Difluorine

   Difluorine is a pale yellow gas with a characteristic smell similar to that of O₃ or Cl₂. It is extremely corrosive, being easily the most reactive element known. Difluorine is handled in Teflon or special steel vessels, although glass (see below) apparatus can be used if the gas is freed of HF by passage through sodium fluoride (equation 1.1).

                                                      (1.1)

   The synthesis of F₂ cannot be carried out in aqueous media because F₂ decomposes water, liberating ozonized oxygen (i.e. O₂ containing O₃); the oxidizing power of F₂ is apparent from the E^o value listed in Table 16.1. The decomposition of a few high oxidation state metal fluorides generates F₂, but the only efficient alternative to the electrolytic method used industrially is reaction 1.2. However, F₂ is commercially available in cylinders, making laboratory synthesis generally unnecessary.

                   (1.2)

   Difluorine combines directly with all elements except O₂, N₂ and the lighter noble gases; reactions tend to be very violent. Combustion in compressed F₂ ( fluorine bomb calorimetry) is a suitable method for determining values of Δ_fH^o for many binary metal fluorides. However, many metals are passivated by the formation of a layer of nonvolatile metal fluoride. Silica is thermodynamically unstable with respect to reaction 1.3, but, unless the SiO₂ is powdered, the reaction is slow provided that HF is absent; the latter sets up the chain reaction 1.4.

                                                  (1.3)

                                               (1.4)

   The high reactivity of F₂ arises partly from the low bond dissociation energy (Figure 1.1) and partly from the strength of the bonds formed with other elements.