Q6. Why is aluminium not found as a free element? Explain the chemistry of the borax bead test.
- Answer: Aluminium is highly reactive and readily combines with oxygen to form aluminium oxide (Al₂O₃), which is a stable compound. This prevents aluminium from being found in a free, metallic state in nature.
The borax bead test is used for detecting metal ions. Borax (Na₂B₄O₇·10H₂O) melts into a clear glassy bead at high temperatures. When certain metal oxides are heated in the bead, they react with the borax to produce characteristic colors. Aluminium doesn’t form distinct colors in this test.
Q7. How does orthoboric acid react with:
(a) Sodium hydroxide
(b) Ethyl alcohol
- Answer:
(a) With Sodium Hydroxide (NaOH): Orthoboric acid reacts with NaOH to form sodium borate (Na₂B₄O₇), as shown in the reaction:
B(OH)₃ + NaOH → NaBO₂ + 2H₂O
(b) With Ethyl Alcohol: Orthoboric acid reacts with ethyl alcohol in the presence of concentrated sulfuric acid to form triethyl borate:
B(OH)₃ + 3C₂H₅OH → B(OC₂H₅)₃ + 3H₂O
Q8. How will you convert boric acid into borax and vice versa?
- Answer:
- Boric acid to borax: Boric acid (H₃BO₃) can be converted into borax by heating it with sodium carbonate (Na₂CO₃), which forms sodium tetraborate (borax) and water:
4H₃BO₃ + Na₂CO₃ → Na₂B₄O₇ + 6H₂O + CO₂ - Borax to boric acid: Borax reacts with a strong acid like hydrochloric acid (HCl) to produce boric acid:
Na₂B₄O₇ + 2HCl + 5H₂O → 4H₃BO₃ + 2NaCl
Q9. Why are liquid silicones preferred over ordinary organic lubricants?
- Answer: Liquid silicones are preferred over organic lubricants due to their higher thermal stability, resistance to oxidation, chemical inertness, water repellency, and low-temperature fluidity. They also have low vapor pressure, making them suitable for high-temperature applications.
Q10. Explain:
(a) CO₂ is non-polar in nature:
- CO₂ has a linear molecular geometry with equal bond dipoles in opposite directions. The symmetry of the molecule causes the dipoles to cancel out, making CO₂ non-polar.
(b) CO₂ is acidic in character: - When CO₂ dissolves in water, it reacts to form carbonic acid (H₂CO₃), which dissociates to release hydrogen ions (H⁺), demonstrating its acidic nature:
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
Q11. Why is CO₂ a gas at room temperature while SiO₂ is a solid?
- Answer: CO₂ is a small, simple molecule with weak van der Waals forces between molecules, allowing it to remain in the gaseous state at room temperature. In contrast, SiO₂ forms a giant covalent network structure with strong Si-O bonds, making it a solid with a high melting point.
Q12. Give the names and the formulas of different acids of boron.
- Answer:
- Boric acid (H₃BO₃)
- Tetrahydroxyboric acid (H₄BO₄)
- Metaboric acid (HBO₂)
Q13. What is the importance of oxides of lead in paints?
- Answer: Lead oxides, such as lead(II) oxide (PbO) and lead(IV) oxide (Pb₃O₄), are used in paints because they enhance durability, increase resistance to corrosion, and provide brilliant color to the pigments. However, due to toxicity, their use is now limited.
Q14. Give the names, electronic configurations, and occurrence of Group-IIIA elements of the periodic table.
- Answer: The Group-IIIA elements are:
- Boron (B): 1s² 2s² 2p¹
- Aluminium (Al): [Ne] 3s² 3p¹
- Gallium (Ga): [Ar] 3d¹⁰ 4s² 4p¹
- Indium (In): [Kr] 4d¹⁰ 5s² 5p¹
- Thallium (Tl): [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
These elements are found in various ores and minerals such as borates, bauxite (Al), and zinc ores (Ga and In).
Q15. Discuss the peculiar behavior of boron with respect to the other members of Group-IIIA elements.
- Answer: Boron exhibits several unique properties compared to other Group-IIIA elements (Al, Ga, In, Tl):
- Non-metallic Character: Boron is a metalloid, while the others are metals.
- Covalency: Boron forms covalent bonds, whereas the other elements can form ionic compounds.
- High Ionization Energy: Boron has a much higher ionization energy compared to the heavier Group-IIIA elements.
- Lack of Metallic Conductivity: Unlike the metals in its group, boron is a poor conductor of electricity.
- Allotropes: Boron exists in various allotropic forms, unlike aluminium and other members.
Q16. (a) What is borax?
(b) Describe its commercial preparation.
(c) Outline the principal uses of borax.
(d) How does borax serve as a water-softening agent?
- Answer:
(a) Borax is a sodium salt of boric acid with the formula Na₂B₄O₇·10H₂O.
(b) Commercial Preparation: Borax is prepared by extracting borate minerals from lake beds and then treating them with soda ash (Na₂CO₃) to form borax. - Na₂CO₃ + CaB₄O₇ + H₂O → Na₂B₄O₇ + CaCO₃
(c) Uses of Borax: - Used in glass and ceramic production.
- As a flux in metallurgy.
- In detergents and soaps as a water softener.
- As an antiseptic and preservative.
(d) Water Softening: Borax helps soften water by binding to calcium and magnesium ions, preventing them from interfering with soap action. This allows soap to lather better in hard water.
Q17. (a) What is boric acid?
(b) How is boric acid prepared in the laboratory?
(c) Give properties and uses of boric acid.
- Answer:
(a) Boric Acid (H₃BO₃) is a weak monobasic acid derived from boron.
(b) Laboratory Preparation: Boric acid can be prepared by adding hydrochloric acid to a hot, concentrated solution of borax:
Na₂B₄O₇ + 2HCl + 5H₂O → 4H₃BO₃ + 2NaCl
(c) Properties: - It is a white crystalline solid, soluble in water.
- Boric acid is a weak acid, antiseptic, and has a low vapor pressure.
Uses: - In antiseptic solutions for minor cuts.
- In the manufacture of glass and ceramics.
- In insecticides to kill ants and roaches.
- As a pH buffer in swimming pools.
Q18. (a) Give the names along with the formulas of three important ores of aluminium.
(b) How and under what conditions does aluminium react with the following:
i) Oxygen ii) Hydrogen iii) Halogens iv) Acids v) Alkalies
- Answer:
(a) Ores of Aluminium: - Bauxite: Al₂O₃·2H₂O
- Cryolite: Na₃AlF₆
- Corundum: Al₂O₃
(b) Reactions of Aluminium: - With Oxygen: Aluminium reacts readily with oxygen, forming a protective layer of aluminium oxide:
4Al + 3O₂ → 2Al₂O₃ - With Hydrogen: Aluminium does not react with hydrogen directly.
- With Halogens: Aluminium reacts with halogens to form aluminium halides:
2Al + 3Cl₂ → 2AlCl₃ - With Acids: Aluminium dissolves in acids to release hydrogen gas:
2Al + 6HCl → 2AlCl₃ + 3H₂ - With Alkalies: Aluminium reacts with alkalis to form aluminates and hydrogen gas:
2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂
Q19. Give the names, electronic configurations, and occurrence of Group-IVA elements of the periodic table.
- Answer: The Group-IVA elements are:
- Carbon (C): 1s² 2s² 2p²
- Silicon (Si): [Ne] 3s² 3p²
- Germanium (Ge): [Ar] 3d¹⁰ 4s² 4p²
- Tin (Sn): [Kr] 4d¹⁰ 5s² 5p²
- Lead (Pb): [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p²
These elements are found in nature in various forms, such as carbon in coal, silicon in sand (SiO₂), and lead in galena (PbS).
Q20. Discuss the peculiar behavior of carbon with respect to the other members of Group-IVA of the periodic table.
- Answer:
- Catenation: Carbon uniquely forms long chains and rings of atoms due to its strong C-C bonds. Other Group-IVA elements do not show such extensive catenation.
- Allotropes: Carbon exists in various allotropes like graphite, diamond, and fullerenes, whereas silicon and others have fewer allotropic forms.
- Multiple Bonds: Carbon forms stable double and triple bonds (C=C and C≡C), unlike heavier elements, which rarely form multiple bonds.
- Non-metallic Nature: Carbon is a non-metal, whereas the others in the group (like Sn and Pb) exhibit metallic properties.
Q21. (a) What are silicones?
(b) Give a brief summary of the principal properties of silicones.
(c) Outline the uses of silicones.
(d) What are silicates?
(e) Describe the structures of silicates.
- Answer:
(a) Silicones are synthetic polymers made up of repeating units of siloxane (Si-O-Si) with organic side groups attached to the silicon atoms.
(b) Properties of Silicones: - High thermal stability
- Water repellency
- Electrical insulating properties
- Flexibility and low toxicity
(c) Uses of Silicones: - Used as sealants and adhesives
- In medical devices and implants
- As lubricants and insulating materials
- In cosmetics and personal care products
(d) Silicates: Silicates are minerals containing silicon and oxygen, commonly combined with other elements like magnesium, iron, and aluminium. Examples include quartz (SiO₂) and feldspar.
(e) Structures of Silicates: Silicates have a tetrahedral structure, where one silicon atom is surrounded by four oxygen atoms. These tetrahedra can link in various ways to form different types of silicates, such as isolated, chain, sheet, and framework silicates.
