4. BORON & ITS COMPOUNDS

8. 2. By passing electric discharge at low pressure through a mixture of BCl 3 or BBr 3 and excess of hydrogen. 3. By reacting LiH with BF 3

9. PROPERTIES:

10. e) At low temperature an addition product is obtained with NH 3 B 2 H 6 + 2NH 3 B 2 H 6.2 NH 3 When the addition product is heated up to 200 0 C, borazine is formed, which is also known as inorganic benzene. 3B 2 H 6.2 NH 3 2B 3 N 3 H 6 +12H 2

11. Boric acid (H 3 BO 3 ): 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. Na 2 B 4 O 7 + 2HCl + 5H 2 O 4H 3 BO 3 +2NaCl From hydrolysis of boron compounds: It can also be prepared by the hydrolysis of boron compounds. BCl 3 + 3H 2 O H 3 BO 3 +3HCl B 2 H 6 + 6 H 2 O H 3 BO 3 +6H 2

12. 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. Action of heat :

13. BORAX (Na2B4O7.10 H2O) From boric acid: Borax can also be obtained in small quantities from boric acid by neutralising it with Na 2 CO 3. 4H 3 BO 3 + Na 2 CO 3 Na 2 B 4 O 7 + 6 H 2 O + CO 2 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. Na 2 B 4 O 7 + 2 H 2 O H 2 B 4 O 7 + 2NaOH

14. 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. Na 2 B 4 O 7.10H 2 O heat Na 2 B 4 O 7 + 10H 2 O Na 2 B 4 O 7 heat 2NaBO 2 + B 2 O 3 Sodium Metaborate Boron oxide MnO + B 2 O 3 heat Mn(BO 2 ) 2 Manganese Metaborate (pink)

15. CoO + B 2 O 3 heat Co(BO 2 ) 2 Cobalt Metaborate ( blue) d.Action with NaOH: With NaOH it forms sodium metaborates Na 2 B 4 O 7 + 2 NaOH 4NaBO 2 + H 2 O e. Action with sulphuric acid: (Calculated amount of acid) Na 2 B 4 O 7 + H 2 SO 4 Na 2 SO 4 + H 2 B 4 O 7 H 2 B 4 O 7 + H 2 O 4H 3 BO 3 Tetra boric acid

16. 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.