Note on Boron and Silicon

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Boron

Boron is a Group 13 element which is borderline between metals and non-metals. It is better to term it as semiconductor than a conductor. Chemically, it is closer to silicon than to aluminium, gallium, indium, and thallium.

Crystalline boron is inert chemically. When finely divided, it is attacked slowly by hot concentrated nitric acid.

  • Name: Boron
  • Symbol: B
  • Atomic number: 10.811
  • Standard state: solid at 298 K
  • Colour: Black
  • Classification: semi metallic
  • Group in periodic table: 13
  • Period in periodic table: 2
  • Block in periodic table: p-block

Extraction

Boron is extracted from borax {Na2B4O7.10H2O}. When borax is boiled with concentrated HCl & followed by hydrolysis then, boric acid is obtained. On heating boric acid, it produces boric anhydride on reduction with Hg produce boron.

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

$$2H_3BO_3\longrightarrow B_2O_3+3H_2O$$

$$B_2O_3+3Mg\longrightarrow 2B+3MgO$$

Properties

  • Boron is second hardest after diamond.
  • It does not conduct electricity.
  • It has 2 isotopic forms.

Isotope

Mass/

Natural abundance (atom %)

10B

10.012 937 0(4)

19.9 (7)

11B

11.009 305 5(5)

80.1 (7)

  • It exists in two allotropic forms i.e. crystalline and amorphous.

Uses

  • It is used in steel industries for increasing the hardness of steel.
  • It is used in the semiconductor.
  • It is used for growth of the plant.

Borax (Na2B4O7.10H2O)

Dehydrated sodium tetraborate is borax. The natural deposit of borax is called tincal. Tincal is 55% borax.

Preparation

It is prepared by heating colemanite with sodium carbonate and water.

$$2Ca_2B_6O_{11}+3Na_2CO_3+H_2O\longrightarrow 2Na_2B_4O_7+3CaCO_3+Ca(OH)_2$$

Finally, crystallization is done and crystals of borax can be found.

Properties

  1. White crystalline solid
  2. Soluble in water
  3. Exist in 3 forms
  • Ordinary or Monoclinic borax (Na2B4O7•10H2O)
  • Octahedral or jewelry borax (Na2B4O7•5H2O)
  • Borax glass (Na2B4O7)

On heating, it loses water of crystallization forming anhydrous and then sodium metaborate and finally changes to boric oxide.

$$Na_2B_4O_710H_2O\longrightarrow Na_2B_4O_7\longrightarrow 2NaBO_2+B_2O_3$$

Uses

  • Used in making a candle.
  • Used as the preservative.
  • Used in the qualitative test of borax.
  • Used as antiseptic.

Boric Acid {H3BO3}

It is also known as orthoboric acid. It can be prepared as:

  1. $$HBO_2+H_2O\longrightarrow H_3BO_3$$
  2. $$H_2B_4O_7+5H_2O\longrightarrow 4H_3BO_3$$

Properties

  • White crystalline in nature.
  • Soapy to touch.
  • Poor acidic in nature.
  • On heating boric acid, boric acid dehydrates and finally gives boric anhydride.

Uses

  • Used as preservatives in industries.
  • Used as antiseptic.
  • Used for manufacture of bora.

Diborane (B2H6)

  • Due to electron deficient nature of boron, it is found in dimer form.

Structure

diborane structure
source:www.meritnation.com Fig:diborane structure

Uses

  • Diborane is a colourless gas.
  • The boiling point is -93°C.
  • The melting point is -165°C.
  • Vapour Density is 0.96 (air = 1.0). Diborane gas is lighter than air.
  • Flash Point of diborane is to be -90°C.
  • Auto-ignition temperature is 38°C to 52°C.
  • Exposure limit is 0.1 p.p.m.
  • The odour of diborane is not possible to be detected below the permissible exposure limit.

Silicon

It was discovered on 1824 by Jons Jacob Berzelius. The name is derived from the Latin 'silex' or 'silicis'. The earth is constructed of silicon, and silicon is very useful in daily life as we use it in the form of glass and pottery. The pure silicon is a solid and a blue-grey metal. Silica (SiO2) in the form of sharp flints were used by ancestors as the weapon.

Group

14

Melting point

1414°C, 2577°F, 1687 K

Period

3

Boiling point

3265°C, 5909°F, 3538 K

Block

p

Density (g cm−3)

2.3296

Atomic number

14

The isotopic form of silicon are

Isotope

Atomic mass

Natural abundance (%)

28Si

27.977

92.223

29Si

28.976

4.685

30Si

29.974

3.092

Uses

  • Used in metallurgy.
  • Used in building material.
  • Used in manufacture of porcelain basin cement etc.
  • Silica gel (SiO2•H2O) is used in chromatography.
  • Used to make alloys including aluminium-silicon and ferrosilicon.
  • Used as a semiconductor.
  • Granite is a complex silicate.

Silicon dioxide (SiO2)

Structure

silicon dioxide
source:ibchem.com
fig:silicon dioxide

Preparation of Silicon Dioxide

Silica is prepared by the hydrolysis of silicon tetrachloride, heating the hydrated oxide silicon dioxide is formed.

$$SiCl_4+4H_2O↔SiO_2⋅2H_2O+4HCl$$

Properties of Silicon Dioxide

Silica is a transparent solid and has high melting point.

Silica is inert to most chemical reagents but reacts with hydrogen fluoride and this help in the formation of silicon tetrafluoride, which is a colourless gas at room temperature and water.

Silica is an acidic oxide and is capable to reacts with alkalis to form salt and water.

$$SiO_2+2NaOH↔Na_2SiO_3+H_2O$$

Properties of Silicon Dioxide

Silicon dioxide is a crystalline compound. It is used to transmits visible and ultraviolet light. Many crystalline forms exist, the most common is Quartz.

Uses of Silicon Dioxide

Pure silicon dioxide is an important material in the electronics industry.

When silica reacts with solid sodium carbonate and is melted, sodium silicate is formed.

$$SiO_2+Na_2CO_3↔Na_2SiO_3+CO_2$$

Reference

pulse, Tracy. Introduction to chemistry. u.s.a: flex book, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.

  • On heating boric acid, boric acid dehydrates and finally gives boric anhydride.
  • Borax is used as antiseptic.
  • Boric acid is also known as orthoboric acid due to electron deficient nature of boron. Diborane is found in dimer form.
  • Silica is prepared by the hydrolysis of silicon tetrachloride, heating the hydrated oxide silicon dioxide is formed.

 

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