Kinetics and thermochemistry of the reactions of alkyl radicals (CH sub 3 , C sub 2 H sub 5 , i-C sub 3 H sub 7 , s-C sub 4 H sub 9 , and t-C sub 4 H sub 9 ) with HI: A reconciliation of the alkyl radical heats of formation

Journal Article · Wed Feb 14 00:00:00 EST 1990 · Journal of the American Chemical Society; (USA)

DOI:https://doi.org/10.1021/ja00160a009· OSTI ID:6717631

Seetula, J A; Russell, J J; Gutman, D ^[1]

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1. Catholic Univ. of America, Washington, DC (USA)

The kinetics of the reactions of CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, s-C{sub 4}H{sub 9}, and t-C{sub 4}H{sub 9} with HI were studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (typically between 295 and 648 K) to determine Arrhenius parameters. These results were combined with determinations of the rate constants of the reverse reactions (I + hydrocarbon) determined previously by others to obtain equilibrium constants for the following reaction: R + HI {leftrightarrow} R-H + I. Second and Third Law based analyses using these equilibrium constants yielded heats of formation for the five alkyl radicals whose R + HI reactions were studied. The Third Law heats of formation (obtained using calculated entropies) are extremely accurate, within {plus minus} 2 kJ mol{sup {minus}1} of the current best values. The cause of the long-standing disparity that has existed between the heats of formation of the alkyl radicals derived from studies of R + HI {leftrightarrow} R-H + I equilibria and those obtained from investigation of dissociation-recombination equilibria has been identified. It is the difference between the assumed generic activation energy of R + HI rate constants (4 kJ mol{sup {minus}1}) that had been used in all prior thermochemical calculations and the actual values of these activation energies. A complex mechanism for R + HI reactions that is consistent with the observed kinetic behavior of these reactions is discussed.

• OSTI ID: 6717631
• Journal Information: Journal of the American Chemical Society; (USA), Vol. 112:4; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ALKYL RADICALS
KINETICS
THERMOCHEMICAL PROCESSES
CALCULATION METHODS
DATA ANALYSIS
EQUILIBRIUM
FORMATION HEAT
HYDROGEN
IODINE
MEASURING INSTRUMENTS
MEASURING METHODS
TEMPERATURE EFFECTS
ELEMENTS
ENTHALPY
HALOGENS
NONMETALS
PHYSICAL PROPERTIES
RADICALS
REACTION HEAT
THERMODYNAMIC PROPERTIES
400200* - Inorganic
Organic
& Physical Chemistry