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The Measurement of the Rate Parameters for the Reactions i‐C 3 H 7 • and n ‐C 3 H 7 • + HI → C 3 H 8 + I • over the Temperature Range 293–623 K: Implications for the Standard Heat of Formation of the Propyl Radicals
Author(s) -
Leplat Nicolas,
Rossi Michel J.
Publication year - 2014
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.20832
Subject(s) - chemistry , radical , analytical chemistry (journal) , photoionization , arrhenius equation , atmospheric temperature range , reaction rate constant , kinetic energy , kinetics , ionization , thermodynamics , activation energy , ion , organic chemistry , physics , quantum mechanics
An investigation of the absolute rate constants of the metathesis reactions i ‐C 3 H7 • + HI → C 3 H 8 + I • (1) and n ‐C 3 H7 • + HI → C 3 H 8 + I • (2) was performed using a newly developed apparatus. The obtained Arrhenius expressions are k 1 = 1.94 (±0.42) × 10 −11 exp(−6.190 (±0.855)/ RT ) and k 2 = 6.49 (±1.86) × 10 −11 exp(−11.084 (±0.913)/ RT ) ( R = 8.314 J K −1 mol −1 ) over the temperature range 293–623 K ( A in cm 3 molec −1 s −1 , E a in kJ mol −1 ). Two Knudsen reactors of different geometry coupled to single‐photon (VUV) photoionization mass spectrometry (SPIMS) were successively used. The two propyl isomers were generated externally to and upstream of the reactor and led to thermalized i‐ and n‐C 3 H7 • free radicals. A minor correction to k 2 for the wall loss of n‐C 3 H7 • ( k w ) was applied throughout the temperature range, whereas no significant wall loss of i‐C 3 H7 • could be measured. The obtained results are based on the disappearance of propyl radicals with increasing amounts of HI. Thermochemical parameters of propyl free radicals resulting from the present kinetic measurements are discussed and point toward a slightly lower value for the standard heat of formation of Δ f H 298 0 (i‐C 3 H7 • ) compared to the currently accepted value, whereas no significant adjustment for n‐C 3 H7 • seems to be necessary. The recommended values are Δ f H 298 0 (i‐C 3 H7 • ) = 86.6 ± 2.1 kJ mol −1 and Δ f H 298 0 (n‐C 3 H7 • ) = 101.1 ± 2.1 kJ mol −1 resulting from a third law evaluation using S 298 0 (i‐C 3 H7 • ) = 289.9 and S 298 0 (n‐C 3 H7 • ) = 290.3 J K −1 mol −1 , both values being very close to the values measured by W. Tsang, namely 88 ± 2 and 100 ± 2 kJ mol −1 , respectively.