Cells, reversible heat change

It is seen from equation (4) that if the e.m.f. of a reversible cell, i.e., E, and its temperature coefficient, SEIdTj at constant pressure are known, it is possible to evaluate the heat change of the reaction occurring in the cell. 

Irreversible losses cause a difference in the efficiency of reversible and real processes. These losses can be described and quantified by their irreversible entropy production. The consideration of the ohmic losses shows that the irreversible entropy production in a SOFC is smaller than in another low-temperature fuel cell. This is caused by the lower irreversible entropy production of the heat dissipated at a higher temperature. The effects of the irreversible mixing of reactants and products lead to an irreversible entropy production as well that reduce the cell voltage. The changes in the Nernst voltage can be understood by the analysis of the fuel utilisation. 

For example, consider a system in which metallic zinc is immersed in a solution of copper(II) ions. Copper in the solution is replaced by zinc which is dissolved and metallic copper is deposited on the zinc. The entire change of enthalpy in this process is converted to heat. If, however, this reaction is carried out by immersing a zinc rod into a solution of zinc ions and a copper rod into a solution of copper ions and the solutions are brought into contact (e.g. across a porous diaphragm, to prevent mixing), then zinc will pass into the solution of zinc ions and copper will be deposited from the solution of copper ions only when both metals are connected externally by a conductor so that there is a closed circuit. The cell can then carry out work in the external part of the circuit. In the first arrangement, reversible reaction is impossible but it becomes possible in the second, provided that the other conditions for reversibility are fulfilled. 

This reaction, having equal number of mols of gas reactants and products, has a negligible change in entropy and thus a negligible heat effect if carried out reversibly at constant temperature. The maximum work available from a fuel cell under these circumstances would then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the... 

The enthalpy change associated with a cell reaction is a state function and hence is independent of whether the reaction is being carried out reversibly or not. If an exothermic reaction is carried out completely irreversibly, the total enthalpy change AH will appear as heat given out to the surroundings ... 

The reaction entropy ACS is a result of the different opportunities of the species to save thermal energy between the absolute zero level of temperature and the temperature level of the reactor. Concerning the energy balance of a fuel cell (Figure 2.1), the heat <2FCrev has to be transferred reversibly from the fuel cell to the environment. 0FCrev is defined as a positive value if the reversible change in entropy is... 

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