Entropy change from cell measurements

From the figures given in Table IV the increase of entropy during the reaction is 8.5 units. As a test of the third law of thermodynamics this value may be compared with a determination of the entropy change of this reaction made by Gerke,21 from the potentials of galvanic cells, in a research already referred to. He measured the potential, E, and the change of the potential with temperature, AE/AT, of cells of the type... 

Other thermodynamic quantities can be derived from electrochemical measurements now that we have linked the potential difference across the cell to the free energy. For example, the entropy change in the cell reaction is given by the temperature dependence of AG.-... 

Therefore, if the standard emf for a cell at a certain temperature is given, its value at another temperature can be determined using Equation 13.6 if the enthalpy change for the cell reaction is known. On the other hand, if the emf of a cell can be determined at two or more different temperatures. Equation 13.6 can be used to estimate the enthalpy change AH° for a cell reaction. Also, because AG° can be determined from one of the iE° measurements, the entropy change A.S° can be determined using AG° = AH° — TAS° (Equation 8.35). Thus, the standard enthalpy and entropy of a reaction can be determined directly from the emf as a function of temperature, which gives us a noncalorimetric method for the calculation of these important thermodynamic quantities. 

In these researches, a mainly purpose is to acquire EPHs of cell or half-cell reactions. The EPH could be considered as a basic issue of TEC. Before the identification of this problem there had been two puzzled questions, one is that the heat effects for a reversible reaction, Q can be calculated by the formula Q = TAS where AS is the entropy change of this reaction and T temperature in Kelvin. However, this formula that is valid for most reactions is not viable at least for a reversible single electrode reaction in aqueous solution. For a reversible single electrode reaction, the experimental value of the heat effect is not in agreement with that calculated on the current thermodynamic databank of ions, that is, with which, the product of the calculated entropy change and the temperature of the electrode reaction always differs from the experimental measurements [2]. For example, for the electrode reaction at the standard state ... 

Once A,G has been measured, we can use thermodynamic relations to determine other properties. For instance, the entropy of the cell reaction can be obtained from the change in the potential with temperature ... 

The differences in the hydration of a solnte in H2O and D2O have been extensively stndied by measnring their thermodynamic properties, the change of free energy (AG°t), enthalpy (A//°t), and entropy (AY°t) at the transfer of 1 mol of solnte from a highly dilute solution in H2O to the same concentration in D2O under reversible conditions (mostly 25 °C and atmospheric pressure). Greyson measured the electromotive force (emf) of electrochemical cells of several alkali halides containing heavy and normal water solutions. The cell potentials had been combined with available heat of solution data to determine the entropy of transfer of the salts between the isotopic solvents. The thermodynamic properties for the transfer from H2O to D2O and the solubilities of alkali halides at 25° in H2O and D2O are shown in Table 4. 

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