BORON AND ITS COMPOUNDS

STRUCTURE OF BORON

STRUCTURE OF BORON

BORON & ITS COMPOUNDSBORON: Atomic Number: 5

Atomic Symbol: B Group: 13th

Period: 2

Electronic Configuration: 1s22s22p1 (Ground state)

1s22s12p2 (Excited state)

Other elements of the group: B, Al, Ga, In, Tl

Dissimilarities of Boron with other elements of this group:

•Boron is a non-metal, while all other elements of this group are metals.

•Boron forms only covalent compounds, while other elements of this group form both covalent and ionic compounds.

•Boron shows a maximum covalency of four, while other elements of this group show a maximum valency of six due to absence of vacant d- orbitals.

CHEMICAL PROPERTIES OF BORON:

Boron

4B + 3O2 2B2O3(Boric anhydride)

2B + 3N2 2BN(Boron nitride)

2B + 3X2 2BX3(Boron trihalide)

2B+ 6NaOH 2Na3BO3 ( Sodium Borate) + 3H2

2B + 3 HNO3 H3BO3(boric acid) + 3NO2

2B + 3 H2SO4 2H3BO3(boric acid) + 3SO2

SOME IMPORTANT COMPOUNDS OF BORON

BORON HYDRIDES (BORANES):

•At room temperature boranes are gaseous substances.•The general formula of the two important series of boranes are BnHn+4 and BnHn+6

•In view of boron having three valence electrons we would expect it to form a simple hydride BH3. However the simplest stable boron hydride is diborane (B2H6).

PREPARATION:1. By the action of LiAlH4 on BCl3

4 BCl3 + 3 LiAlH4 Ether 2B2H6 + 3LiCl +3AlCl3

2. By passing electric discharge at low pressure through a mixture of BCl3 or BBr3 and excess of hydrogen.

2 BCl3 + 6H2 silent electric discharge B2H6 + 6 HCl

3. By reacting LiH with BF3

8BF3 + 6 LiH B2H6 + 6 LiBF4

PROPERTIES:a) It is a colourless gas which is stable at low temperature. At higher temperature it decomposes to give Boron and Hydrogen

B2H6 2B + 3H2

b) It burns in oxygen to give B2O3

B2H6 + 3O2 B2O3 + 3H2O

c) It readily reacts with water liberating H2 gas B2H6 + 6H2O 2H3BO3 + 6H2

d) It reacts with Cl2 forming boron trichloride

B2H6 + 3Cl2 2 BCl3 + 3HCl

e) At low temperature an addition product is obtained with NH3

B2H6 + 2NH3 B2H6.2 NH3

When the addition product is heated up to 2000C, borazine is formed, which is also known as inorganic benzene. 3B2H6.2 NH3 2B3N3H6 +12H2

USES OF DIBORANE:•a rocket fuel for supersonic aeroplanes•a starting material for preparing higher boranes, metal boron hydrides such as LiBH4, NaBH4 etc.•a catalyst in polymerisation reactions•a reducing agent in organic reactions.

Boric acid (H3BO3):

PREPARATION:

•From Boric Acid: Boric acid is obtained by treating a hot conc. solution of borax with HCl or sulphuric acid. The resulting solution on concentration and cooling gives crystals of boric acid.

Na2B4O7 + 2HCl + 5H2O 4H3BO3 +2NaCl

•From hydrolysis of boron compounds: It can also be prepared by the hydrolysis of boron compounds.

BCl3 + 3H2O H3BO 3

+3HCl

B2H6 + 6 H2O H3BO 3

+6H2

PROPERTIES:•Sparingly soluble in cold water but highly soluble in hot water.•It acts as a weak acid.•White crystalline solid with a soft soapy touch.

H3BO3 370 K HBO2 + H2O

410 K

HBO2 H2B4O7 ( Tetra boric acid)

H2B4O7 Red heat 2B2O3 + H2O

•Action of heat:

BORAX (Na2B4O7.10 H2O)

From boric acid: Borax can also be obtained in small quantities from boric acid by neutralising it with Na2CO3.

4H3BO3 + Na2CO3 Na2B4O7 + 6 H2O + CO2

Physical & Chemical Properties:a)White crystalline solid.

b)Its aqueous solution is basic in nature. Borax is therefore used as a water softener and cleansing agent.

Na2B4O7 + 2 H2O H2B4O7 +

2NaOH

c. Action of heat: On heating it loses its water of crystallisation. On further heating, it gives a transparent glassy bead of boron oxide and sodium Metaborate.

Na2B4O7.10H2Oheat Na2B4O7 +

10H2O

Na2B4O7heat 2NaBO2 + B2O3

Sodium Metaborate Boron oxideThis transparent glassy bead is commonly known as Borax- bead and is used in the qualitative analysis of certain coloured ions such as Nickel, cobalt, chromium, cupric ions etc.

This test is performed on a platinum wire with the salts containing these cations. On heating the salts are converted into their oxides which form coloured metaborates with B2O3.

MnO + B2O3 heat Mn(BO2)2

Manganese Metaborate (pink)

CoO + B2O3 heat Co(BO2)2

Cobalt Metaborate ( blue)

d.Action with NaOH: With NaOH it forms sodium metaborates

Na2B4O7 + 2 NaOH 4NaBO2 + H2O

e. Action with sulphuric acid: (Calculated amount of acid)

Na2B4O7 + H2SO4 Na2SO4 +

H2B4O7

H2B4O7 + H2O 4H3BO3

Tetra boric acid

SOME IMPORTANT QUESTIONS:

1. Why boron halides are Lewis acids?2. Why boric acid is considered as non protic acid?3. What name is given to the bond present in

diborane?4. Why boron cannot extend its covalency beyond

four?5. What will be nature of aqueous solution of borax

and why?6. What happens when borax is heated? Write the

chemical equation.