Thermodynamic Considerations and Definitions

Let us consider the cell in Fig. 2 that represents a closed thermodynamic system yotpp a 5. Electrochemical reactions occur in the interfacial regions ot/P and a /P between metallic phases and electrolyte. These regions have chemical, electric, and hydrodynamic properties different from those existing in the depths of the phases. Electrons are transferred across the interfaces a/y and 5/a between metallic phases. The electrode a is chosen as anode, electrode a as cathode. The external circuit connecting the terminals 1 and 2 is shown as a wire for simplicity. The following reactions take place at the interfaces in the case of the hydrogen-oxygen cell  

The phases in which reactants and products are located are given in parentheses. It is assumed in the thermodynamic considerations that the 

The contribution of the electron transfer between metallic phases disappears from reaction 5 if the metals y and 5 are identical. The arrangement that 5 is identical with y and a represents a special case of this identity. If an infinitesimal number dn of moles of electrons is transferred from phase y to phase 5 in the time interval d t by the external circuit the current I is  

Here F denotes the faraday. If only one electrochemical reaction is considered, the current is counted positive for an anodic reaction and negative for a cathodic one. The current density i is the ratio between / and apparent surface area A. 

The work done by the surroundings on the system is for the infinitesimal process  

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