Introduction to Boron, Silicon and periodic Position

Boron

Introduction and Periodic Position

Boron is placed in group III-A of the periodic table. The valence shell configuration of group III-A element is ns²np¹. Therefore, these elements are p-block elements. The members of group III-A are ₅B (boron), ₁₃Al (aluminum), ₃₁Ga (gallium), ₄₉In (indium) and ₈₁Ti (thallium). As we move down in the group, the metallic character goes on increasing and m.p. goes on decreasing. The density goes on increasing. Boron has the highest value of ionization energy (800 KJ mol^-1). B and Al exhibit +3 O.S. while other members show +1 and +3 O.S. Oxides and hydroxides of B are acidic, those of Al and Ga are amphoteric, while those of In and Tl are basic. Due to high I.P. values, B and Al tend to form covalent bonds. Boron trihalides and aluminium chloride, having an incomplete octet, acts as Lewis acids. Boron, unlike other members, forms a large number of covalent hydrides like B₂H₆, B₄H₁₀ etc.

Occurrence of Boron

Boron does not occur in the free state. Important ores of boron are borax or tincal(Na₂B₄O₇.10H₂O) and colemanite (Ca₂B₆O₁₁5H₂O). Boron also occurs in the minerals kernite or razorite(Na₂B₄O₇.4H₂O), boracite (2Mg₃B₈O₁₅MgCl₂), boro calcite (CaB₄O₇.4H₂O) and in some plants and coal ash.

Extraction of Boron

1. Isolation from boric anhydride:The finely powdered borax is boiled with conc.HCl when sparingly soluble boric acid separates out.

$$Na_2B_4O_7+2HCl+5H_2O\xrightarrow\Delta{2NaCl+4H_3BO_3}$$

The boric acid on strong heating decomposes to form a boric anhydride.

$$2H_3BO_3\xrightarrow\Delta{B_2O_3+3H_2O}$$

Alternatively, if finely powdered colemanite is dissolved in the boiling water and SO2 gas is passed through the solution, less soluble boric acid separates out on cooling. Boric acid on strong heating gives basic anhydride.

$$Ca_2B_6O_{11}+11H_2O+4SO_2\xrightarrow\Delta{2Ca(HSO_3)_2+6H_3BO_3}$$

The boric anhydride thus obtained is mixed with sodium, potassium or magnesium powder and heated in a covered crucible so that it's reduced to boron.

$$B_2O_3+3Mg\xrightarrow\Delta{2B+3MgO}$$

The heated mass is stirred with an iron rod so as to oxidize the excess of Na, K or Mg. The mass is now boiled with water, then with hydrochloric acid and finally with hydrofluoric acid, when the dark brown powder of amorphous boron only remains as residue and the rest everything is dissolved. This residue is now washed with water and dried.

To obtain the crystalline boron, the boric anhydride is reduced with aluminium powder, i.e., the aluminothermic process is applied for the reduction.

$$B_2O_3+Al\xrightarrow\Delta{2B+Al_2O_3}$$

The crystals are treated with NaOH solution to remove the impurity of Aluminium.

$$2Al+5NaOH\longrightarrow{2Na_3AlO_3+3H_2}$$

$$2Al+2NaOH+2H_2O\longrightarrow{2NaAlO_2+3H_2}$$

2. Electrolytic Method: Nowadays boron is obtained by the electrolysis of a fused mixture of boric anhydride, magnesium oxide, and magnesium fluoride. The electrolysis is called out in a carbon crucible which itself acts as anode and cathode are of iron which remains at the centre of the crucible. The magnesium obtained by electrolysis of its oxide reduces boric anhydride to boron which is deposited at the cathode. Magnesium fluoride lowers the melting temperature.

$$2MgO\xrightarrow\Delta{2Mg+O_2}$$

$$B_2O_3+3Mg\xrightarrow\Delta{2B+3MgO}$$

Purification

Boron obtained from processes still, contains a number of impurities. It is purified by heating it electrically in vacuum at 1100°C at this temperature; the impurities are volatilized off leaving behind pure boron.

Properties

1. Boron exists in two allotropic forms.
   a) Amorphous form having a density 2.45. It melts at 2300°C but vaporises before reaching this temperature.
   b) Crystalline form having the metallic lustre.
2. It has low electrical conductivity.
3. Crystalline boron is unreactive while amorphous boron is reactive. Pure boron does not react with water.
4. Boron burns in air at 700°C to give boron trioxide and boron nitride.
   $$4B+3O_2\longrightarrow{2B_2O_3}$$
   $$2B+N_2\longrightarrow{2BN}$$
5. There is no action of hydrochloric acid on boron, however, it is oxidized with nitric acid and sulphuric acid to give boric acid.
   $$B+3HNO_3\longrightarrow{H_3BO_3+3NO_2}$$
   $$2B+3H_2SO_4\longrightarrow{2H_3BO_3+3SO_2}$$
   No reaction with crystalline boron.
6. When heated with nitric oxide-boron gives boron nitride.
   $$5B+3NO\longrightarrow{B_2O_3+3BN}$$
7. It reacts with alkalies forming borates and liberating hydrogen.
   $$2B+6NaOH\longrightarrow{2Na_3BO_3+3H_2↑}$$
   $$2B+6KOH\longrightarrow{2K_3BO_3+3H_2↑}$$
8. When heated with Mg metal, boron forms boride.
   $$3Mg+2B\xrightarrow\Delta{Mg_3B_2}$$
9. At high temperature, it combines with carbon to give boron carbide, which is the next hardest substance then diamond.
   $$4B+C\xrightarrow\Delta{B_4C}$$
10. At high temperature, it combines with halogens giving trihalides.
    $$2B+3Cl_2\xrightarrow\Delta{2BCl_3}$$
    $$2B+3Br_2\xrightarrow\Delta{2BBr_3}$$
    Boron halides are electron deficient compounds and behave as Lewis acids.
11. It is a powerful reducing agent and it reduces SiO₂ and CO₂.
    $$3SiO_2+4B\longrightarrow{3Si+2B_2O_3}$$
    $$3CO_2+4B\longrightarrow{3C+2B_2O_3}$$

Applications

1. As de-oxidiser for metals.
2. In the manufacture of the special type of steel known as boron steel and used as control rods in atomic reactors.
3. It is used as a semiconductor for making electrical devices.
4. Boron filaments are used in making light and composite material for aircraft.
5. It is used as a moderator in nuclear reactors and as a refractory material.
6. Boron is used in medicine.

Compounds of Boron

1. Boranes

Boron forms a number of hydrides (such as BH₃, B₂H₆, B₄H₁₀, B₅H₉ etc). These are called boranes. BH₃ is unstable, B₂H₆ has a stable bridged structure which is known as diborane. Boranes possessing the general formula B_nH_n+4 and B_nH_n+6 are well known. Among these the most important borane is diborane. Boranes are volatile and covalent in nature. The stability of diborane is ascribed to its structure.

The structure of diborane has been determined by electron diffraction. The two boron atoms are joined by two bridging hydrogen atoms, with four terminal hydrogen atoms in a plane at right angles to that containing the bridging groups.

The two boron atoms and four hydrogen atoms lie in the same plane, the two bridging hydrogen atoms lie symmetrically one above and the other below in a plane perpendicular to this plane. B₂H₆ has a three center-electron pair bond. It has two types of B-H bond.

(i) B-H_f. It is the normal covalent bond (two center electron pair bond).
(ii) B-H_b. The bond between three atoms, B-H_b-B (three centers two electron pair bond).

Applications

1. Diborane is used as rocket fuel for supersonic aeroplanes.
2. For the preparation of higher boranes, metal boron hydrides such as LiBH₄, NaBH₄ etc.
3. As a catalyst in polymerisation reaction.
4. As a reducing agent.

2. Boric Acid

A number of boric acids as are known which are obtained from boron trioxide with different amounts of water. For example,

a) Ortho boric acid, B₂O₃.3H₂O

b) Meta boric acid, B₂O₃.H₂O

c) Tetra boric acid, 2B₂O₃.H₂O

d) Pyroboric acid, 2B₂O₃.3H₂O

Among these, ortho boric acid is one of the most important acids, which is simply known as boric acid.

Ortho Boric Acid, B₂O₃.3H₂O or H₃BO₃

Preparation

1. From Borax

Boric acid is obtained by the reaction of conc. sulphuric acid or hydrochloric acid on hot and conc. a solution of borax. On cooling, the reaction mixture, crystals of boric acid are obtained.
$$Na_2B_4O_7+H_2SO_4+5H_2O\longrightarrow{4H_3BO_3+Na_2SO_4}$$
$$Na_2B_4O_7+2HCl+5H_2O\longrightarrow{4H_3BO_3+2NaCl}$$

2. From colemanite

Colemanite is first crushed to a fine powder. It is suspended in boiling water and sulphur dioxide is bubbled through the mixture.

$$Ca_2B_6O_11+2SO_2+9H_2O\longrightarrow{2CaSO_3+6H_3BO_3}$$

The liquid is filtered and evaporated by the heat and the sun. Crystals of pure boric acid are formed. This is the commercial method for obtaining boric acid.

Physical Properties

Boric acid is a white crystalline solid and soapy in touch. Slightly soluble in cold water, fairly soluble in hot water and steam volatile. It is a weak acid.

Chemical Properties

1. Action of heat

On heating at 100°C meta boric acid is formed, at 160°C tetra boric acid and at red hot it gives B₂O₃.

$$4H_3BO_3\xrightarrow{100°C\;-H_2O}{4HBO_2}\xrightarrow{160°C\;-H_2O}{H_2B_4O_7}\xrightarrow{Red\;hot\;-H_2O}{2B_2O_3}$$

2. Acidic character

In aqueous solution, it behaves as a weak acid. Its solution turns litmus to a wine red but has no effect on methyl orange. It ionises as a mono basic acid. It acts as Lewis acid by accepting OH^- ion from H₂O.
$$B(OH)_3+H_2O\longrightarrow{[B(OH)_4]^- + H^+}$$

3. Action with alkalies

With strong alkalies, boric acid forms salt known as meta borates.
$$B(OH)_3+NaOH\longrightarrow{NaBO_2+2H_2O}$$

Applications

1. In the form of aq. the solution it is used as an antiseptic and in eye wash.
2. For making enamels and pottery glazes.
3. It is used in the glass industry.
4. In the manufacture of borax.
5. For preserving food.

Structure

Structure of BO₃^3- ion

Boric acid is covalently bonded compound and contains planar triangular BO₃^3- units, which are bonded together through hydrogen bonds into two-dimensional sheets.

Each boron atom of each BO₃^3-unit is bonded to three oxygen atoms and each oxygen atom in this unit is bonded to a hydrogen atom through hydrogen bonds. The H-atom acts as a bridge between the two O-atoms. Thus boric acid consists of a number of layers, which are linked together by weak van-der-Waals forces.

Borax, Na₂B₄O₇.10H₂O

Sodium Tetraborate:Naturally, borax occurs ass tincal in certain dried-up lakes of India, Tibet, Ceylon, California. Impure borax is known as tincal. Native tincal contains about 54-55% of borax. Its actual formula can be written as Na₂[B₄O₅(OH)₄].8H₂O.

Preparation of borax

1. From boric acid

When boric acid is boiled with sodium carbonate solution, borax is obtained.

$$4H_3BO_3+Na_2CO_3\xrightarrow\Delta{Na_2B_4O_7+6H_2O+CO_2}$$

The liquid obtained is crystallized to give the crystals of borax.

2. From colemanite

Large quantities of borax are manufactured from colemanite (Ca₂B₆O₁₁.5H₂O) by boiling with conc. a solution of soda ash.

$$Ca_2B_6O_{11}+2Na_2CO_3\xrightarrow\Delta{2CaCO_3↓+Na_2B_4O_7+2NaBO_2}$$

The solution is filtered when CaCO₃ is filtered off and the filtrate containing borax and meta borate is concentrated when crystals of borax separate. The mother liquor contains sodium meta borate in which current of CO₂ is passed to convert sodium meta borate into borax.

$$4NaBO_2+CO_2\longrightarrow{Na_2CO_3+Na_2B_4O_7}$$

Properties

1. Borax exists in three forms:

(a) Prismatic borax

It is decahydrate (Na₂B₄O₇.10H₂O), it is obtained by crystallization of salt solution at room temperature.

(b) Octahedral borax (Na₂B₄O₇.5H₂O)

It is obtained by crystallization of salt solution above 60°C .

(c) Borax glass

It is the anhydrous form of borax and is obtained by heating the borax above its melting point until all the molecules of water of crystallization are removed.

It is a colourless substance having density 2.37. It absorbs moisture and converts into decahydrate form. It is less soluble in cold water but dissolves readily in hot water.

2. Its solution in water is alkaline due to the formation of NaOH.

$$Na_2B_4O_7+3H_2O\longrightarrow{2NaBO_2+2H_3BO_3}$$

$$NaBO_2+2H_2O\longrightarrow{NaOH+H_3BO_3]2}$$

$$Na_2B_4O_7+7H_2O\longrightarrow{2NaOH+4H_3BO_3}$$

Sodium hydroxide is a strong base but boric acid is a weak acid, hence the solution becomes alkaline.

3. Borax reacts with strong acids (conc. Hcl or conc. H₂SO₄) to form boric acid.

$$Na_2B_4O_7+2HCl+5H_2O\longrightarrow{2NaCl+4H_3BO_3}$$

4. When borax is heated, it loses water of crystallization and form borax glass (opaque mass). Borax glass combines with metallic oxides forming different coloured meta borates (borax bead test) e.g. cobalt meta borate is of intense blue colour formed by the action of CoO with B₂O₃.

$$Na_2B_4O_7.10H_2O\xrightarrow\Delta{Na_2B_4O_7}\xrightarrow\Delta{2NaBO_2+B_2O_3}B_2O_3+CoO\xrightarrow\Delta{Ca(BO_2)_2}$$

5. Borax reacts with barium chloride solution to give a white precipitate of barium meta borate.

$$Na_2B_4O_7+BaCl_2+3H_2O\longrightarrow{Ba(BO_2)_2+2H_3BO_3+2NaCl}$$

6. Borax on heating with NH₄Cl gives boron nitride.

$$Na_2B_4O_7+2NH_4Cl\longrightarrow{2NaCl+B_2O_3+2BN+4H_2O}$$

Applications

1. In borax bead test for the detection of basic radicals.
2. As a flux in soldering and welding.
3. In the preparation of borosilicate glass (Pyrex-glass) which is heat resistant.
4. As a preservative of food.
5. In the leather industry for cleaning hides and skins.
6. In making enamels and glazes for pottery.

Structure

In borax, two boron atoms are triangular and two boron atoms are in tetrahedral in geometry. In this ion eight water molecules are associated with two sodium ions and the formula of borax is represented as Na₂[B₄O₅(OH)₄].8H₂O.

Silicon

Introduction

Silicon (Si) is the member of group IVA in the periodic table with atomic number 14. Silicon exists in two allotropic forms: amorphous form and crystalline adamantine form.

Occurrence

Silicon is the element which is next to oxygen in abundance in the earth's crust. It's an important constituent of igneous rocks which consist of silicates of magnesium, aluminium, potassium or iron. Silicon is widely present as silica (SiO₂) in the form of sand, quartz, and flint.

Preparation of Silicon

1. An amorphous variety of silicon is prepared by the reduction of quartz or sand with highly pure coke in an electric arc furnace. More volatile silicon is separated from the graphite formed due to decomposition of silicon carbide.

$$SiO_2+C\xrightarrow\Delta{Si+2CO}$$

The excess of silica prevents the formation of silicon carbide (or carborundum, SiC).

$$2SiC+SiO_2\xrightarrow\Delta{3Si+2CO}$$

2. Amorphous silicon can be extracted when well-powdered quartz is mixed with magnesium powder and heated in a fire clay crucible along with the certain quantity of calcined magnesia (MgO).

$$SiO_2+2Mg\xrightarrow\Delta{Si+2MgO}$$

The unreacted SiO₂ reacts with calcined MgO to give a fusible mass slags the upper molten layer and hence contamination of silicon by SiO₂ is prevented. The magnesium oxide is dissolved in dilute HCl; which then leaves silicon. Pure silicon can be extracted by this method. Silicon extracted from above methods is refined by zone refining method.

3. When amorphous silicon is strongly heated in a strong crucible, it fuses and on cooling solidifies to the dense crystalline silicon.

4. If potassium silicofluoride is heated in an iron crucible with aluminium to a red heat (1000°C), a pale yellow crystalline of silicon is obtained.

$$3K_2SiF_6+4Al\xrightarrow\Delta{6KF+2AlF_3+3Si}$$

Silicon remains dissolved in the molten aluminum present in excess. The solid solution is then treated with HF to remove fluorides of Al and K. Silicon is then refined by zone refining method. Silicon thus obtained is graphatoidal with six-sided plates and is in the ultra pure state which is used as a semiconductor.

Properties of Silicon

Amorphous silicon (sp. gr. 2.25) is a brownish powder. Silicon possesses more non-metallic character and very few metallic characters. So, silicon is regenerated as a typical non-metal crystalline silicon crystals (sp. gr. 2.49). It's very hard and scratches glass. The melting point and boiling point of silicon are respectively 1410°C and 3280°C.

Chemical properties

1.Action with air:Amorphous silicon burns brilliantly in oxygen but superficially in air to form silica.

$$Si+O_2\xrightarrow\Delta{SiO_2}$$

The crystalline variety doesn't burn in air or oxygen even on strong heating.

2. Action with halogens:Silicon combines with halogens to give tetrahalides. Amorphous silicon ignites spontaneously and crystalline silicon combines in cold spontaneously with fluorine to give silicon tetrafluoride.

$$Si+2F_2\longrightarrow{SiF_4}$$

Both the varieties burn in chlorine when heated to give the tetrachloride.

$$Si+2Cl_2\xrightarrow\Delta{SiCl_4}$$

The reactions with bromine and iodine need more heat. Silicon tetrahalides are hydrolyzed by water to give the silicic acid.

$$SiF_4+4H_2O\longrightarrow{Si(OH)_4\;(i.e\;H_2SiO_3\;H_2O)+4HF}$$

3. Action with water:Silicon is not attacked by cold water. The amorphous variety is attacked by steam at a red heat and the crystalline variety reacts with boiling water or steam to liberate hydrogen gas.

$$Si+2H_2O\xrightarrow\Delta{SiO_2+2H_2↑}$$

4. Action with alkalis:Both the varieties of silicon dissolve in hot and conc. caustic alkalis to liberate hydrogen gas and corresponding silicates are formed.

$$Si+2KOH+H_2O\xrightarrow\Delta{K_2SiO_3+2H_2↑}$$

$$Si+2NaOH+H_2O\xrightarrow\Delta{Na_2SiO_3+2H_2↑}$$

Silicon undergoes fusion with sodium carbonate to give sodium silicate and black carbon particles.

$$Si+Na_2CO_3\xrightarrow\Delta{Na_2SiO_3+C}$$

Uses of Silicon and Silicates

Silicon is used:

1. As a semiconductor material in transistors, solar cells etc. For this purpose, ultrapure Si is used.
2. As silica gel (SiO₂ H₂O) which functions as an absorbent, and a desiccant (to absorb moisture).
3. As a deoxidizer in copper alloys.
4. To manufacture silicates which have various applications and organo-silicon polymer, called silicones.
5. As sand in building constructions. Considerable amounts of silica are applied in metallurgy as flux and larger amounts of silica are applied for making mortar, concrete, and glass.

Uses of Silicates:

1. Artificial silicates, glass, and cement have household uses. Glass is applied for making laboratory apparatus, windows, lenses etc. Cement is used in constructions.
2. Sodium silicate or water glass (Na2SiO3 with the excess of silica) is used in fire proofing of wood, preservation of eggs, the weighting of silk in soap industry as a filler for a cheaper variety of soap in plant industry and as a detergent.
3. Asbestos is used to make fire proof materials and in wire gauzes used in the laboratory.
4. Clays are used in ceramic industry to manufacture bricks, tiles, earthenware and porcelain china ware.
5. Similarly various aluminium silicates mica (infurnace windows, as an insulator), lapis lazuli (blue color, topaz (as the gem), ultramarines (as paints), feldspar (KAlSi₃O₈, as an insulator) have broad uses.

Silicones

Silicones are the organ-silicon polymers. The structure of silicone is:

Where, R is an alkyl or aryl group.

The complex cross-linked silicone has the following type of structure:

Silicones have high thermal stability, resistance to oxidation and chemical reagents, lubricating and water repellant characters and may exist in the form of oily or viscous liquids, resins or rubber-like three-dimensional solids.

Silicones are used:

1. For making high-temperature oil baths, high vacuum pumps.
2. For making low-temperature lubrications.
3. For making water-proof cloth and paper.
4. As the insulating material in electric appliances since they can withstand high temperature without charring.
5. As a silicone rubber in the number of the way since after vulcanization, it means its shape and elasticity permanently.

Similarities of Boron with Silicon

Boron, being diagonally opposite to silicon in the next group in the periodic table, is more similar to silicon than to the members of its own group.

1. Both B and Si are typical non-metals and have high m.p. and low density.
2. Both exhibits allotropy.
3. Both are bad conductors of electricity.
4. Both form stable covalent hydrides e.g. B₂H₆ (diborane), SiH₄ (silane).
5. Both dissolve in alkalis to form borates and silicates respectively liberating H₂ gas.
   $$2B+6NaOH\xrightarrow\Delta{2Na_3BO_3+3H_2↑}$$
   $$Si+2NaOH+H_2O\xrightarrow\Delta{Na_2SiO_3+2H_2↑}$$
6. Fluorides of both are colorless fuming gases and chlorides of both are colorless fuming liquids.
7. Carbides of both (B₄C & SiC) are very hard and are used as abrasives.

Bibliography

Gautam, Shree Dhar and Manju Pant. Comprehensive Chemistry. sixth edition. Vol. 1. Kathmandu: Heritage Publishers and Distributors Pvt. Ltd., 2072.