Argon entropy

Values extracted and in some cases rounded off from those cited in Rabinovich (ed.), Theimophysical Fropeities of Neon, Argon, Krypton and Xenon, Standards Press, Moscow, 1976. This source contains an exhaustive tabulation of values, v = specific volume, mVkg h = specific enthalpy, kj/kg s = specific entropy, kJ/(kg-K). The notation 6.76.—3 signifies 6.76 x 10 . This book was published in English translation by Hemisphere, New York, 1988 (604 pp.). 

Self-Test 7.6A Calculate the standard entropy of vaporization of argon at its boiling point (see Table 6.3). 

The third law of thermodynamics makes it possible to measure the absolute entropy of any substance at any temperature. Figure 14-12 shows the stages of a pure substance, in this case argon, as the temperature is raised from 7 =0,S = 0toJ = 298 K. As temperature increases, entropy increases steadily. 

More energy is required to heat one mole of argon gas from 87.3 to 298 K, and this energy gets dispersed among the argon atoms, increasing the entropy still further. 

C14-0050. Table lists molar enthalpies of fusion of several substances. Calculate the molar entropy of fusion at its normal melting point for each of the following (a) argon (b) methane (c) ethanol and (d) mercury. 

FIG. 1 The calculated surface tension of an argon fluid represented as a Lennard-Jones fluid is shown as a function of temperature. The GvdW(HS-B2)-functional is used in all cases. The filled squares correspond to step function profile and local entropy, the filled circles to tanh profile with local entropy, and the open circles to tanh profile with nonlocal entropy. The latter data are in good agreement with experiment. 

When the volume occupied by one mole of Ar at 0°C is halved, there is a decrease in entropy (dispersal of energy), as signified by the negative sign of the entropy change, -5.76 J/(mol rxn)-K. In the smaller volume there are fewer energy levels available for the argon molecules to occupy and so, there is a decrease in entropy in the smaller volume. 

The results of the accurate work of Orr (23) on the adsorption of argon on KC1 and Csl crystals have been mentioned above, and it was shown that when the surface was half covered, the adsorbate had an entropy equivalent to that of a two-dimensional gas. The curves given... 

Argon adsorbed on charcoal at 215°K. has still a slight supermobile character 197), the entropy contribution of the perpendicular vibration being 2 entropy units. Its frequency is... 

Despite the somewhat larger heat of adsorption and the lower temperature, the time of adsorption of argon on charcoal therefore is practically the same as the corresponding figure for xenon on mercury. The higher the entropy, hence the more mobile the adsorbed molecule is, the longer is its time of adsorption, other quantities, such as heat of adsorption and temperature, being equal. 

Argon on graphite, at higher 0 values, shows the same behavior as nitrogen on single-crystal copper, as has been described above 223). The mutual van der Waals attraction forces may, in this case, lead to two-dimensional condensation, as is also shown by the entropy data. We shall discuss two-dimensional condensation in Sec. VIII,4. 

Heat and Entropy Functions. Both differential and integral heats and entropies of adsorption were evaluated from the data for argon adsorption on muscovite (3). The variations in these functions with coverage for the potassium and barium muscovite were discussed in terms of localized adsorption on the various kinds of geometrical sites on the mica surface. On the cesium mica, however, the heat of adsorption of argon was higher and more uniform than on the potassium and barium micas, and the entropy functions showed a monotonic decrease with coverage. 

We may assume that both argon and krypton adsorbed on mica are essentially two-dimensional liquids at the completion of the first monolayer (2, 4). The rise in the entropy functions for argon adsorption on potassium and barium mica as the monolayer point is approached may then reflect the transition from substantially localized adsorption at lower coverages to a mobile film. No such phenomenon is observed with krypton, suggesting that there is no change in the behavior of the adsorbed phase during formation of most of the first monolayer. 

When the data on the recombination of atoms are employed (Table XII.9) to calculate fc 4 + fci, it is found that the values of fc 4 + ky may be from 100 to 1000 times greater than simple collision frequencies. Thus Marshall and Davidson S have calculated that h + kv is l.l X 10 liters/ mole-sec for I2 in argon, l.G X 10 in neopentane, and 1.8 X 10 in pentane." This has been commented on by Rice, who pointed out that such anomalously high values may be accounted for in terms of a large positive entropy of activation for the halogens arising from the increase in number of vibrational levels in the critical complex, the increase in electronic states and the increase in the moment of inertia. In the case of H2 a factor of... 

Seshadrl, Nlmon and White (2) observed the infrared spectra of KBOg In an argon matrix and made a complete assignment of all six fundamentals which are adopted In the tabulation. The uncertainty of the calculated statistical entropy at 298.15 K is estimated to be 1 eu which Is due to the magnitude of the matrix shifts from the free molecule spectrum, particularly for the lower frequency modes. 

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