Chemistry Class 11 NCERT Solutions: Chapter 11 the p Block Elements Part 1

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Q: 1. Discuss the pattern of variation in the oxidation states of

(i)

(ii) C to Pb

Answer:

The electric configuration of group elements is . Therefore, the most common oxidation state exhibited by them should be . However, it is only boron and aluminium which practically show the +3-oxidation state. The remaining elements, i.e., Ga, In, Tl, show both the and +3 oxidation states. On moving down the group, the +1 state becomes more stable. For example, is more stable than Tl This is because of the inert pair effect. The two electrons present in the s-shell are strongly attracted by the nucleus and do not participate in bonding. This inert pair effect becomes more and more prominent on moving down the group. Hence, is unstable, is fairly stable, and is very stable.

Element and Oxidation State

Group 13 Element

Oxidation State

B

+3

Al

+3

Ga, In, Tl

+1, +3

The stability of the oxidation state decreases on moving down the group

(ii) C to Pb

The electronic configuration of group 14 elements is . Therefore, the most common oxidation state becomes more and more common on moving down the group. C and Si mostly show the state. On moving down the group, the higher oxidation state becomes less stable. This is because of the inert pair effect. Thus, although Ge, Sn, and Pb show both the and +4 states, the stability of the lower oxidation state increases and that of the higher oxidation state decreases on moving down the group.

Elements and Oxidation State

Group 14 Element

Oxidation State

C

+4

Si

+4

Ge, Sn, Pb

+2, +4

Q: 2. How can you explain higher stability of as compared to ?

Answer:

Boron and thallium belong to group 13 of the periodic table. In this group, the oxidation state becomes more stable on moving down the group. is more stable than because the oxidation state of B is more stable than the oxidation state of . In the state is highly oxidising and it reverts back to the more stable state.

Q: 3. Why does boron trifluoride behave as a Lewis acid?

Answer:

The electric configuration of boron is . It has three electrons in its valence shell. Thus, it can form only three covalent bonds. This means that there are only six electrons around boron and its octet remains incomplete. When one atom of boron combines with three fluorine atoms, its octet remains incomplete. Hence, boron trifluoride remains electron – deficient and acts as a Lewis acid.

Q: 4. Consider the compounds, and .How will they behave with water? Justify.

Answer

Being a Lewis acid readily undergoes hydrolysis. acid is formed as a result.

completely resists hydrolysis. Carbon does not have any vacant orbital. Hence, it cannot accept electrons from water to form an intermediate. When and water are mixed, they form separate layers.

Q: 5. Is boric acid a protic acid? Explain.

Answer:

Boric acid is not a protic acid. It is a weak monobasic acid, behaving as a Lewis acid

It behaves as an acid by accepting a pair of electrons from ion.

Q: 6. Explain what happens when boric acid is heated.

Answer:

On heating orthoboric acid at or above, it changes to metaboric acid . On further heating, this yields boric oxide