Pulsed laser powered homogeneous pyrolysis for reaction kinetics studies: Probe laser measurement of reaction time and temperature

Pulsed laser powered homogeneous pyrolysis (LPHP) is a technique which can be used to measure rate parameters for purely homogeneous unimolecular decomposition reactions at high temperatures (600-1500 K). The reaction temperature in pulsed LPHP may be obtained from the speed of sound in the reacting gas, which may be measured by observing the thermal lens effect of the gas on a probe laser beam. The reaction time may be obtained directly from the thermal lens measurements. In this work experiments were performed using ethyl acetate (EtAc) and isopropyl chloride (2-ClPr), two reactants whose unimolecular decomposition rate parameters are well established. This allowed us to assess the accuracy and precision attainable with this technique. Pulsed LPHP proved capable of providing rate parameters in good agreement with those in the literature. The results for which the measured activation energies were closest to the literature values gave the temperature dependence of the rate constants as log(6_EtAc) = (12.0 ± 0.9) - 47.7 ± 4.4(kcal/mol)/2.303RT and log(k_2-ClPr) = (13.3 ± 1.0) - 50.8 ± 4.8(kcal/mol)/2.303RT. These may be compared with the literature recommendations, log(k_EtAc) = 12.6 - 48.0(kcal/mol)/2.303RT and log(k_2-ClPr) = 13.6 - 51.1 (kcal/mol)/2.303RT. In all cases the measured rate parameters agreed with the recommended values to within the error limits of the measured values. Potential sources of error in the temperature measurement and the kinetic parameters are explored. The expected accuracy of the experiments is assessed, and possible improvements in the experiment are suggested.