The enthalpy change and the detailed rate coefficients of the equilibrium reaction OH+C2H2=MHOC2H2 over the temperature range 627–713 K

Abstract

The reaction between OH and C2H2 in He has been studied over the pressure range 414–864 Torr and the temperature range 627–713 K by means of the laser-photolysis/laser-induced-fluorescence technique. Analysis of the temporal profile of [OH] yielded the forward and the reverse rate coefficients, hence the equilibrium constant for the reaction OH+C2H2+M=HOC2H2+M. The temperature dependence of the equilibrium constant yielded the enthalpy of reaction ΔH(670±43 K)=−(34.2±1.0) kcal mol−1 and the entropy of reaction ΔS(670±43 K)=−(30.5±1.2) cal K−1 mol−1 near 670 K; consequently, ΔH0 (298 K)=−(33.6±1.2) kcal mol−1 and ΔS0 (298 K)=−(28.9±1.5) cal K−1 mol−1 were derived for the reaction, in agreement with theoretical prediction. The temperature dependence of the rate coefficients for the forward, the reverse, and the adduct-loss (isomerization/decomposition other than the reverse) reactions near 1 atm have been determined to be kf = (1.1 ± 0.2) × 10−13 exp[(1460 ± 500)/T] cm3 molecule−1 s−1, kr = (2.2 ± 0.4) × 1012 exp[ − (15 100 ± 600)/T] s−1, and ka= (7.4+0.2−5.6) × 1010 exp[ − (13 200+700−1700)/T] s−1, respectively.