Investigation on the mechanism and applications of the reaction Cl(2+)2HBr=2HCl+Br-2

The mechanism of reaction Cl-2+2HBr=2HCl+Br-2 has been carefully investigat ed with density functional theory (DFT) at B3LYP/6-311G** level. A series o f three-centred and four-centred transition states have been obtained. The activation energy (138.96 and 147.24 kJ/mol, respectively) of two bimolecul ar elementary reactions Cl-2+HBr --> HCl+BrCl and BrCl+HBr --> HCl+Br-2 is smaller than the dissociation energy of Cl-2, HBr and BrCl, indicating that it is favorable for the title reaction occurring in the bimolecular form. The reaction has been applied to the chemical engineering process of recycl ing Br-2 from HBr. Gaseous Cl-2 directly reacts with HBr gas, which produce s gaseous mixtures containing Br-2, and liquid Br-2 and HCl are obtained by cooling the mixtures and further separated by absorption with CCl4. The re covery percentage of Br-2 is more than 95%, and the Cl-2 remaining in liqui d Br-2 is less than 3.0%. The paper provides a good example of solving the difficult problem in chemical engineering with basic theory.