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 (k_H2O ≈ 2k_D2O). 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 k^I(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.