Heterogeneous Reaction HOCl + HBr → BrCl + H2O on Ice Films

The heterogeneous reaction HOCl + HBr(s) → BrCl + H 2 O(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 × 10 -7 to 6.6 × 10 -5 Torr, the reaction probability (γ 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 × 10 -7 to 1.3 × 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 (γ 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.