Reactions of atomic hydrogen with hydrogen chloride and nitrosyl chloride

Abstract

The kinetics of reaction of hydrogen chloride with atomic hydrogen have been investigated in a discharge-flow system at total pressures near 1 mm Hg, and from 195 to 373°K. The rate of removal of hydrogen atoms was first-order in [H] and [HCl], H+HCl→H₂+Cl. (1) The rate constant k₁ is satisfactorily represented by the expression k₁=(3.5±1.5)× 10¹¹T^½ exp [(–2900±300)/RT] cm³ mole^–1 sec^–1, in this temperature range.

Combination of these data with values found for k₁ at higher temperatures shows that all the results can be fitted satisfactorily either (i) by a simple collision theory temperature-dependence, k₁=(6.2±3.5)× 10¹¹T^½ exp [(–3100±400)/RT] cm³ mole^–1 sec^–1, with a steric factor of about 0.025, or (ii) by various models for the transition complex HHCl, where the temperature-dependence has a more complex form. The correlation of the data for k₁ with the simple Arrhenius expression A exp (–E/RT) is not satisfactory. The best fit of the data is to two semi-empirical, linear models proposed by Bigeleisen et al.³ for the HHCl transition complex, in which there is strong H—H bonding, and to a triangular model, but the limits of experimental error do not rule out several other models.

The reaction of hydrogen atoms with nitrosyl chloride was both stoichiometric, H+NOCl→NO+HCl (2) and very rapid, k₂ > 1 × 10¹² cm³ mole^–1 sec^–1 at 300°K. The reaction can be used as a gas phase titration for the measurement of hydrogen atom concentrations.