How is boron obtained from borax ? Give the chemical reactions involved. Draw the structure of `B_(2)H_(6)` and give its reaction with `HCl`.

Explanation

Finely ground borax is heated with concentrated hydrochloric acid or concentrated sulphuric acid when sparingly soluble orthoboric acid separates out.
`Na_(2)B_(4)O_(7) + 2HCl rarr 2NaCl + H_(2)B_(4)O_(7)`
`Na_(2)B_(4)O_(7) + H_(2)SO_(4) rarr Na_(2)SO_(4) + H_(2)B_(4)O_(7)`
`H_(2)B_($)O_(7) + 5H_(2)O rarr 4H_(3)BO_(3)`
Orthoboric acid is strongly heated to get `B_(2)O_(3)`.
`2H_(3)BO_(3) rarr B_(2)O_(3) + 3H_(2)O`
The reduction of boric anhydride `(B_(2)0_(3))` can be done with sodium, potassium, or magnesium.
`B_(2)O_(3) + 6K rarr 2B + 3K_(2)O`
`B_(2)O_(3) + 3Mg rarr 2B + 3MgO`
Boron is obtained these days by the electrolysis of a fused mixture containing boric anhydride, magnesium oxide, and magnesium fluoride. The electrolysis is done in carbon crucible, which acts as the anode and iron rod is used as the cathode. The magnesium discharge at the cathode reduces `B_(2)O_(3)` to boron. The structure of diborane is given below :

There are two types of hydrogen atoms. Four hydrogen atoms are terminal hydrogens. The other two hydrogen atom are known as bridge hydrogens. The two boron atoms and the four terminal `H` atoms lie in the same plane, while the two bridging `H` atoms are in the plane perpendicular to the rest of the molecule. The terminal hydrogens are bonded by normal covalent bonds. The bridged hydrogens are bonded by three-centre electrons pair bonds. Which involve two electrons but bind three atoms. There are two such types of bonds in diborane. The boron atom undergoes `sp^(3)` - hybridisation. Two hybrid orbitals of each `B` atom overlaps with `1s` orbitals of two `H` atoms.
The hybrid orbital containing an unpaired electron of one `N` atom and the vacant hybrid orbital of the second `B` atom overlap simultaneously with `1 s` orbital of an `H` atom to from `B-H-B` bridge bond , this bond is called three-centre electron pair bond. It is also called banana bond.
`B_(2)H_(6) + HCl rarr underset ("Chlorine") (B_(2)H_(5) Cl) + H_(2)`.