4OH), NH4+ - Hydrolysed, Cl- - Hydrated

NH4+(aq)+2H2O(l)⇌ NH4OH(aq)+H3+O(aq)

Salt

Base

Nature of base

Cation

Hydrolysis

Ca(CH3COO)2

Ca(OH)2

Strong base

Ca2+

no 

NH4Cl

NH4OH

Weak base

NH4+

yes

NaCl

NaOH

Strong base

Na+

no 

Mg(HCOO)2

Mg(OH)2

Strong base

Mg2+

no 

Na2(COO)2

NaOH

Strong base

Na+

no 

C5H5NHCl

C5H5NHOH

Weak Base

C5H5NH+

yes

Practice Problems

Q1. Arrange the solution of salt AlCl3, GaCl3 and InCl3in order of decreasing pH.
Answer: These hydroxides of these metals are weak bases. Salts containing the strong acid HCl will undergo cationic hydrolysis. The charge and size of the cation influence hydrolysis. The greater the size and charge, the greater the hydrolysis to produce hydrogen ions. Because they all have the same charge, aluminum has the smallest size and is thus highly hydrolyzed, resulting in a more acidic solution.

The pH sequence will be Al < Ga < Tl

Ions

Origin

Cl-

HCl - Strong acid

Al3+

Al(OH)3 - Weak base

Ga3+

Ga(OH)3 - Weak base

In3+

In(OH)3 - Weak base

 

Q2. Comment for the pH of table salt when added in water whether it is acidic or basic.
Answer: Table salt : NaCl

image

Ions

Origin

Hydrolysis

Cl-

HCl- Strong acid

No

Na+

NaOH - Strong base

No

NaCl is a combination of strong acid and strong base. Hence, hydration of respective ions takes place. Hence, the pH of solution should be approaching 7 and the nature of salt should be neutral.

Q3. Comment for the pH of KClO4 salt when added in water

image

Ions

Origin

Hydrolysis

ClO4-

HClO4 - Strong acid

No

K+

KOH - Strong base

No

Answer: KClO4 is a combination of strong acid and strong base. Hence, hydration of respective ions takes place. Hence, the pH of solution should be approached to 7 and the nature of salt should be neutral.

Q4. Predict whether Mg(CN)salt undergoes cationic hydrolysis or not ?

image

Ions

Origin

Hydrolysis

CN-

HCN - Weak acid

yes

Mg2+

Mg(OH)2 - Strong base

No

Answer: Mg(CN)salt is a combination of weak acid and strong base. It will not undergo cationic hydrolysis as cation is from a strong base. Hence cation gets hydrated rather than hydrolysed.

Q5. Comment for the pH of KCl salt when added in water

image

Ions

Origin

Hydrolysis

Cl-

HCl - Strong acid

No

K+

KOH - Strong base

No

Answer:  KCl is a combination of strong acid and strong base. Hence, hydration of respective ions takes place. Hence, the pH of solution should be approached to 7 and the nature of salt should be neutral.

Frequently Asked Questions -FAQ

Question 1. What are basic salts?
Answer: The salt which is formed by combination of weak acid and strong base is known as basic salts. For example : CaCO3 is a basic salt.

Question 2. What are acidic salts?
Answer: 
The salt which is formed by combination of strong acid and weak base is known as acidic salts. For example : NH4ClO4(explosive) is an acidic salt.

Question 3. Is washing soda an acid, base or salt ?
Answer: 
Washing soda (Na2CO3) is a combination of strong base (NaOH) and weak acid (H2CO3). Hence it is a basic salt.

Question 4. Why table salt (NaCl) will not form HCl & NaOH when it is added to water ?
Answer: 
NaCl (HCl+NaOH)is a combination of strong acid and strong base. Hence, Na+and Cl- get hydrated rather than hydrolysed .They are stable in ionic form and don't want to react further.

Question 5. Is Baking soda an acid, base or salt ?
Answer: 
baking soda (NaHCO3) is a combination of strong base (NaOH) and weak acid (H2CO3). Hence it is a basic salt.

Related topics

Anionic Hydrolysis

Hydrolysis of salt weak acid and weak base_SEOABS11CHE09CON010

Hydrolysis of salts- salt of weak acid and strong base_

Acids, Bases and Salts

Hydrolysis of salts - salt of weak acid and strong base_SEOABS11CHE09CON009

Theory of Indicators_SEOABS11CHE09CON014

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