Class 12 Chemistry Chapter 7 The p-Block Elements

Chapter 7 of Class 12 Chemistry, titled “The p-Block Elements,” explores the chemistry of elements in groups 13 to 18 of the periodic table. These elements exhibit a wide range of properties and include metals, non-metals, and metalloids. The chapter covers the general characteristics, oxidation states, trends, and specific compounds of these elements.

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The chapter begins with Group 13 elements (Boron family), discussing their physical and chemical properties, trends in reactivity, and important compounds like borax, boric acid, and diborane. It then moves to Group 14 elements (Carbon family), emphasizing the unique properties of carbon and silicon, as well as their important compounds like carbon monoxide, carbon dioxide, silicones, and silicates.

The discussion of Group 15 elements (Nitrogen family) includes the properties of nitrogen and phosphorus, their oxides and acids, and industrial processes like the Haber process for ammonia synthesis and the Ostwald process for nitric acid production. Group 16 elements (Oxygen family) cover oxygen and sulfur, along with their compounds like oxides, sulfuric acid, and sulfates.

The chapter then delves into Group 17 elements (Halogens), focusing on fluorine, chlorine, bromine, and iodine, their physical and chemical properties, and their important compounds such as hydrogen halides, halogen acids, and interhalogen compounds. Finally, Group 18 elements (Noble Gases) are discussed, highlighting their inert nature, the discovery of compounds like xenon fluorides, and their uses in various fields.

In summary, Chapter 7 provides an in-depth understanding of the p-block elements, their trends, and their importance in various chemical reactions and industrial processes.

Questions with Answers

1. What are p-block elements?

P-block elements are those elements in which the last electron enters the p-orbital. They belong to groups 13 to 18 of the periodic table and include metals, non-metals, and metalloids.

2. Why do p-block elements show variable oxidation states?

P-block elements show variable oxidation states due to the involvement of both s and p orbitals in bonding, and the availability of vacant d orbitals in heavier elements.

3. Explain the inert pair effect.

The inert pair effect refers to the tendency of the s-electrons of the valence shell to remain paired and not participate in bonding, leading to lower oxidation states in heavier p-block elements.

4. Why does boron form covalent bonds rather than ionic bonds?

Boron forms covalent bonds because it has a small size and high ionization energy, which makes the formation of B³⁺ ions unfavorable.

5. What is the significance of the diagonal relationship between boron and silicon?

The diagonal relationship between boron and silicon arises due to their similar sizes and electronegativity, leading to comparable chemical properties, such as the ability to form covalent compounds.

6. Describe the structure of diborane (B₂H₆).

Diborane has a unique structure with four terminal hydrogen atoms bonded to two boron atoms, and two bridging hydrogen atoms that form three-center two-electron bonds.

7. What is the role of alumina in the Hall-Héroult process?

In the Hall-Héroult process, alumina (Al₂O₃) is dissolved in molten cryolite to lower its melting point and improve the conductivity, enabling the electrolytic extraction of aluminium.

8. Explain the amphoteric nature of aluminium.

Aluminium is amphoteric, meaning it can react with both acids and bases, forming salts and hydrogen gas with acids and aluminates with bases.

9. What is the catenation property of carbon?

Catenation is the ability of carbon atoms to form long chains or rings by bonding with other carbon atoms. This property is due to the small size of carbon and the strength of the C-C bond.

10. Describe the hybridization in the carbon atom of methane.

In methane (CH₄), carbon undergoes sp³ hybridization, where one s orbital and three p orbitals mix to form four equivalent sp³ hybrid orbitals, each forming a sigma bond with a hydrogen atom.

11. What is the significance of the hydrogen bond in water?

The hydrogen bond in water leads to its high boiling point, surface tension, and the anomalous expansion on freezing, making water essential for life.

12. Explain the industrial preparation of ammonia by the Haber process.

In the Haber process, ammonia is synthesized by reacting nitrogen with hydrogen in the presence of an iron catalyst at high temperature and pressure, optimizing the yield of ammonia.

13. Why is phosphoric acid considered a tribasic acid?

Phosphoric acid (H₃PO₄) is considered tribasic because it has three replaceable hydrogen atoms, each capable of dissociating to donate a proton in an aqueous solution.

14. What are interhalogen compounds?

Interhalogen compounds are molecules formed by the combination of two different halogens, such as ClF, BrF₃, and ICl₃, and they generally have higher reactivity than individual halogens.

15. Why are noble gases chemically inert?

Noble gases are chemically inert because they have a complete octet in their valence shells, making them stable and unlikely to participate in chemical reactions.

16. Describe the preparation and properties of sulfuric acid.

Sulfuric acid is prepared by the contact process, involving the oxidation of sulfur dioxide to sulfur trioxide, which is then absorbed in water. Sulfuric acid is a strong acid, a good dehydrating agent, and has a wide range of industrial applications.

17. What is the importance of ozone in the atmosphere?

Ozone in the stratosphere forms the ozone layer, which absorbs harmful ultraviolet (UV) radiation from the sun, protecting living organisms from its damaging effects.

18. Explain the structure and bonding in xenon fluorides.

Xenon fluorides, such as XeF₂ and XeF₄, exhibit a linear or square planar structure due to the involvement of p and d orbitals in hybridization, with xenon forming multiple bonds with fluorine.

19. What are silicones, and what are their uses?

Silicones are polymers with a repeating unit of siloxane (Si-O) chains. They are used in lubricants, sealants, and insulation materials due to their stability, flexibility, and water-repellent properties.

20. Why do halogens exhibit high reactivity?

Halogens exhibit high reactivity because they have high electronegativity and a strong tendency to gain an electron to achieve a stable noble gas configuration.