Notes-Part-2-Class-11-Science-Chemistry-Chapter-9-Elements of Group 13, 14 and 15-Maharashtra Board

Allotropy :

Allotropy : When a solid element exists in different crystalline forms with different physical properties (such as colour, density, melting point etc.) the phenomenon is called allotropy. Individual forms are called allotropes.

• In the p-block elements other than carbon, elements such as boron, silicon, bismuth also exhibit allotropy.

Allotropes of Carbon :

The important allotropes of carbon are (i) Diamond (ii) Graphite (iii) Fullerenes (iv) Carbon nanotubes (v) Graphene.

(i) Diamond :

Structure of diamond :

• Diamond has three dimensional structure.
• In this, carbon atom undergoes sp³-hybridisation.
• Each carbon atom is bonded tetrahedrally to four other carbon atoms which occupy the four corners of the tetrahedron, by overlapping of sp³-hybrid orbitals. The C—C bond distance is 154 pm.
• Each of these four carbon atoms are covalently bonded to four more carbon atoms in the Similar fashion and the pattern is repeated in the entire crystal lattice. These tetrahedra are linked together into a three dimensional network to form a giant molecule of diamond.

(ii) Graphite

Graphite is thermodynamically most stable form (allotrope) of carbon.

Structure of graphite :

• Planar hexagonal rings of carbon atoms are arranged in layers in a two-dimensional sheet-like structure called graphite.
• As a result of sp2-hybridization, each carbon atom in a hexagonal ring forms three sigma bonds with the other three carbon atoms. Graphite has a C-C bond length of 141.5 pm.
• All carbon atoms share the fourth electron in the unhybrid p-orbital of each carbon atom.
• The distance between the various layers, which is 335 pm, is held by weak van der Waals forces.
• Graphite is very soft and slippery because it cleaves easily between the layers. It is therefore a very effective dry lubricant.
• The electrons in graphite are delocalised over whole sheet. Due to presence of mobile electrons graphite is a good conductor of electricity.

(iii) Fullerenes :

• Fullerene is an allotrope of carbon with definite numbers of carbon atoms.
• Graphite is heated in an electric arc under an inert atmosphere of argon or helium to produce fullerenes. Fullerenes C₃₂, C₅₀, C₇₀, and C₈₄ are only present in trace amounts in the soot that is produced.
• C₆₀, also known as Buckminster fullerene or bucky ball, has a shape similar to a soccer ball. It has fused carbon atom rings that are twenty hexagonal and twelve pentagonal in shape. The distance between the nearby carbon atoms is 143.5 and 138.3 pm.
• Fullerenes are covalent and soluble in organic solvents.
• Fullerene C₆₀ reacts with group 1 metals forming solid such as K₃C₆₀ which behaves as a super conductor below 18 K. It carries electric current with zero resistance.

(iv) Carbon nanotubes :

• Carbon nanotubes are cylindrical in shape consisting of rolled up graphite sheets.
• Nanotubes can be single walled (SWNTs) with a diameter less than 1 nm or multi walled (MWNTs) with diameter upto 100 nm. Their length range from several micrometers to milimeters.
• Carbon nanotubes are robust. They can be bent, and when released spring back to the original shape.
• They have high electrical or heat conductivity and highest strength to wait ratio (for any known material).
• Researchers at NASA are combining carbon nanotubes with other materials which can be used to build light weight space craft.

(v) Graphene :

• Isolated layer of graphite is called graphene (Fig.).
• Graphene sheet is a two dimensional solid.
• Graphene has unique electronic properties.

Allotropes of phosphorus :

Phosphorus is found in different allotropic forms, the important ones being white and red phosphorus.

(i) White (yellow) phosphorus :

Structure and properties :

• White (yellow) phosphorus consists of discrete tetrahedral P4 molecules. The P-—P—P bond angle is 60°.
• It is less stable and more reactive because of angular strain in the P₄ molecule.
• It is translucent white waxy solid which glows in dark (Chemiluminescence).
• It is insoluble in water but dissolves in boiling NaOH solution.
• It is poisonous.

(ii) Red phosphorus :

• Red phosphorus consists of chains of P₄ tetrahedra linked together by covalent bonds. Thus it is polymeric in nature.

• It is stable and less reactive.

• It is odourless. It possesses iron grey lustre
• It does not glow in dark.
• It is insoluble in water.
• It is non poisonous.

Remember : Red phosphorus is prepared by heating white phosphorus at 573 K in an inert atmosphere.

Molecular structures of some important compounds of the group 13, 14 and 15 elements :

(i) Boron trichloride (BCl₃) :

• Bond angle Cl —B — Cl is 120°.
• Boron trichloride is covalent.
• Boron atom undergoes spz-hybridisation with one vacant unhybridised p-orbital.
• Boron in BCl₃ has incomplete octet.
• BCl₃ molecule is non polar trigonal plannar molecule.

(ii) Aluminium Chloride (AlCl₃) :

• Aluminium atom in AlCl₃ is sp²-hybridised with one vacant unhybrid p-orbital.
• AlCl₃ exists as a dimer, AlCl₆.
• Al₂Cl₆ is formed by the overlap of vacant 3d orbital of Al with lone pairs of electron of Cl.
• AlCl₃ is an incomplete octet molecule.

(iii) Orthoboric acid / boric acid (H₃BO₃) :

• Orthoboric acid has central boron atom bound to three — OH groups.
• The solid orthoboric acid has layered crystal structure.
• The geometry of the molecule is trigonal planar with B(OH)₃ units are joined together by hydrogen bonds.

(iv) Diborane (B₂H₆) :

• Boron has electronic configuration, ₅B : 1s² 2s² 2p¹
• Boron has only three valence electron.
• In diborane each boron atom is sp³
• Three of such hybrid orbitals are half filled, the fourth sp³ hybrid orbital is vacant.
• Two of the sp³-orbitals of each B atom overlap axially with 1s—orbital of two terminal hydrogen atoms forming four terminal B-H, normal covalent sigma bonds. They are called two-centre, two-electron bonds or (2c — 2e) bonds.
• Two boron atoms are linked through two B—H_b—B bridge bonds, each of which is formed by overlapping of sp³ orbitals of two boron atoms and s-orbital of hydrogen atom. The bridge bonds are weak bonds.
• These bonds are called three centre two electron (3c-2e) bonds, since they involve three nuclei and two electrons. Due to their resemblance to banana, they are called banana bonds.
• In diborane molecule, four H_t and two B atoms lie in the same plane while one H_b lies above and another H_b below the plane.
• B₂H₆ molecule involves 8 bonds and 12 bonding electrons namely 6 electrons from two B atoms and 6 electrons from 6H atoms. Hence B₂H₆ an electron deficient molecule.

(v) Silicon dioxide (SiO₂) :

• Silicon dioxide (SiO₂) is commonly named as silica.
• SiO₂ is a 3-dimensional network solid.
• Each silicon atom is covalently bonded with four oxygen atoms tetrahedrally and each oxygen atom is covalently bonded to another silicon atom.
• Each corner is shared with another tetrahedron.
• The entire structure is considered as a giant molecule in which eight membered rings are formed with alternate Si and O atoms.
• Quartz, cristobalite and tridymite are the different crystalline forms of silica.

Nitric acid (HNO₃) :

• Nitric acid is a strong, oxidising mineral acid.
• The central nitrogen atom is sp²
• HNO₃ exhibits resonance phenomenon.

Orthophosphoric acid/phosphoric acid (H₃PO₄) :

• Phosphorus forms number of oxyacids.
• Orthophosphoric acid is a strong non toxic mineral acid.
• It contains three ionizableacidic hydrogens.
• The central phosphorous atom is tetrahedral.

Chemistry of notable compounds of elements of groups 13, 14 and 15

(i) Borax (Na₂B₄O₇) :

• It is one of the most important boron compound.
• The crystalline borax has formula Na₂B₄O₇.10H₂O or Na₂[B₄O₅(OH)₄].8H₂

Preparation :

• Borax is prepared from the mineral colemanite by boiling it with a solution of sodium carbonate.

Ca₂B₆O₁₁ (Colemanite) + 2Na₂CO₃ \(\underrightarrow{Δ}\) Na₂B₄O₇ (Borax) + 2NaBO₂ + 2CaCO₃ ↓

• On crystallisation it forms Na₂B₄O₇.10H₂O (It is a white solid).

Uses of borax :

Borax is used :

• In the manufacture of optical and hard borosilicate glass.
• As a flux for soldering and welding.
• As a mild antiseptic in the preparation of medicinal soaps.
• In qualitative analysis for borax bead test.
• As a brightener in washing powder.

(ii) Silicones :

• Silicones represent organosilicon polymers where {R₂SiO} is a repeating unit.
• These are held together by linkage. 
• Silicones have empirical formula R₂SiO (R = CH₃ or C₆H₅ group). This is similar to that of ketone (R₂CO) and hence the name silicones.

Preparation :

• The starting materials for the manufacture of silicones are alkyl or aryl substituted silicon chlorides of general formula R_nSiCl_{(4_n)}, where R is an alkyl or aryl group.
• When silicon (Si) and methyl chloride react in the presence of copper catalyst at 573 K, various types of alkyl or methyl substituted chlorosilanes like MeSiCl₃, Me₂SiCl₂, Me₃SiCl and Me₄Si (tertramethyl silane TMS) are obtained.
• The hydrolysis of dimethyl chlorosilane (CH₃)₂SiCl₂ followed by condensation polymerisation give straight chain polymers called silicones.
• The formation of silicones can be explained by following reactions :

• The chain length of polymer can be controlled by adding (CH₃) ₃SiCl which blocks the ends of the chain. ((CH₃) ₃SiOH is used for planar silicones)

Uses of silicones

They are used as

• insulating material for electrical appliances
• water proofing of fabrics
• sealant
• high temperature lubricants
• For mixing in paints and enamels to make them resistant to high temperature, sunlight and chemicals.

(iii) Ammonia (NH₃) :

Preparation

• Ammonia is present in small quantities in air and soil where it is formed by the decomposition of nitrogeneous organic matter such as urea.

NH₂CONH₂ (urea) + 2H₂O → (NH₄)₂CO₃ ⇌ 2NH₃ + H₂O + CO₂

• Ammonia is prepared on laboratory scale, by decomposition of the ammonium salts with calcium hydroxide or caustic soda.

2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O

(NH₄)₂SO₄ + 2NaOH → 2NH₃ + Na₂SO₄ + 2H₂O

• On the large scale ammonia is prepared by direct combination of dinitrogen and dihydrogen. (Haber’s process)
• In the process, dinitrogen and dihydrogen react directly at high pressure of 200 x 10⁵ Pa (200 atmosphere) and temperature of 700 K.
• The catalyst used is iron oxide with traces of K₂O and Al₂O₃ as promoters. Under these conditions equilibrium is attained rapidly.

N₂(g) + 2H₂(g) ⇌ 2NH₃(g);  Δ_fH^ο = −46.1 kJ mol^-1

Uses :

Ammonia is used in

• Manufacture of fertilizers such as urea, diammonium phosphate, ammonium nitrate, ammonium sulphate etc.
• Manufacture of some inorganic compounds like nitric acid.
• Refrigerant (liq. ammonia).
• Laboratory reagent in qualitative and quantitative analysis (aq. solution of ammonia)

Remember : Ammonia gives brown ppt with Nessler’s reagent (alkaline solution of K₂HgI₄).

2KI + HgCl₂ → HgI₂ + 2KCl

2KI + HgI₂ → K₂HgI₄ (Nesseler’s reagent)

2K₂HgI₄ + NH₃ + 3 KOH → H₂N-HgO-HgI (Millon’s base-Brown ppt) +7KI + 2H₂O