No abstract

Rate constants for the reaction of atomic chlorine with hydrogen have been measured from 200-500 K using the flash photolysis-resonance fluoreLcence technique. The rate constants obey the Arrhenius equation k-(2.66 + 0.42) x 10-11 exp(-2230+60/T) cm 3 molecule-1 s-1 , The re g ijits are compared with previous work and are discussed with parts-. cular reference to the equilibrium constant for the reaction and to relative rate data for chlorine atom reactions. Theoretical calculations, using the BEBO method with tunneling, give excellent agreement with experiment.

Absolute rate constants for the reaction of hydrogen atoms with acetaldehyde were obtained over the temperature range 298-500 K using the flash photolysis-resonance fluorescence technique. The rate constants gave the Arrhenius expression k = (2.23 ±0.39)X 10-11 exp( -3300± 120/ 1.987 T) cm 3 molecule-Is-I. Independent experiments with a discharge flow system employing resonance fluorescence detection of H gave a room temperature rate constant in agreement with this equation. These results are discussed and comparisons are made with previous work and theoretical predictions.

The reaction of hydrogen atoms with chlorine has been studied in a discharge-flow system using electron spin resonance detection of H-atoms. Chlorine consumption and hydrogen chloride production, measured by on-line mass spectrometry, were much lower than expected. This is explained by the enhanced rate of the reaction H + HC1+ H2 + C1 due to vibrational excitation of the HCl produced in the initial step. Based on a mechanism including this enhanced reaction rate an Arrhenius relation, (4.6k 1.3) x 1Olo exp[-(5 9404 840) J mol-l/RT] dm3 mol-' s-l, is obtained for the rate constant of the initial step over the temperature range 292-434 K.The reaction between hydrogen atoms and chlorine is an important reaction in the H,+C12 system.l Comprehensive studies have been made by Polanyi and co-workers on the energy distribution of the reaction products. Surprisingly few measurements have been made of the absolute rate constant of the r e a ~t i o n . ~ An Arrhenius equation k = 3.7 x loll exp[ -(7 500+ 1 300) J mol-l/RT] dm3 mol-l s-l was obtained by a mass spectrometric method under conditions of high H/C12 ratio. A discharge-flow, time of flight mass spectrometric measurement yielded a value of k = 2.1 x lofo dm3 mol-1 s-l, again in the presence of excess H-atoms.It is desirable to obtain independent confirmation of the Arrhenius parameters for the rate constant using the present e.s.r. reaction system because of the potential importance of the rate of the H + CI2 reaction in a series of chlorine atom abstraction systems under investigation in this laboratory.