Kinetics and Products of the Gas-Phase Reaction of SO₃ with Water

The kinetics of the gas-phase reactions of SO 3 with H 2 O and D 2 O were studied over the temperature range 250−360 K in N 2 with a laminar flow reactor coupled to a chemical ionization mass spectrometer. The SO 3 loss is second order in the water concentration, is independent of pressure (20−80 Torr N 2, 300 K), and has a strong negative temperature dependence and a significant H/D isotope effect ( k H 2 O ≈ 2 k D 2 O ). The yield of sulfuric acid is 1.0 ± 0.5 per SO 3 consumed. These observations are consistent with the rapid association of SO 3 and H 2 O to form the adduct H 2 OSO 3 which reacts with water to produce sulfuric acid. The first-order rate coefficients for loss of SO 3 by reaction with H 2 O and D 2 O are given by k I (s -1 ) = (2.26 ± 0.85) × 10 -43 T exp((6544 ± 106)/ T )[H 2 O] 2 and (9.45 ± 2.68) × 10 -44 T exp((6573 ± 82)/ T )[D 2 O] 2, where T ≡ K and [H 2 O, D 2 O] ≡ molecule cm -3 . The errors are the uncertainty at the 95% confidence level for precision only. Analysis of the temperature dependence of the SO 3 loss yields an upper limit for the H 2 O−SO 3 bond enthalpy of 13 kcal mol -1 .