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

Abstract

The kinetics of the gas-phase reactions of SO3 with H2O and D2O were studied over the temperature range 250−360 K in N2 with a laminar flow reactor coupled to a chemical ionization mass spectrometer. The SO3 loss is second order in the water concentration, is independent of pressure (20−80 Torr N2, 300 K), and has a strong negative temperature dependence and a significant H/D isotope effect (kH2O ≈ 2kD2O). The yield of sulfuric acid is 1.0 ± 0.5 per SO3 consumed. These observations are consistent with the rapid association of SO3 and H2O to form the adduct H2OSO3 which reacts with water to produce sulfuric acid. The first-order rate coefficients for loss of SO3 by reaction with H2O and D2O are given by kI(s-1) = (2.26 ± 0.85) × 10-43T exp((6544 ± 106)/T)[H2O]2 and (9.45 ± 2.68) × 10-44T exp((6573 ± 82)/T)[D2O]2, where T ≡ K and [H2O, D2O] ≡ molecule cm-3. The errors are the uncertainty at the 95% confidence level for precision only. Analysis of the temperature dependence of the SO3 loss yields an upper limit for the H2O−SO3 bond enthalpy of 13 kcal mol-1.