![]() In order to measure gas viscosities, laminar flow is assumed in the capillary. A calibration flask of known volume is provided to determine the volume of the system. The change in pressure of the system is measured with a manometer as a function of time. ApparatusĪ gas sample is drawn through a thin capillary tube. The viscosity of a gas provides a means for determining molecular diameters, as viscosity arises from collisions among molecules. The rate of gas effusion provides a means of determining the average molecular velocity, as faster molecules will strike the pinhole area more frequently and therefore effuse more rapidly. Thus, the ratio of specific heats should be (3/2 R + 2/2 R + R) / (3/2 R + 2/2 R) = (7/2 R) / (5/2 R) = 1.4, which is in very close agreement with experiment.īoth gas effusion and gas viscosity experiments validate the kinetic theory of gases and provide access to microscopic information from macroscopic measurements. This required the advent of quantum mechanics, which explained that the degrees of freedom for molecular vibration and rotation around the axis of a linear molecule were to be neglected because the excited quantum states for these motions were too high in energy to be accessed at room temperatures. As a result, Maxwell proclaimed that the kinetic theory "could not possible satisfy the known relation between the two specific heats of a gas" and "the result of the dynamical theory, being at variance with experiment, overturns the whole hypothesis, however satisfactory the other results might be." Despite considerable effort, Maxwell was never able to reconcile the kinetic theory of gases with the specific heats experimental result. Since Cp = Cv + R, the ratio of specific heats was predicted to be (3/2 R + 3/2 R + R + R) / (3/2 R + 3/2 R + R) = 5 R / 4 R = 1.2, which differed from observed values for oxygen, hydrogen, and nitrogen of about 1.4. Translational and rotational degrees of freedom each contribute 1/2 R to the specific heat, and vibrational degrees of freedom contribute R to the specific heat. A diatomic molecule has three translational, three rotational, and one vibrational degree of freedom. ![]() The results were reported in 1866, reconciling his kinetic theory of gases with observed gas viscosities.Īs an aside, Maxwell was never able to reconcile his kinetic theory of gases with the observed ratio of specific heats, C p/ C v, for diatomic gases. These measurements were made using an apparatus in attic of their house, and the temperature was controlled through appropriate stoking of the fireplace. He was troubled with his own proposal, however, because it had "the curious result" that viscosity is independent of pressure which was "certainly very unexpected." Maxwell and his wife made the first reliable measurements of gas viscosities in order to determine the dependence of gas viscosity on temperature and pressure. Although now fully accepted, this proposal went against the conventional theory of the time that a range of velocities would be equalized by molecular collisions. James Clerk Maxwell, who is famous for the Maxwell-Boltzmann distribution of molecular velocities and Maxwell's Equations of electromagnetic radiation, proposed in 1860 that gases possess a distribution of velocities. Laidler, The World of Physical Chemistry, Oxford University Press, 1993, p. ![]() The viscosity of gases played an important role in the historical development of the kinetic theory of gases (see Keith J. Thus, an understanding of liquid and gas viscosity is essential for engineering a chemical process. Viscosity of fluids is the key physical property that dictates the design of pipelines to transport material. Introductionįluid flow through pipes is of immense importance to chemical engineers, who must design appropriate methods for transporting chemicals to and from reaction vessels.
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