Entropy of gases

The entropies of gases are often corrected to the ideal gas condition.2 The correction AS rr is made by following the cycle shown in Figure 4.5 with... 

The standard molar entropies of gases are higher than those of comparable... 

C ° = 1 M and p° = 1 bar, we can simplify the equations for entropies of gases and solutes when their concentrations differ from standard conditions, obtaining Equation ... 

To facilitate the use of Table II, we provide equations for the entropies of gases. For a linear molecules,... 

GENERAL EQUATIONS FOR THE ENTROPY OF GASES Entropy of the Ideal Gas... 

So knowing the states of the reactants and products in a chemical reaction should allow us to predict whether the reaction is accompanied by an increase or a decrease in entropy. Consider, for example, the reaction 2Na(s) + Cl fg) -> 2NaCl(s). We know that the entropies of solids are very much smaller than the entropies of gases and, because this reaction results in a decrease in the number of moles of gaseous molecules (from 1 to 0), the entropy will decrease. Similarly, we would predict an increase in entropy for the reaction CaCOjfs) -> CaO(s) + CO g) because there is an increase in the number of moles of gaseous molecules (from 0 to if However, the entropy change for the reaction CaSiOjfs) CaO(s) + SiO s) is difficult to predict because the reactants and products are solids and are likely to have very similar entropy values. All we can say is that the entropy change is likely to be small. 

Harold Clayton Urey, 1893-. Professor of chemistry at the Institute for Nuclear Studies at the University of Chicago and at the University of California. In 1931 Dr. Urey and his collaborators discovered deuterium, the heavy isotope of hydrogen. Pie has carried out notable researches on the entropy of gases and on the properties and separation of isotopes and has studied the chemical evidence of the earth s origin. 

It is important to note that the entropies of gases are larger than those of liquids or solids. This is because entropy is a function of the degree of randomness or disorder at the molecular level. Customarily, S° values are given in joules per kelvin per mole, whereas AG° and AH° are given in kilojoules remember to multiply the latter by 1000 when doing calculations. The entropy of formation of liquid H20 (reaction 2.14) is then... 

Standard molar entropies increase as the complexity of a substance increases. The standard molar entropies of gases are higher than those of comparable solids and liquids at the same temperature. 

Because of the increase in entropy accompanying fusion and vaporisation, the standard molal entropies of gases are usually higher than those of liquids of similar molecular complexity, which are in turn higher than those of solids. As the figures show, the generalisation is approximate nevertheless it is very useful. 

Entropies of gases tend to be larger than those of solids or liquids S > Snqu,d or 5soiid), as expected from the association of entropy with randomness and disorder. 

Entropy of gases dominates change in reaction entropy. 

TABLE XIX. STANDABD ENTROPIES OF GASES AT 25" C IN CAL. PER DEGREE PER MOLE... 

Usually with increasing molar mass the entropies of gases increases. Thus molar entropy of SbH3 should... 

Conversions for entropies of gases (from JANAF Tables)... 

Entropies of gases are larger than those of liquids, which are larger than those of solids. This is a consequence of Eq. (9.57). 

The entropy of gases increases logarithmically with the mass this is illustrated by the monatomic gases, or the series of diatomics, HF, HCl, HBr, HI. 

Since the entropy of gases is much larger than the entropy of condensed phases, there is a large decrease in entropy in this reaction a gas, hydrogen, is consumed to form condensed materials. Conversely, in reactions in which a gas is formed at the expense of condensed materials, the entropy will increase markedly. 

In statistical thermodynamics, it is established that we can compute from the partition function several properties. In tables of textbooks, the entropy of gases... 

The standard molar entropies of gases are greater than those of liquids and solids, consistent with our interpretation of experimental observations, as represented in Figure 19.13. 

If we had to rely on classical thermodynamics, we would know little more than we have already said about entropy. It is a parameter, with a method of measurement, which increases in spontaneous processes, even when no energy changes are possible, that is, in isolated systems. We would also notice, after measuring the entropy of many substances, that the entropies of gases are relatively large, those of solids relatively small, and those of liquids somewhere in between, but we would probably not have any mental picture of what entropy represents physically. 

Steeper slope, because the entropy of gases is always much greater than that of liquids. 

Katsanos, N.A. Kapolos, J. Gavril, D. Bakaoukas, N. Loukopoulos, V. Koliadima, A. Karaiskakis, G. Time distribution of adsorption entropy of gases on heterogeneous surfaces by inverse gas chromatography. J. Chromatogr. A, 2006,1127, 221 227. 

Enthalpy and entropy of gases at low pressure can be calculated accurately from the Soave-Redlich-Kwong, Peng-Robinson or Patel-Teja equations of state at moderate and high pressure the Peng-Robinson or Patel-Teja equations of state are recommended. On the other hand, for liquid phase enthalpy and entropy none of the cubic equations of state can provide precise results. Empirical correlations, such as that proposed by Lee-Kesler, are much more precise. 

In the surroundings, for an exothermic reaction, ASsun > 0 for an endothermic reaction, AS surr < 0. —> For reaction entropy, the entropy of gases dominates changes. 

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