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Hydrogen bromide (HBr) is very soluble in water (Annex VII Section 7.7 Solubility in water) forming hydrobromic acid which dissociates to ions (H+and Br-). It forms an an azeotropic mixture in water. A saturated solution is 66.5 % at 25 ºC. HBr in water acts as a strong mineral acid (i.e., hydrobromic acid) and dissociates readily to form hydrogen and bromide ions.

HBr undergoes photolysis in air. Falconer and Sunder (1968) showed that the photolysis reaction of HBr could progress via one or more of the following reaction schemes:

[1]       HBr +hν → HBr*

[2]      HBr* + M → HBr + M

[3]       HBr* → H(2S) + (Br(2P3/2)

[4]       [HBr + HBr]c+hν → [HBr…H…Br]c

[5]       [HBr…H…Br]c→ HBr + HBr

[6]       [HBr…H…Br]c→ H2+ 2[Br]c

[7]       [HBr…H…Br]c→ HBr + H+ [Br]c

[8]       H + H + M → H2+ M

[9]       H + HBr → H2+ [Br]c

[13]     [Br2+ HBr]c+hν → [HBr…H…Br]c

[14]     [HBr…H…Br]c→ Br2+ HBr

[15]     [HBr…H…Br]c→ H + Br2+ [Br]c

[16]     H + Br2→ HBr + Br

Dotted lines indicate complexes and the subscript c indicates caged or matrix restrained species. 

Baumfalk R, et al (1997) measured kinetic energies the dissociation reaction and obtained values of 1.36 ± n0.06 eV (Br) and 0.90 ± 0.05 eV (Br*) which were in good agreement with the calculated values of 1.35 eV and 0.89 eV obtained using the following equation:

              hν(243.1nm) –D0(HBr) =Ekin(H) +Ekin(Br) +Eint(Br)

WhereD0is the dissociation energy of the HBr molecule andEkinandEintthe kinetic and internal energy of the fragment atoms.

The H atom photofragmented with zero kinetic energy after the photo-dissociation