KINETICS OF THE REACTIONS OF CHLORINE ATOMS WITH C2H4 (K(1)) AND C2H2(K(2)) - A DETERMINATION OF DELTA-H-F,298-DEGREES FOR C2H3

The rate constant (k(1)) for the reaction of Cl atoms with C2H4 has be en measured as a function of pressure (0.2-100 Torr) at 297 K using th e relative rate technique in two reactors with either FTIR or GC analy sis. The results of these and previous experiments (100-3000 Torr) can be described to within +/-10% by a Tree expression with the limiting rate constants k(1)(infinity) = (3.2 +/- 0.15) x 10(-10) cm(3) molecul e(-1) s(-1); and k(1)(0) = (1.42 +/- 0.05) x 10(-29) cm(6) molecule(-2 ) s(-1) using F-cent = 0.6. The stated uncertainties are statistical o nly, and the true uncertainties in the limiting rate constants must in clude uncertainty in F-cent. Temperature-dependent (297-383 K) measure ments of k(1) were carried out in the low-pressure regime (0.2-2 Torr) yielding the rate constant expression k(1)(0) = (1.7 +/- 0.3) x 10(-2 9)(T/298)(-3.28) cm(3) molecule(-1) s(-1). The rate constant, k(1b), f or the abstraction channel of reaction 1 to form the vinyl radical was determined at 300, 343, and 383 K. This measurement of k(1b) in combi nation with literature values of k(-1b) allows a determination of the heat of formation of the vinyl radical by the third law method [Delta H(f,298)degrees(C2H3) = 70.6 +/- 0.4 kcal/mol]. Using the same data wi th the second law yields Delta H(f,298)degrees(C2H3) = 69.6 +/- 1.6 kc al/mol. These measurements agree satisfactorily with a recent negative -ion photoelectron spectroscopy determination of this quantity (71.6 /- 0.8 kcal mol(-1)). The pressure dependence of the rate constant (k( 2)) for the reaction of Cl with C2H2 was determined over the range 0.3 -700 Torr. The results of these and previous experiments (100-6000 Tor r) can be described to within +/-10% by a Tree expression with the lim iting rate constants k(2)(infinity) = (2.0 +/- 0.1) x 10(-10) cm(3) mo lecule(-1) s(-1) and k(1)(0) = (6.1 +/- 0.2) x 10(-30) cm(6) molecule( -2) s(-1) using F-cent = 0.6.