Solved Exercise, Chem-12, Ch-03

GROUP IIIA & GROUP IVA ELEMENTS

FILL IN THE BLANKS

Q.01: Fill in the blanks:

(i) ________ acid has the chemical formula HBO₂. (Metaboric acid)

(ii) Aluminium normally occurs as_________ minerals found in the rocks of outer portion of the earth. (aluminosilicate)

(iii) Aluminium gives ____ gas with hot concentrated H₂SO₄. (SO₂)

(iv) Boron forms ————— bond in its compounds. (covalent)

(v) The chemical formula of white lead is ______. (2PbCO₃.Pb(OH)₂)

(vi) ______ is the only element with less than four electrons in the outer most shell that is not a metal. (Boron)

(vii) In the Group IIIA of the periodic table, ________ is a semimetal. (boron)

(viii) Borax that occurs as a natural deposit, is called _______. (tincal)

(ix) Cryolite is an important mineral of aluminium and its formula is________. (Na₃AlF₆)

(x) A _______ is a substance that has different resistances to the passage of an electric current under different circumstances. (semi-conductor)

TRUE/FALSE

Q.02: Indicate True or False:

(i) Boron always uses all the three of its valence electrons for bonding purposes. (TRUE)

(ii) Diaspore is an ore of carbon. (FALSE)

CORRECTION: Diaspore is an ore of aluminium.

(iii) Emerald is an ore of aluminium that has the chemical formula Al₂O₃.2SiO₂.2H₂O. (FALSE)

CORRECTION: Emerald is an ore of aluminium that has the chemical formula Al₂F₄SiO₄.

(iv) An aqueous solution of borax is feebly acidic in nature. (FALSE)

CORRECTION: An aqueous solution of borax is weakly basic in nature.

(v) In case of borax bead test, of cupric oxide, the beads are coloured blue in the reducing flame. (FALSE)

CORRECTION: In case of borax bead test, of cupric oxide, the beads are coloured blue in the oxidizing flame.

(vi) Boric acid can be titrated with sodium hydroxide. (TRUE)

(vii) Carbon and silicon are the only non-metals in Group IVA. (TRUE)

(viii) PbO is commonly known as litharge. (TRUE)

(ix) Basic lead carbonate is a reddish-brown pigment. (FALSE)

CORRECTION: Basic lead carbonate is a white pigment.

(x) Aluminium oxide (Al₂O₃) is also called bauxite. (FALSE)

CORRECTION: Aluminium oxide (Al₂O₃) is also called alumina/corundum.

MCQs

Q.03: Multiple choice questions.

(i) Which metal is used in thermite process because of its activity?       

(a) Iron                         

(b) Copper

(c) Aluminium               

(d) Zinc

ANSWER: (c) Aluminium

EXPLANATION: Aluminium is used in thermite process because its reaction with air is highly exothermic, producing a lot of energy which is used for welding purpose.

(ii) Aluminium oxide is:     

(a) Acidic oxide             

(b) Basic oxide

(c) Amphoteric oxide     

(d) None of these

ANSWER: (c) Amphoteric oxide

EXPLANATION: The oxide of aluminium is amphoteric and reacts with both acids and bases.

(iii) Chemical composition of colemanite is:                            

(a) Ca₂B₆O₁₁. 5H₂O        

(b) CaB₄O₇. 4H₂O

(c) Na₂B₄O₇.4H₂O          

(d) CaNaB₅O₉.8H₂O

ANSWER: (a) Ca₂B₆O₁₁. 5H₂O

EXPLANATION: Chemically, colemanite is actually calcium borate or calcium borate hydroxide pentahydrate.

(iv) Which element forms an ion with charge +3?                       

(a) Beryllium                 

(b) Aluminium

(c) Carbon                     

(d) Silicon

ANSWER: (b) Aluminium

EXPLANATION: Among these, Aluminium is a metal which can lose its valence three electrons to form a cation with charge +3.

(v) Which electronic configuration corresponds to an element of group-IIIA of the periodic table?

(a) 1s², 2s², 2p⁶, 3s², 3p¹

(b) 1s², 2s², 2p⁶, 3s², 3p⁶, 4s²

(c) 1s², 2s², 2p⁶

(d) 1s², 2s², 2p⁶, 3s², 3p³

ANSWER: (a) 1s², 2s², 2p⁶, 3s², 3p¹

EXPLANATION: The elements of group IIIA have three electrons in their valence shells. So, their valence shell configuration must be ns², np¹, where n is the valence shell number.

(vi) Which element among the followings belongs to group IVA of the periodic table?  

(a) Barium   

(b) Iodine    

(c) Lead      

(d) Oxygen

ANSWER: (c) Lead

EXPLANATION: Lead is the lowermost element of group IVA, present in 6^th period.

(vii) Boric acid can’t be used:              

(a) As antiseptic in medicine

(b) For washing eyes

(c) In soda bottles

(d) For enamels and glazes

ANSWER: (c) In soda bottles

EXPLANATION: Soda bottles are actually carbonated drinks, containing dissolved carbon dioxide. Since, boric acid does not generate CO₂, it cannot be used in soda bottles as a source of carbonation.

(viii) Which of the following elements is not present abundantly in earth crust?

(a) Silicon                     

(b) Aluminium

(c) Sodium                    

(d) Oxygen

ANSWER: (c) Sodium

EXPLANATION: Very small amounts of odium are present in earth crust in the form of minerals. The other three elements i.e., oxygen, silicon & aluminium are the three most abundant elements present in earth crust.

(ix) Tincal is mineral of:         

(a) Al          

(b) B           

(c) Si          

(d) C

ANSWER: (b) B

EXPLANATION: Tincal is the ore of boron because chemically it is a sodium tetraborate or Na₂B₄O₇.10H₂O, also known as borax.

(x) The chief ore of aluminium is:          

(a) Na₃AlF₆                    

(b) Al₂O₃.2H₂O

(c) Al₂O₃                       

(d) Al₂O₃.H₂O

ANSWER: (b) Al₂O₃.2H₂O

EXPLANATION: The chief ore of aluminium is Al₂O₃.2H₂O which is also known as bauxite. It is from bauxite that most of the aluminium is extracted by Halls-Beroult process.

Q.04: What is the action of an aqueous solution of borax on litmus?

Ans: The aqueous solution of borax, being alkaline in nature, turns red litmus to blue.

Q.05: Give equations to represent the following reactions: (a) Borax is heated with CoO. (b) Al₂O₃ is heated with NaOH solution.

ANSWER:

(a) Borax is heated with CoO:

Na₂B₄O₇  ⟶ 2NaBO₂ + B₂O₃

CoO+ B₂O₃  ⟶ Co(BO₂)₂

(b) Al₂O₃ is heated with NaOH solution:

Al₂O₃  + 2NaOH ⟶ 2NaAlO₂ + H₂O

Q.06: Why is aluminium not found as a free element? Explain the chemistry of borax bead test.

ANSWER:

Aluminium is not found naturally in free state. This is because it is very reactive metal. It occurs as alumino-silicate minerals in earth crust as the third most abundant element after oxygen and silicon.

Borax Bead Test: Borax bead test is used to identify certain metal cations. Some powdered borax is taken on a hot loop of platinum wire, and is heated again. Borax first swells up and then melts into a colourless, glass-like bead on the loop. A few grains of the salt are put on the glassy bead, which is then re-heated, first in the oxidizing flame and then in the reducing flame.

Chemistry of Borax Bead Test: Borax when fused is decomposed into sodium metaborate and boric anhydride.

Na₂B₄O₇ ⟶ 2NaBO₂ + B₂O₃

B₂O₃ or boric anhydride combines with the metallic oxide, giving coloured metallic borates. For example, in case of cupric oxide, beads are coloured blue in the oxidizing flame, because cupric borates are blue in colour.

CuO + B₂O₃ ⟶ Cu(BO₂)₂

Q.07: How does orthoboric acid react with: (a) Sodium hydroxide (b) Ethyl alcohol

ANSWER:

(a) Reaction of H₃BO₃ with C₂H₅OH:

H₃BO₃ + 3C₂H₅OH ¾¾® (C₂H₅)₃BO₃ + 3H₂O

(b) Reaction of H₃BO₃ with NaOH:

4H₃BO₃ + 2NaOH ⟶ Na₂B₄O₇ + 7H₂O

Q.08: How will you convert boric acid into borax and vice versa?

ANSWER:

1) Boric Acid into Borax:

4H₃BO₃ + 2NaOH ⟶ Na₂B₄O₇ + 7H₂O

2) Borax into Boric Acid:

Na₂B₄O₇ + H₂SO₄ + 5H₂O ⟶ Na₂SO₄ + 4H₃BO₃

Q.09: Why are liquid silicones preferred over ordinary organic lubricants?

ANSWER:

Liquid silicones are better lubricants than ordinary organic lubricants because they show very small change in viscosity with change in temperature. If the temperature drops from 100^oCto 0^oC, the viscosity of petroleum oil may increase about 100 folds, whereas that of silicone oils less than four folds. Furthermore, in the presence of air at high temperature as 300^oC, silicone oils remain free from acid formation, oxidation, etc.

Q.10: Explain: (a) CO₂ is non-polar in nature. (b) CO₂ is acidic in character.

ANSWER:

(a) In CO₂, carbon makes two double bonds with two O-atoms. There is no lone pair of electrons on the central atom. So, two double bonds arrange at 180^o, and the molecule becomes linear. The dipole moments of these two double bonds are in opposite directions, so cancel each other. The net dipole moment becomes zero. That’s why the molecule of CO₂ is non-polar in nature.

(b) CO₂ is acidic in character because it is the oxide of a non-metal. When it is dissolved in water, it forms an acid.                                          

CO₂ + H₂O  ⟶  H₂CO₃

Q.11: Why is CO₂ a gas at room temperature while SiO₂ is a solid?

ANSWER:

CO₂ is a gas at room temperature. The reason is that carbon atom is of very small size. It cannot make four single bonds with four oxygen atoms around it to form a giant molecule, due to the increased repulsion of electron pairs at close distances. Rather, it makes two double bonds with two oxygen atoms to form a small, linear, non-polar CO₂ molecule, which has very weak intermolecular forces, and hence is a gas at room temperature.   

On the other hand, SiO₂ is a solid at room temperature. This is because silicon atom has greater size. It can make four single bonds with four oxygen atoms in a tetrahedral manner. Each oxygen atom is further bonded to a silicon atom on the other side, to form Si-O-Si bond. This continues in three dimensions until a giant molecule of SiO₂ is formed, which is a solid at room temperature.

Q.12: Give the names and the formulas of different acids of boron.

ANSWER:

The names and formulas of four important oxyacids of boron are:

(1) Orthoboric acid: H₃BO₃         

(2) Metaboric acid: HBO₂

(3) Tetraboric acid: H₂B₄O₇              

(4) Pyroboric acid: H₆B₄O₉

Q.13: What is the importance of oxides of lead in paints?

ANSWER:

The oxides of lead are coloured compounds, so they are used as pigments of different colours in paints.

Examples:

1. Lead suboxide, Pb₂O, is a black pigment.
2. Lead monoxide (Litharge & Massicot), PbO, occurs from pale yellow to reddish yellow colour.
3. Triplumbictetraoxide (Red Lead or Minium), Pb₃O₄ is a bright scarlet crystalline powder.
4. Lead dioxide, PbO₂, is a reddish-brown powder.

Q.14: Give the names, electronic configurations occurrence of Group-IIIA elements of the periodic table.

ANSWER:

Consult the textbook at page 38.

Q.15: Discuss the peculiar behaviour of boron with respect to the other members of Group- IIIA elements.  

ANSWER:

Peculiar Behaviour of Boron: Boron shows many dissimilarities with the members of its own group. These dissimilarities are due to the large difference in their sizes and ionization energies. Some of these peculiarities of boron are:

1. Boron is the only non-metallic element in group IIIA.
2. It is the only element which has less than four electrons in the valence shell, but is not a metal.
3. Boron always uses its three valence electrons in bonding, showing +3 & -3 oxidation states.
4. Boron can form molecular addition compounds, while other members cannot.
5. Boron does not form ionic compounds with sulphates, nitrates or other anions because it does not form a stable cation.

Q.16: (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 the sodium salt of tetraboric acid. Chemically, it is sodium tetraborate or Na₂B₄O₇.10H₂O. It is found as a natural deposit Tincal, in dried up lakes of Tibet and California.

(b) Describe its commercial preparation.

(b) Commercial Preparation:

(1) From Boric Acid: Formally, borax was prepared by treating hot boric acid solution with soda ash.

4H₃BO₃ + Na₂CO₃ ⟶ Na₂B₄O₇ + 6H₂O + CO₂

(2) From Colemanite: Now-a-days, borax is obtained by boiling finely powdered colemanite with Na₂CO₃ solution. CaCO₃ precipitates out and a mixture of borax and sodium metaborate is formed.

Ca₂B₆O₁₁ + 2Na₂CO₃  ⟶ 2CaCO₃ + Na₂B₄O₇ + 2NaBO₂

The clear solution from the top is separated. Borax is crystallized out. To get more borax, CO₂ is blown through the mother-liquor. Sodium metaborate is converted into borax as:

4NaBO₂ + CO₂  ⟶ Na₂CO₃ + Na₂B₄O₇

(c) Outline the principal uses of borax.

(c) Uses of Borax:

(1) It is used to prepare borate glass which is heat resistant.

(2) It is used in softening of water.

(3) It is used as a flux in welding and in metallurgy.

(4) It is used in leather industry for tanning and dyeing.

(d) How does borax serve as a water softening agent?

(d) Hardness in water is due to the presence of Ca²⁺ and Mg²⁺ ions. Borax, when dissolved in hard water, converts these ions into insoluble calcium and magnesium tetraborates, which are then removed easily. Thus, borax can be used as a water-softening agent.

Ca²⁺(aq)+ Na₂B₄O₇ ⟶ CaB₄O₇¯+ 2Na⁺(aq)

Mg²⁺(aq)+ Na₂B₄O₇ ⟶ MgB₄O₇¯+ 2Na⁺(aq)

Q.17: (a) What is boric acid? (b) How is boric acid prepared in laboratory? (c) Give properties and uses of boric acid.

ANSWER:

(a) BORIC ACID: Boric acid or orthoboric acid (H₃BO₃) is a white crystalline chemical substance (triclinic), sparingly soluble in cold water (2.6% at 40 0 C) but dissolves readily in hot water (37% at 107°C).

(b) Boric acid is prepared in the laboratory by treating hot concentrated solution of borax with H₂SO₄. The crystals of H₃BO₃ separate out on cooling.

Na₂B₄O₇ + H₂SO₄ + 5H₂O ⟶ Na₂SO₄ + 4H₃BO₃  

(c) Uses of Ortho Boric Acid:

(1) Boric acid is used in medicines as an antiseptic, e.g., dusting powder, boric ointment, and boric solution as an eye wash.

(2) It is used in pottery as a glaze.

(3) It is also used in candle industry for stiffening of wicks.

Q.18: (a) Give the names along with the formulas of three important ores of aluminium.

ANSWER:

(1) Feldspar — KAlSi₃O₈

(2) Mica — KH₂Al₃(SiO₄)₃

(3) Bauxite — Al₂O₃.2H₂O

(b) How and under what conditions does aluminium react with the following: i) Oxygen ii) Hydrogen iii) Halogens iv) Acids v) Alkalies

ANSWER:

Reactions of Aluminium With:

(i) Oxygen: 4Al + 3O₂ ⟶ 2Al₂O₃

(ii) Hydrogen: 2Al + 3H₂ ⟶ 2AlH₃

(iii) Halogens: 2Al + 3Cl₂ ⟶ 2AlCl₃

(iv) Acids: 2Al_(s) + 6HCl_(aq) ⟶ 2AlCl_3(aq) + 3H_2(g)

2Al_(s) + 6H₂SO_4(aq) ⟶ Al₂(SO₄)_3(aq) + 6H₂O_(ℓ) + 3SO_2(g)

(v) Alkalies: 2Al_(s) + 2NaOH_(aq) + 6H₂O_(ℓ) ⟶ 2NaAl(OH)_4(aq) + 3H_2(g)

Q.19: Give the names, electronic configurations and occurrence of Group-IVA elements of the periodic table.

ANSWER:

Consult the textbook at page 44-45.

Q.20: Discuss the peculiar behaviour of carbon with respect to the other members of Group-IVA of the periodic table.

ANSWER:

Peculiar Behaviour of Carbon:

Carbon differs from other elements of group IVA in the following respects:

1. Carbon and silicon are non-metals while other members of the family are metalloids or metals.
2. Carbon has the tendency to form long chains of identical atoms. This self-linkage property is called catenation. This property is maximum in carbon, while decreases down the group.

Q.21: (a) What are silicones? (b) Give a brief summary of the principal properties of silicones. (c) Outline the uses of silicones. (d) Describe the important uses of silicates.

ANSWER:

(b) Properties of Silicones:

Silicones are a class of synthetic polymers with unique properties:

1. Heat Resistance: Silicones can withstand a wide range of temperatures, from very low to high, without significant degradation.
2. Chemical Stability: They are resistant to chemicals, making them durable in various environments.
3. Water Repellency: Silicones are water-resistant and do not easily absorb water.
4. Flexibility: Silicones are flexible and elastic, allowing them to maintain their properties over a broad temperature range.
5. Electrical Insulation: They exhibit excellent electrical insulation properties, making them suitable for electrical applications.
6. Low Toxicity: Silicones are generally non-toxic and biocompatible, making them suitable for use in medical and cosmetic applications.
7. Low Surface Tension: They have a low surface tension, providing them with good spreading and wetting properties.
8. Inertness: Silicones are inert and do not react with many substances, contributing to their stability.

These properties make silicones versatile materials used in various industries, including electronics, construction, automotive, medical, and personal care products.

(c) Important Uses of Silicones:

1. Methyl silicones are used as lubricants in bearings, gears and hydraulic brakes.
2. Methyl silicones of high molecular mass are used in making rubber like tubing and sheets.
3. They are used in electrical insulation due to their resinous properties.
4. They are used as water repellents, on the surfaces of electrical insulators, cloth, glass, leather, paper, etc.

(d) Silicates: “The compounds derived from silicic acids are termed as silicates.” e.g., sodium silicate (Na₂SiO₃), magnesium silicate or talc, Mg₃H₂(SiO₃)₄.

(e) Uses of Sodium Silicate:

1. It is used as a filler for soap in soap industry.
2. It is used in textile as a fire proof.
3. It is used as furniture polish.
4. It is used in calico printing.