Equilibrium Constant for the Reaction ClO + ClO ↔ ClOOCl Between 250 and 206 K

Abstract

The chlorine peroxide molecule, ClOOCl, is an important participant in the chlorine-catalyzed destruction of ozone in the stratosphere. Very few laboratory measurements have been made for the partitioning between monomer ClO and dimer ClOOCl at temperatures lower than 250 K. This paper reports absorption spectra for both ClO and ClOOCl when they are in equilibrium at 1 atm and temperatures down to 206 K. The very low ClO concentrations involved requires measuring and calibrating a differential cross section, ΔσClO, for the 10-0 band of ClO. A third law fit of the new results gives Keq = [(2.01 ± 0.17) 10–27 cm3 molecule–1] e(8554∓21)K/T, where the error limits reflect the uncertainty in the entropy change. The resulting equilibrium constants are slightly lower than currently recommended. The slope of the van’t Hoff plot yields a value for the enthalpy of formation of ClOOCl at 298 K, ΔHfo, of 129.8 ± 0.6 kJ mol–1. Uncertainties in the absolute ultraviolet cross sections of ClOOCl and ClO appear to be the limiting factors in these measurements. The new Keq parameters are consistent with the measurements of Santee et al.42 in the stratosphere.

The kinetics of the ClOOCl catalytic cycle

“…The recent laboratory determination of K EQ by Hume et al . [], on which JPL 2015 is based, is a notable exception: their laboratory effort extended to 206 K, temperatures relevant to stratospheric chemistry. Furthermore, Hume et al .…”

“…Furthermore, Hume et al . [] show agreement, within uncertainty, between a third law fit of their data and a second law fit, providing additional confidence in the accuracy of their results. Earlier studies [ Cox and Hayman , ; Nickolaisen et al ., ] of K EQ reported inconsistencies between their respective second and third law analyses.…”

“…The most recent results from Hume et al . [], and hence JPL 2015, agree well with K EQ derived from the SOLVE data.…”

“…A salient feature of most laboratory determinations of K EQ , including the recent work of Ferracci and Rowley [2010], is that these observations are generally made at temperatures considerably warmer than 200 K. Hence, the lines shown in Figure 6 (left) represent an extrapolation to colder conditions than most experimental setups. The recent laboratory determination of K EQ by Hume et al [2015], on which JPL 2015 is based, is a notable exception: their laboratory effort extended to 206 K, temperatures relevant to stratospheric chemistry. Furthermore, Hume et al [2015] show agreement, within uncertainty, between a third law fit of their data and a second law fit, providing additional confidence in the accuracy of their results.…”

“…The recent laboratory determination of K EQ by Hume et al [2015], on which JPL 2015 is based, is a notable exception: their laboratory effort extended to 206 K, temperatures relevant to stratospheric chemistry. Furthermore, Hume et al [2015] show agreement, within uncertainty, between a third law fit of their data and a second law fit, providing additional confidence in the accuracy of their results. Earlier studies [Cox and Hayman, 1988;Nickolaisen et al, 1994] of K EQ reported inconsistencies between their respective second and third law analyses.…”