Pressure and temperature dependence of the reaction NO2 + NO3 + M yields N2O5 + M

The kinetics of the reaction NO/sub 2/ + NO/sub 3/ + M ..-->.. N/sub 2/O/sub 5/ + M (1) were studied with flash photolysis/visible absorption by monitoring the disappearance of NO/sub 3/ under pseudo-first-order conditions over the temperature range 236-358 K and pressure range 20-700 torr of M (helium and nitrogen). Reaction 1 was found to be in the falloff region between second- and third-order kinetics in this range. Falloff parameters were obtained by fitting the experimental data to the falloff equation of Troe and co-workers. When one uses a published temperature-dependent broadening factor (F/sub c/), the data are described by k/sub 0/(N/sub 2/) = 4.5 x 10/sup -30/ cm/sub 6/ molecule/sup -1/ s/sup -1/ and k/sub infinity/ - 1.65 x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ at 298 K with T dependencies of T/sup -3.4/ and T/sup -0.4/ for k/sub 0/ and K/sub infinity/, respectively. This expression was used in conjunction with recent measurements of the rate constant for N/sub 2/O/sub 5/ thermal decomposition to obtain an improved measurement ..delta..H/sub f//sup 0/ (298) for NO/sub 3/, which was determined to be 17.3 +/- 0.7 kcal mol/sup -1/. From these data the recommended equilibrium constant expression is K/submore » eq/ = k/sub 1//K/sub -1/ = 2.31 x 10/sup -8/ exp(11350/T) (atm/sup -1/). 31 references, 6 figures, 2 tables.« less