THE ENTHALPY CHANGE AND THE DETAILED RATE COEFFICIENTS OF THE EQUILIBRIUM REACTION OH+C2H2M-REVERSIBLE-ARROW-HOC2H2 OVER THE TEMPERATURE-RANGE 627-713 K

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 half arrow right over half arrow left HOC2H2+M. The temperature dependence of the equilibrium constant yielded the enthalpy of reaction DELTA-H(670 +/- 43 K) = - (34.2 +/- 1.0) kcal mol-1 and the entropy of reaction DELTA-S(670 +/- 43 K) = - (30.5 +/- 1.2) cal K-1 mol-1 near 670 K; consequently, DELTA-H-0 (298 K) = - (33.6 +/- 1.2) kcal mol-1 and DELTA-S0 (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 k(f) = (1.1 +/- 0.2) X 10(-13) exp[(1460 +/-500)/T] cm3 molecule-1 s-1, k(r) = (2.2 +/- 0.4) X 10(12) exp[ - (15 100 +/- 600)/T] s-1, and k(a) (7.4(+0.2/-5.6)) X 10(10) exp[ - (13 200(+700/-1700))/T] s-1, respectively.