Heterogeneous reaction HOCl+HBr→BrCl+H2O on ice films

The heterogeneous reaction HOCl + HBr(s) --> BrCl + H2O(s) on the ice surface at 189 and 220 K has been investigated in a flow reactor interfaced with a differentially pumped quadrupole mass spectrometer. Pseudo first-order decay of HOCl over the HBr-treated ice surfaces has been determined under the condition of P-HOCl < P-HBr. For the HBr partial pressure in the range of 1.1 x 10(-7) to 6.6 x 10(-5) Torr, the reaction probability (gamma(g)) was determined in the range of 0.06 to 0.38 at 189 K. The reaction probability is in the range of 0.01 to 0.07 at 220 K for HBr partial pressure from 7.2 x 10(-7) to 1.3 x 10(-5) Torr. The reaction probability was found to be strongly dependent on the ice surface temperature. The reaction probability is higher at the lower temperature than that at the warmer temperature and a mechanistic explanation is provided. The "true" reaction probabilities (gamma(t)) of the reaction were calculated using a pore diffusion model. The kinetic analysis indicates that the heterogeneous reaction of HOCl + HBr may follow the Eley-Rideal type of mechanism. Also, the heat of uptake of HOCl on ice was determined to be about -8.5 +/- 2 kcal/mol.