Chemistry Class-11: Chapter –11. The p-Block Elements Part – 4

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III. Short Answer Type

Question: 22

Draw the structures of (dimer).

Answer:

Question: 23

Explain the nature of boric acid as a Lewis acid in water.

Answer:

Boric acid acts as Lewis acid in water by accepting a pair of electrons from a hydroxyl ion:

Question: 24

Draw the structure of boric acid showing hydrogen bonding. Which species is present in water? What is the hybridisation of boron in this species?

Answer:

Species present in water is . Boron is hybridised.

Question: 25

Explain why the following compounds behave as Lewis acids?

(i)

(ii)

Answer:

and being electron deficient due to incomplete octet of central metal atom behave as Lewis acids.

Question: 26

Give reasons for the following:

(i) is immiscible in water, whereas is easily hydrolysed.

(ii) Carbon has a strong tendency for catenation compared to silicon.

Answer:

is a covalent compound. Hence, insoluble in water whereas is soluble because atom in can accomodate the lone pair of electrons obtained from oxygen atom of water molecule in d-orbitals.

Question: 27

Explain the following:

(i) is a gas whereas is a solid.

(ii) Silicon forms ion whereas corresponding fluoro compound of carbon is not known.

Answer:

(i) Very high bond enthalpy and ionic character of bond.

(ii) Vacant orbitals are available on atom to accomodate electrons and expand coordination number up to 6.

Question: 28

The oxidation state in group and oxidation state in group 14 becomes more and more stable with increasing atomic number. Explain.

Question: 29

Carbon and silicon both belong to the group 14, but inspite of the stoichiometric similarity, the dioxides, (i.e., carbon dioxide and silicon dioxide), differ in their structures. Comment.

Answer:

[Hint: In , carbon is sp hybridised and it is a linear molecule. In , is tetrahedrally bonded to four oxygen atoms.]

Question: 30

If a trivalent atom replaces a few silicon atoms in three dimensional network of silicon dioxide, what would be the type of charge on overall structure?

Answer:

Negative

Question: 31

When is treated with water, it hydrolyses and forms only whereas in acidified aqueous solution forms . Explain what is the hybridisation of boron and aluminium in these species?

Answer:

Question: 32

Aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character. A piece of aluminium foil is treated with dilute hydrochloric acid or dilute sodium hydroxide solution in a test tube and on bringing a burning matchstick near the mouth of the test tube, a pop sound indicates the evolution of hydrogen gas. The same activity when performed with concentrated nitric acid, reaction doesn’t proceed. Explain the reason.

Answer:

[Hint: Conc. renders aluminium passive by forming a protective oxide layer on the surface.]

Question: 33

Explain the following:

(i) Gallium has higher ionisation enthalpy than aluminium.

(ii) Boron does not exist as .

(iii) Aluminium forms but boron does not form .

(iv) is more stable than .

(v) as an oxidising agent but acts as a reducing agent.

(vi) Electron gain enthalpy of chlorine is more negative as compared to fluorine.

(vii) acts as an oxidising agent.

(viii) Carbon shows catenation property but lead does not.

(ix) BF3 does not hydrolyse.

(x) Why does the element silicon, not form a graphite like structure whereas carbon does.

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