Chemistry Class 11 NCERT Solutions: Chapter 9 Hydrogen Part 5

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Q: 20. Complete the following chemical reactions.

(i)

(ii)

(iii)

(iv)

(v)

Classify the above into

(a) hydrolysis

(b) redox and

(c) hydration reactions

Answer:

(i)

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

(ii)

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

(iii)

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

(iv)

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

(v)

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

called hydrolysis reactions. The given reaction represents hydrolysis of .

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:

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

surrounded tetrahedrally by four other oxygen atoms at a distance of . 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 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 and anions present in water by 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 group. The

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

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

and ions.

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

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 ions neutralize the 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

The reaction takes place as:

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

(2) Reaction with

The reaction takes place as:

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

(3) Self-ionization of water

In the reaction, two water molecules react as: