Standard Reaction Entropies

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. 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

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

Use data in Table 7.3 or Appendix 2A to calculate the standard reaction entropy for each of the following reactions at 25°C. For each reaction, interpret the sign and magnitude of the reaction entropy, (a) The formation of... 

Calculate the standard reaction entropy, enthalpy, and Gibbs free energy for each of the following reactions from data found in Appendix 2A ... 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Perhaps the two most important outcomes of the first and the second laws of thermodynamics for chemistry are representedby equation 2.54, which relates the standard Gibbs energy (ArG°) with the equilibrium constant (K) of a chemical reaction at a given temperature, and by equation 2.55, which relates ArG° with the standard reaction enthalpy (A rH°) and the standard reaction entropy (ArA°). 

Once Ar// is determined at a given temperature, equations 2.54 and 2.55 can be used to obtain Ar,V. at the same temperature (but that is often found in practice to be unreliable). To calculate the standard reaction enthalpy at 298.15 K, we can use the procedures described after equations 2.10 or 2.14. On the other hand, the standard reaction entropy at 298.15 K can be derived from... 

The standard reaction enthalpy A,H°9g l5 and the standard reaction entropy ArS29g l5 at 298.15 K and the standard reaction specific heat are calculated from ... 

The depletion of ozone in the stratosphere can be summarized by the net equation 2 03(g) - 3 02(g). (a) From values in Appendix 2A, calculate the standard reaction free energy and the standard reaction entropy for the reaction, (b) What is the equilibrium constant of the reaction at 25°C What is the significance of your answers with regard to ozone depletion ... 

If the change in heat capacity in a chemical reaction is equal to zero, the enthalpy of the reaction is independent of temperature, and the equilibrium constant of the chemical reaction can be readily calculated over a range of temperature without making an integration, as described in Section 3.7. In general, the enthalpy of a chemical reaction is a function of temperature and ionic strength. When ArG° and ArH° are known, the standard reaction entropy ArS° can be calculated... 

ArN change in binding of hydrogen ions in a ArS° standard reaction entropy (J K-1 mol-1)... 

The change of the cell voltage as a function of the cell temperature (temperature coefficient) dEf fdT = -0.83xl0-4 VK 1 and the relationship between the standard reaction entropy and the standard free enthalpy of reaction (3AG /3T)p = -AS the value for the cell reaction can be calculated as -AS = -16.02 J K-1 mol-1. Refs. [i] Bockris JO M, Reddy AKN, Gamboa-Aldeco M (2000) Modern electrochemistry, vol 2A. Kluwer, Dordrecht, p 1356 [ii] Holze R (1989) Leitfaden der Elektrochemie. Teubner, Stuttgart [iii] Handel S (1971) A dictionary of electronics. Penguin Books, London [iv] Rieger PH (1994) Electrochemistry. Chapman-Hall, New York, p 2... 

Standard reaction entropies, hS° -> Determine similarly to calculation of standard enthalpy of... 

The thermodynamic data in the selected set refer to a temperature of 298.15 K (25.00°C), but they can be recalculated to other temperatures if the corresponding data (enthalpies, entropies, heat capacities) are available [97PU1/RAR]. For example, the temperature dependence of the standard reaction Gibbs energy as a function of the standard reaction entropy at the reference temperature (7o= 298.15 K), and of the heat capacity function is ... 

Here AH is the standard enthalpy change of reaction AS the standard reaction entropy change R the gas constant T the temperature /(eq the equilibrium constant for the reaction, given simply by the product of concentrations (activities in reality) of all the products to the power of their stoichiometric coefficients over the same product for reactants m the number of products / / the forward rate constant kr the reverse rate constant n, the stoichiometric coefficient of species i and 1 the number of reactants. A AG value below zero indicates a reaction with an equilibrium point where there is an excess of products over reactants, a... 

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