Measurements have been made, at temperatures between 35 and 100° C, of the viscosities and thermal conductivities of gaseous methane, ethane, ethylene, propane, propylene, cyclo -propane, n -butane, cis -2-butene, trans -2-butene, 1:3-butadiene, iso -butane, n -pentane, iso -pentane, neo -pentane, cyclo -pentane, n -hexane, neo -hexane, cyclo -hexane, benzene, n -heptane, 2:4-dimethyl-pentane, n -octane, 2:3:4-trimethyl-pentane and of carbon tetrafluoride. Viscosities were measured by observing the damping of a pendulum swinging in the vapour and by a capillary-flow viscometer. Thermal conductivities were measured by the hot-wire method. The observed values of the ratio between the transport properties are discussed in the light of the kinetic theory of gases. I t is found that highly flexible molecules show systematically lower values of the ratio, k/n , than other molecules, and it is suggested th at this is due to a special type of intermolecular collision, involving some degree of intercoiling of the two molecules. This view is relevant to the interpretation of recent work on the vibrational activation of the ethylene molecule by collision with other hydrocarbons, and on the kinetics of the thermal decomposition of gaseous hydrocarbons. Inferences can also be drawn about the ease of interchange of translational energy with vibrational and with rotational energy in collisions; these are discussed in relation to information obtained from ultrasonic dispersion measurements.
The velocity of ultrasonic waves has been measured in a number of gases at 25°C and for values of the ratio, ultrasonic frequency/pressure, ranging from 2 x 10 5 to 2 x 10 7 c s -1 atm -1 . Dispersion, corresponding to a single vibrational relaxation process was shown by acetylene, CD 3 Br and hexafluoro-ethane; and, to a double relaxation process, by ethane. Incipient dispersion was shown by propane, ethyl chloride, ethyl fluoride and dimethyl ether. No dispersion was shown by 1.1-difluoro-ethane, n -butane, iso -butane, neo -pentane and ammonia. Correlation of these with previous results leads to the conclusion that: ( а ) For molecules with a distribution of fundamental frequencies, such that there is only a small gap between the lowest and the remaining frequencies, vibrational activation enters via the lowest mode and spreads rapidly to the other modes, giving rise to a single relaxation process involving the whole of the vibrational energy. The chief factors determining the probability of excitation of the lowest mode are its frequency and the presence or absence of hydrogen atoms in the molecule. Molecules containing two or more hydrogen atoms suffer translational-vibrational energy transfer very much more easily than other molecules. Deuterium has almost the same effect as hydrogen. ( b ) For molecules, in which there is a large gap between the lowest and the remaining fundamental frequencies, a double relaxation process occurs. The complex energy transfer probabilities involved do not fit the same quantitative functional relation with vibrational frequency as in ( a ) above. ( c ) Torsional oscillations due to hindered internal rotation behave similarly to other fundamental modes. For molecules in which there is a large gap between the torsional frequency and the other modes (e. g. ethane) a double relaxation process occurs as in ( b ). Where there is no such gap, vibrational energy enters all modes via the torsional mode as in ( a ).
The compressibilities of a num ber of organic vapours have been measured at pressures up to 1 atm. and temperatures ranging from 40 to 130° C. The observed second virial coefficients are compared with values calculated from the critical data by the Berthelot equation. The results show two distinct classes of behaviour. Class I is shown by ethane, ethylene, n -hexane, cyclohexane, benzene, diethyl ether, ethyl chloride, chloroform and carbon tetrachloride, where the measured second virial coefficients are in agreement with the calculated values. Class II by acetaldehyde, acetone, acetonitrile, methyl alcohol, where the measured second virial coefficients are consistently very much higher than the calculated values. It is concluded that the vapours of polar substances for which the energy of attraction between molecules, due to dipole interaction or to hydrogen bonding, is larger than kT undergo dim erization. This view is supported by thermal conductivity data. The range of validity of the Berthelot equation for both non-polar and polar vapours is examined.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.