Anamalous Nature of Fluorine
Fluorine is the most reactive element among halogen. This is due to the minimum value of F-F bond dissociation energy.
Fluorine decomposes cold dilute alkalies liberating OF2 and with conc. alkali, O2 is liberated. Under similar conditions, the other halogens will give rise to the hypohalites and halates respectively.
It has the greatest affinity for hydrogen, forming HF which is associated due to the hydrogen bonding. Hydrofluoric acid is a weak acid whereas the other hydrohalic acids are strong acids.
...... H- F...... H- F..... H-F.
It differs markedly from the other halogens in that it can form two types of salts with metals. NaF and NaHF2.
The salts of HF differ from the corresponding salts of other hydracids. AgF is soluble in water while the other AgX are insoluble.
Being strongly electronegative it can have only a negative oxidation state
while the other halogens can have negative as well as positive oxidation state.
HF attacks glass while others do not.
Fluorine, because of the absence of d-orbitals in its valence shell does not form any polyhalides. Thus we have I3 -, Br3-, Cl3- ions but no F3- ion.
Industrially, hydrogen fluoride is obtained by heating fluorspar (CaF2) with concentrated H2SO4 in a lead vessel.
CaF2 + H2SO4® CaSO4 + 2HF.
HF distils over and the vapours are condensed in water in a lead receiver.
Aqueous HF thus obtained is stored in wax bottles. It cannot be stored in glass or silica bottles as it attacks silicates and silica.
Na2 SiO3 + 6HF ® Na2SiF6 + 3H2O
SiO2 + 4HF ® SiF4 + 2H2O
The action of hydrofluoric acid on silica and silicates is used for etching glass. The glass article is first covered with a film on wax. The design to be etched is now drawn on the waxed surface and is then exposed to the action of hydrofluoric acid. Now the glass can be very soon etched. The wax is finally washed off with turpentine.