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

You know that the sea water tastes salty due to the good amount of dissolved salts and mostly of sodium chloride. It means that the sea water contains sodium ions and chloride ions as present in the crystal salt.

So, the water does not react with these ions and form hydrogen chloride (HCl) and sodium hydroxide (NaOH). You may know that unlike sodium chloride, ammonium chloride reacts with water to form a basic solution by cationic hydrolysis.

You can gargle with water containing sodium chloride or vinegar far soar throat. Can you gargle with sodium acetate ?

gargle

Why study this concept?

There are four major categories in which thousands of salt exists in our nature or we use them in our daily life. For example baking soda, washing soda, ammonium nitrate in fertilizers and many more. In this topic, we are going to discuss how these salts formed and some interesting aspects particularly of salts that undergo what is called as anionic hydrolysis.

Table of content

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

What are salts?

Salts basically consist of positive ion of a base and negative ion of an acid

Cl^-in NaCl from HCl (acid) and Na⁺ in NaCl from NaOH (base)

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

The cations and anions of salts produced by the reaction of strong acids and strong bases are hydrated rather than hydrolysed. 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.

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 rather than hydrolysis.

*Salt*	*Examples*	*Hydration of*	*Hydrolysis of*
Salt of SASB	NaCl(HCl+NaOH)	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 an example of CH₃COONa; CH₃COO^-- Hydrolysed, Na⁺- Hydrated

Salt Hydrolysis

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

Salt Hydrolysis can be categorized into two types of hydrolysis.

1. Cationic Hydrolysis
2. Anionic Hydrolysis

We will discuss Anionic hydrolysis here

Anionic Hydrolysis

The anion of salt always comes from acid and if it is from weak acid then it will react with water to give OH-ions. The solution becomes basic in nature. Hence, pH of the solution increases by increasing OH-ions. When the anion of a weak acid is hydrolysed, this is known as Anionic Hydrolysis.

If we consider anion A^- from weak acid AH, then

A^-(aq)+H2O(l) ⇌ AH(aq)+OH^-(aq)

Let's take example of CH₃COONa (CH₃COOH+NaOH), CH₃COO^- - Hydrolysed, Na⁺ - Hydrated

Hydration of Na+ and Hydrolysis of CH3COO-

*Salt*	*Acid*	*Nature of acid*	*Anion*	*Hydrolysis*
NaCN	HCN	Weak acid	CN^-	yes
KClO₄	HClO₄	Strong acid	ClO₄^-	no
MgSO₄	H2SO₄	Strong acid	SO₄^2-	no
KNO₂	HNO₂	Weak acid	NO₂^-	yes
NaNO₃	HNO₃	Strong acid	NO₃^-	no
CH₃COONa	CH₃COOH	Weak acid	CH₃COO^-	yes
CaCO₃	H₂CO₃	Weak acid	CO₃^2-	yes
FeS	H₂S	Weak acid	S^2-	yes

Practice Problems

Q1. Comment for the pH and nature of NaCN salt when added in water whether it is acidic or basic.

Ions

Origin

Hydrolysis

CN^-

HCN - Weak acid

Yes,

CN^-(aq)+H₂O(l) ⇌ HCN(aq)+OH^-(aq)

Na⁺

NaOH - Strong base

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

Q2. Comment for the pH of K₂CO₃ salt when added in water whether it is acidic or basic.

Ions

Origin

Hydrolysis

CO₃^2-

H₂CO₃ - Weak acid

Yes,

CO₃^2-(aq)+2H₂O(l) ⇌ H₂CO₃(aq)+2OH^-(aq)

K⁺

KOH - Strong base

K₂CO₃ is a combination of weak acid and strong base. In this salt, upon reaction with water produce OH- ions. This will increase the concentration of OH^- in the solution and hence pH of solution increases.

Q3. Comment for the pH of NaClO₄ salt when added in water

*Ions*	*Origin*	*Hydrolysis*
ClO₄^-	HClO₄ - Strong acid	No
Na⁺	NaOH - Strong base	No

NaClO4 is a combination of strong acid and strong base. In this salt, only 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 NaCN salt undergoes anionic hydrolysis or not ?

Ions

Origin

Hydrolysis

CN^-

HCN - Weak acid

Yes,

CN^-(aq)+H₂O(l) ⇌ HCN(aq)+OH^-(aq)

Na⁺

NaOH - Strong base

NaCN is a combination of weak acid and strong base. It will undergo anionic hydrolysis as anion is from weak acid and hence, it will react with water and get hydrolysed.

Frequently Asked Questions -FAQ

Question 1. What is the difference between hydrolysis and hydration?
Answer: In hydration, the ions remain as ions in the solutions and hence do not affect the pH of the solution. Whereas in Hydrolysis, ions react with water and release OH^- and H⁺ions as per the nature of salt. Hence in hydrolysis, the pH of the solution varies.

Question 2. What are neutral salts?
Answer: The salt which is formed by a combination of strong acid and strong base is known as neutral salts. For example : A very common example of NaCl, it is a neutral salt.

Question 3. What is the nature of soda which is used to puff the edible cakes?
Answer: Baking soda (NaHCO₃) is used to puff the edible cakes. It is a combination of strong base (NaOH) and weak acid (H₂CO₃). Hence it is a basic salt.

Question 4. Why are ions from strong acid and strong base hydrated rather than hydrolysed?
Answer: Ions from strong acid and strong base are hydrated rather than hydrolysed because they are stable in ionic form and don't want to react further.

Question 5. What is the nature of Fertilizer ammonium nitrate?
Answer: Ammonium Nitrate (NH₄NO₃) is a combination of weak base (ammonium hydroxide) and strong acid (nitric acid). Hence it is an acidic salt.

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