Negative temperature coefficients reaction rate

Therefore further progress in this area depends on the measurement of equilibrium constants. At this stage I simply cannot say how much of the difference of two powers of 10 between the k+Bpl of the alkenes and the styrenes is to be attributed to an intrinsic difference in reactivity and how much to the existence of the P+ G complexes. The negative temperature coefficient of the rate constant for a-methyl styrene found by Chawla Huang (1975) is a strong indication in favour of my view that the propagation is not a simple bimolecular reaction. 

They exhibit negative temperature coefficients of reaction rate. 

The three compounds exhibit the major superficial features of hydrocarbon combustion the pressure-time curves are sigmoidal, and the rate of reaction, as given by (dAp/dt)max, exhibits a region of negative temperature coefficient. 

Cool Flames. Cool flames are confined, roughly speaking, to the temperature regime which exhibits the negative temperature coefficient of the rate. The flames are clearly nonisothermal, and the light emission which is most intense at the end of the maximum rate period is probably caused by radical-radical reactions (27, 28) such as... 

The remarkable fact about the velocity of reaction was the negative temperature coefficient. The following figures are taken from Bodenstein s paper, and illustrate the way in which the rate of reaction decreases as the temperature rises. 

The effect of temperature is unusual as cool flame combustion reactions show a region of temperatures and pressures in which the rate shows anomalous behaviour the rate decreases as the temperature increases - the negative temperature coefficient effect. 

For initial temperatures between q and r a cool flame region exists, but, in contrast to region n, p, the overall rate must decrease with increasing temperatures since at r reaction enters the steady state region. Between q and r reaction has a negative temperature coefficient. 

This scheme provides a reasonable explanation of the negative temperature coefficient. However, it was not possible to explain quantitatively the relative rates of formation of oxidation products and methane by identifying (33) with any single reaction of type (34)—36)... 

The combustion of aliphatic ketones generally resembles that of hydrocarbons, the reactions being autocatalytic and possessing two regimes of slow oxidation, separated by a region of negative temperature coefficient of the rate. Cool flames are also observed under some circumstances. 

This ketone is unique amongst those studied in that it apparently exhibits no region of negative temperature coefficient of the rate and no cool flames have been observed [45]. The pressure—time curves were similar to those of methyl iso-propyl ketone at 310 °C, the reaction accelerating smoothly and then stopping suddenly [46]. Analyses of the combustion products have been made at various stages of the reaction at 270, 310 and 350 °C. Carbon monoxide, carbon dioxide, hydrogen peroxide, iso-butene-1 2-oxide, methanol, methyl ethyl ketone and acetaldehyde were all detected, and towards the end of the reaction iso-butene and methane were also formed. 

Biacetyl and acetylacetone have been studied by Salooja [47(a)] in a flow system. Biacetyl was rather reactive, appreciable reaction beginning at 350 °C and ignition occurring at about 530 °C under the experimental conditions employed acetylacetone began to react above 400 °C but ignited at 480 °C. Biacetyl was anomalous in that it did not appear to exhibit a zone of negative temperature coefficient of the rate of combustion, probably because no stable olefinic intermediates are formed in the oxidation process. Some measurements on the rate of slow combustion of methyl vinyl ketone have also been reported [47(b)]. 

The reaction rate does not show any marked negative-temperature coefficient, and the apparent activation energy calculated from the slope of the log (d[02 ] /dt) versus 1/T graph was about 14 kcal. mole . ... 

Direct evidence for the existence of a negative temperature coefficient of reaction rate, and the distinctions between alkanes of different structure were demonstrated in recent studies of -heptane and 2,2,4-trimethylpen-tane (f-octane) oxidation in a well-stirred flow reactor (Figs 6.1 and 6.2). The experiments were performed by Dagaut et al. [30] at 1 MPa and a mean residence time of Is over the temperature range 550-1150K. The fuel at the reactor inlet was set at a fixed mole fraction, 10 , and the [RH] [O2] ratio was chosen in the proportions 1 36,1 22, 1 11 and 1 7.5. These correspond to the stoichiometric proportions 4> = 0.3, 0.5, 1.0 and 1.5 respectively. The reactants were diluted with an excess of N2 in order to maintain isothermal conditions in the reactor. 

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