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

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

You have heard of baking soda, table salt and salt ammonia? All of them are solid. and salts formed by the reaction between an acid and a base. So are them are similar or different? If different, How?

Let us learn more about them and answer the questions.

There are many salts in nature which are classified into four primary kinds of salts. However, we are only aware of one neutral salt, which is table salt. Right????? There are many more salts that we use on a daily basis, some of which are basic, some of which are acidic, and some of which are neutral in chemical nature.

In particular, ammonium nitrate, an acidic salt, is utilized in fertilizers, and basic salts like baking soda and washing soda are employed in domestic cleaning. In this article, we'll discuss the production of these salts as well as a number of fascinating characteristics, particularly of salts that undergo a process called hydrolysis. Let's investigate them in depth.

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Table of content

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

What are salts?

Salts, consists of a positive ion of a base and a negative ion of acid, which react to form this salt

For example HCl(aq)+NaOH(aq)NaCl(aq)+H2O(aq)

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

Concept of hydration and hydrolysis

The cations and anions of salts produced by the reaction of strong acids and strong bases are hydrated rather than hydrolysed in an aqueous solution. 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 (reaction with water) and ion from strong electrolyte (strong acid and strong base) undergoes hydration rather than hydrolysis.

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. Anionic Hydrolysis
  2. Cationic Hydrolysis

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

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)\rlharBOH(aq)+H3+O(aq)

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

NH4+(aq)+2H2O(l)\rlhar NH4OH(aq)+H3+O(aq)

Hydrolysis of salt of weak acid and weak base(WAWB)

A salt which is formed from the combination of weak acid and weak base is called Salt of weak acid and weak base (WAWB).

Consider a salt of weak acid and weak base CH3COONH4 Here both cation of weak base NH4+ and anion of weak acid CH3COO- undergo hydrolysis and can be written as

NH4+(aq)+CH3COO-(aq)+H2O(l) \rlhar NH4OH(aq)+CH3COOH(aq)

Practice Problems

Q1. Comment for the pH of NaClO4 salt when added in water

Answer: NaClO4(aq)Na+(aq)+ClO4-(aq)

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 approximately near 7 and the nature of salt should be neutral.

Q2. Arrange the solution of salt AlCl3, GaCl3 and InCl3in order of decreasing pH.

Answer: These metals' hydroxides are weak bases. Cationic hydrolysis will occur in salts that contain the strong acid HCl. Hydrolysis is influenced by the cation's charge and size. The rate of hydrolysis to create hydrogen ions increases with increasing size and charge. Aluminium has the smallest size and is therefore the most readily hydrolyzed, resulting in a more acidic solution. This is because they all have the same charge.

The pH sequence will be Al < Ga < Tl

Q3. Comment for the pH of table salt when added to water, whether it is acidic or basic.

Answer: Table salt : NaCl

NaCl(aq)Na+(aq)+Cl-(aq)

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.

Q4. Predict whether Mg(CN)2salt undergoes cationic hydrolysis or not ?

Answer: Mg(CN)2(aq)Mg2+(aq)+2CN-(aq)

Mg(CN)2salt 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 and nature of NaCN salt when added in water whether it is acidic or basic.

Answer: NaCN(aq)Na+(aq)+CN-(aq)

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.

Frequently Asked Questions -FAQs

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

2. 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.

3. What is the composition of the soda used to puff the edible cakes?
Answer:
To puff the edible cakes, baking soda (NaHCO3) is used. It is a mixture of a strong base (NaOH) and a weak acid (H2CO3). As a result, it is a basic salt.

4. Is washing soda a base, an acid, or a salt?
Answer:
Strong base (NaOH) and weak acid (H2CO3) combine to form washing soda (Na2CO3) . It is a basic salt as a result.

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