Oxygen standard entropies

The standard entropies S° of gases are much larger than those of solids and liquids (Section 2.3). This may be understood by the somewhat simplistic view of S° as a measure of disorder at the molecular level. The molecules of gases have much greater freedom of translational motion, and hence are less ordered, than those of liquids and especially solids. Consequently, for oxidation of a solid metal to a solid oxide with consumption of gaseous oxygen... 

E° = E°(cathode) — °(anode). standard enthalpy of combustion AHc° The change of enthalpy per mole of substance when it bums (reacts with oxygen) completely under standard conditions, standard enthalpy of formation AH° The standard reaction enthalpy per mole of compound for the compound s synthesis from its elements in their most stable form at 1 atm and the specified temperature, standard entropy of fusion ASfus° The standard entropy change per mole accompanying fusion (the conversion of a substance from the solid state to the liquid state), standard entropy of vaporization ASvap° The standard entropy change per mole accompanying vaporization (the conversion of a substance from the liquid state to the vapor state). 

EXAMPLE I Calculate the standard entropy change for the reaction of a mole of carbon monoxide with oxygen to produce carbon dioxide at 25°C. 

For a given reaction, AG° and AH° would need to be calculated from standard formation values (graphite, oxygen, and carbon dioxide) first, before plugging into the equation. Also, AS° would need to be calculated from standard entropy values. 

AHt = standard enthalpy of reaction at T K = standard entropy of reaction at T K OR4 = oxygen coordination tetrabedra = molar entropy at 298 K... 

Oxygen gas has many applications, from welders torches to respirators. The gas is sold commercially in pressurized steel tanks. One such tank contains O2 at p = 6.50 bar and T — 298 K. Using standard thermodynamic data, compute the molar entropy of the gas in the tank at 6.50 bar and the change in entropy of a 0.155-mol sample of gas withdrawn from the tank at 1.10 bar and constant temperature. 

Furthermore, pure oxygen gas, whose molar entropy is lower by an amount of / ln 0.2034 than that of gaseous oxygen molecules in the atmospheric wet air (xQl = 0.2034 ) at the standard temperature and pressure, possesses its standard molar exergy given by Eq. 10.26 relative to the atmospheric wet ain... 

This simple reaction is maybe the most studied, and it has been known for a very long time [5], The amount of energy that has to enter in reaction (1) should at least cancel the difference in enthalpy between water and hydrogen and oxygen, which is 5.92 eV at standard conditions. As there are three diatomic molecules on the product side and only two liquid water molecules on the initial side, the entropy... 

By way of illustrations we display in Fig. 1.17.2a plot of the molar heat capacity of oxygen under standard conditions. The plot of Cp vs. In T is then used to determine the entropy of oxygen from the area under the curves. Note that the element in the solid state exists in three distinct allotropic modifications, with transition temperatures close to 23.6 and 43.8 K the melting point occurs at 54.4 K, and the boiling point is at 90.1 K. All the enthalpies of transition at the various phase transformations are accurately known. An extrapolation procedure was employed below 14 K, which in 1929 was about the lower limit that could conveniently be reached in calorimetric measurements. 

Extended virial equations of many terms and constants have been developed for the highly accurate representation of experimental data. Some are developed specifically for standard tables of density and derived thermodynamic functions such as entropy and enthalpy. Bender [31] extended the virial equation to a 20-constant equation to represent argon, oxygen, methane, hydrogen, ethene. 

Tables 7.1 and 7.2 are copied directly from the thermodynamic compilation of Robie, Hemingway and Fisher (1978), abbreviated as RHF. Many of the other standard thermodynamic data sets discussed later in this chapter are arranged in a similar fashion. We can now begin to examine some of the features of these tables a little more closely. First, we have just observed that A/ G° and Ay H° for the formation of 02(g) from the elements is zero at all temperatures because this is just the difference between the G (or H) of oxygen and the G (or H) of the elements making up oxygen, which is the same thing. We have not yet defined the equilibrium constant K (see Chapter 13), but for completeness we should point out that it is 1.0 and log IT = 0 for the reaction for the formation of oxygen from itself, giving us another column of zeros. Note too that the entropy of 02(g), S°t, given in Table 7.1 is not equal to zero at any temperature shown these are absolute entropies, not entropies of formation from the elements, as discussed in Chapter 6 and again later in this chapter. Table 7.1 is typical of data tables for the elements.

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