Chemistry Class 11 NCERT Solutions: Chapter 9 Hydrogen Part 5

Q: 20. Complete the following chemical reactions.

(i) Equation

(ii) Equation

(iii) Equation

(iv) Equation

(v) Equation

Classify the above into

(a) hydrolysis

(b) redox and

(c) hydration reactions

Answer:

(i) Equation

Equation is acting as an oxidizing agent in the reaction. Hence, it is a redox reaction.

(ii) Equation

Equation is acting as a reducing agent in the acidic medium, thereby oxidizing Equation . Hence, the given reaction is a redox reaction.

(iii) Equation

The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis.

(iv) Equation

The reaction in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of Equation .

(v) Equation

The reactions in which a compound reacts with water to produce other compounds are

called hydrolysis reactions. The given reaction represents hydrolysis of Equation .

Q: 21. Describe the structure of the common form of ice.

Answer

Ice is the crystalline form of water. It takes a hexagonal form if crystallized at

atmospheric pressure, but condenses to cubic form if the temperature is very low.

The three-dimensional structure of ice is represented as:

Q 21 Structure of Three-Dimensional Structure of Ice

Q 21 Structure of Three-Dimensional Structure of Ice

Q 21 Structure of Three-Dimensional Structure of Ice

The structure is highly ordered and has hydrogen bonding. Each oxygen atom is

surrounded tetrahedrally by four other oxygen atoms at a distance of Equation . The structure also contains wide holes that can hold molecules of appropriate sizes interstitially.

Q: 22. What causes the temporary and permanent hardness of water?

Answer:

Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates Equation in water. Permanent hardness of water is because of the presence of soluble salts of calcium and magnesium in the form of chlorides in water.

Q: 23. Discuss the principle and method of softening of hard water by synthetic ion-exchange resins.

Answer:

The process of treating permanent hardness of water using synthetic resins is based on

the exchange of cations Equation and anions Equation present in water by Equation respectively.

Synthetic resins are of two types:

(1) Cation exchange resins

(2) Anion exchange resins

Cation exchange resins are large organic molecules that contain the Equation group. The

resin is firstly changed to Equation (from Equation ) by treating it with Equation . This resin then exchanges Equation ions with Equation and Equation ions, thereby making the water soft.

Equation

There are cation exchange resins in Equation form. The resins exchange Equation ions for Equation

and Equation ions.

Equation

Anion exchange resins exchange OH- ions for anions like Equation present in water.

Equation

During the complete process, water first passes through the cation exchange process.

The water obtained after this process is free from mineral cations and is acidic in nature. This acidic water is then passed through the anion exchange process where Equation ions neutralize the Equation ions and de-ionize the water obtained.

Q: 24. Write chemical reactions to show the amphoteric nature of water.

Answer:

The amphoteric nature of water can be described on the basis of the following reactions:

(1) Reaction with Equation

The reaction takes place as:

Equation

In the forward reaction, Equation accepts a proton from Equation Hence, it acts as a Lewis base.

(2) Reaction with Equation

The reaction takes place as:

Q 24 2 Structure of Reaction With NH3

Q 24 2 Structure of Reaction With NH3

Q 24 2 Structure of Reaction With NH3

In the forward reaction, Equation denotes its proton to Equation Hence, it acts as a Lewis acid

(3) Self-ionization of water

In the reaction, two water molecules react as:

Q 24 2 Structure of Self-ionization of water

Q 24 2 Structure of Self-Ionization of Water

Q 24 2 Structure of Self-ionization of water

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