Reaction entropy, molar

This conversion-related quantity is called the molar reaction entropy ArS. In our example, the caveat for small A is unnecessary because only pure substances are participating in the reaction. However, if dissolved substances appear in the conversion formula, we can then only allow small additional conversions A for any arbitrary extent of the reaction. This is to ensure that the composition of the solution and, therefore, the entropy demands of the substances in it do not change noticeably. 

The molar reaction entropy is the change of entropy—Abased upon the conversion— at constant p and T. It equals the sum of the molar entropies of the reaction partners... 

We have taken over this name for lack of a better one in order to distinguish between the effects caused by differences of entropy demands and those caused by entropy generation. In Eq. (8.25), the expression on the left represents the latent molar reaction entropy and the one on the right, the generated molar reaction... 

The temperature coefficient x corresponds to the molar reaction entropy Ar5 introduced in Sect. 8.5, or more generally, to the corresponding entropy A 5 of a transformation ... 

As we have shown in Chap. 9, the temperature coefficient k of the drive corresponds to a molar reaction entropy. In this case, this is the change of entropy... 

J si,o represents the drive of the melting process at the chosen initial state (in this case, a temperature of To and pressure of po). The temperature coefficient aTsi of the drive corresponds to the molar reaction entropy Agi o of the melting process and the pressure coefficient corresponds to the negative molar reaction volume — AsiVo (compare Chap. 9), both at the initial state To, po)- This yields ... 

Some of the most useful experimentally-derived data for thermodynamic calculations are values of standard molar reaction enthalpies, standard molar reaction Gibbs energies, and standard molar reaction entropies. The values of these quantities for a given reaction are related, as we know (Eq. 11.8.21), by... 

Derive an expression for the standard molar reaction entropy of a reaction that can be used to calculate its value from the thermodynamic equilibrium constant and its temperature derivative. Assume that no solute standard states are based on concentration. 

To calculate the change in entropy that accompanies a reaction, we need to know the molar entropies of all the substances taking part then we calculate the difference between the entropies of the products and those of the reactants. More specifically, the standard reaction entropy, AS°, is the difference between the standard molar entropies of the products and those of the reactants, taking into account their stoichiometric coefficients ... 

The standard reaction entropy is the difference between the standard molar entropy of the products and that of the reactants weighted by the amounts of each species taking part in the reaction. It is positive (an increase in entropy) if there is a net production of gas in a reaction it is negative (a decrease) if there is a net consumption of gas. 

Calculate the standard reaction entropy from standard molar entropies (Example 7.9). 

In principle, the free energy change and in turn the equilibrium constant for such reactions can be calculated from conventional thermodynamic data (molar enthalpy, entropy, volume data) on the end-member isotopic species denoted in the reaction. This approach, however, is generally not practicable because of the paucity of thermodynamic data on isotopically pure end-members. Moreover, even if such data were widely available, the Gibbs free energy changes associated with most isotope exchange reactions... 

Naturally, formal symbols corresponding to the various reaction entropies can be introduced, which are more or less self-explanatory. This allows us to abbreviate the three molar entropies introduced above, and their integral counterparts, to ... 

Here, aHr, Asr, and A r denote the molar enthalpy, entropy, and Gibbs function, respectively, of the reversible combustion reaction of the fuel with oxygen. HHV is the molar higher heating value of the fuel, and vjt(v, ) is the stoichiometric coelRcient of the (/ th) substance in this reaction. 

In contrast. Fig. 11.6(b) shows the nonlinearity of the entropy as a function of during the same reaction. The nonUnearity is a eonsequenee of the dependence of the partial molar entropy 5, on the mixture eomposition (Eq. 11.1.24). In the figure, the slope of the eurve at each value of equals Af5 at that point its value ehanges as the reaction advances and the composition of the reaetion mixture ehanges. Consequently, the molar integral reaction entropy A5m(rxn) = A5(rxn)/A approaehes the value of Ar5 only in the limit as A approaches zero. 

A new volume of Landolt-Bomstein appeared in 1961. This deals with calorimetric quantities and is concerned with elements, alloys, and compounds, and with reaction enthalpies. Subjects covered include the experimental and theoretical basis of thermochemistry, standard values of molar enthalpies, entropies, enthalpies of formation, free energies of formation, and enthalpies of phase change. Planck, Einstein, and Debye functions, anharmonicity, and internal rotation are considered. The final section presents thermodynamic data for mixtures and solutions. 

The difference in molar entropy between the products and the reactants in their standard states is called the standard reaction entropy, A S. It can be expressed in terms of the molar entropies of the substances in much the same way as we have already used for the standard reaction enthalpy ... 

The constant-pressure molar heat capacities of Unear gaseous molecules are approximately jR and those of non-Unear gaseous molecules are approximately 4R. Estimate the change in standard reaction entropy of the following two reactions when the temperature is increased by 10 K at constant pressure ... 

We cannot calculate AjG from the standard molar Gibbs energies themselves because these quantities are not known. One practical approach is to calculate the standard reaction enthalpy from standard enthalpies of formation (Section 1.11), the standard reaction entropy from Third-Law entropies (Section 2.5), and then to combine the two quantities by using... 

AS) close to zero, so that even a low reaction enthalpy suffices to promote the polycondensation. In contrast, polyaddition is characterized by a large negative reaction entropy, so that a highly exothermic reaction is needed to effect the polymerization. The requirement of a highly exothermic reaction strongly limits the number of addition reactions suited for a successful polymerization. In the case of polycondensations even a reaction enthalpy around zero may allow for the synthesis of a high molar mass polymer, when the byproduct is transferred into another phase (e.g., evaporization of CO2 or crystallization of NaCl). From the thermodynamic point of view polycondensations of cyclic monomers represent an intermediate case, as discussed in Chap. 9. 

The absolute entropy of one mole of substance in its standard state is called the standard molar entropy, S°. Standard molar entropies of a number of substances at 298.15 K are tabulated in Appendix D. These values may be used to calculate the standard reaction entropy, ArS°,for a reaction. Consider the following general equation for a reaction. 

The standard reaction entropy, AfS°, is calculated from standard molar entropies by applying equation (13.5). Solve... 

PRACTICE EXAMPLE B N2O3 is an unstable oxide that readily decomposes. The standard reaction entropy for the decomposition of N2O3 to nitrogen monoxide and nitrogen dioxide at 25 °C is AjS = 138.5 J mol What is the standard molar entropy of N203(g) at 25 °C ... 

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