Electrode Potentials and Electromotive Forces

As we have seen, acidity and basicity are intimately connected with electron transfer. When the electron transfer involves an integral number of electrons it is customary to refer to the process as a redox reaction. This is not the place fora thorough discussion of the thermodynamics of electrochemistry that may be found in any good textbook of physical chemistry. Rather, we shall investigate the applications of electromotive force (emf) of interest to the inorganic chemist. Nevertheless, a very brief review of the conventions and thermodynamics of electrode potentials and half-reactions will be presented. 

If we construct a cell composed of a hydrogen electrode and a second electrode (M /M) of metal M immersed in a solution of of unit activity, we can measure the potential between the electrodes of the cell. Since the hydrogen electrode was assigned a potential of 0.00 V, the potential of the electrode. 

Accordingly, the sign of the emf of either a half-reaction C electrode ) or the overall redox reaction depends upon the direction in which the equation for the reaction is written (as is true fior any thermodynamic quantity such as enthalpy, entropy, or firee energy). The sign of the reduction electrode is always algebraically the same as that.of the electrostatic potential.  

The Nemsi equation applies 10 the potentials of ixMli haff-ieactions and total redox reaclions  

Reactions resulting in a decrease in free energy (AC 0) are spontaneous. This is a requirement of the second law of thermodynamics. Concomitantly, redox reactions in which 0 are therefore spontaneous. 

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Chapter 10 Chemistry in Aqueous and Nonaqueous Solvents 359 Water 360 Nonaqueous Solvents 360 Molten Salts 374 Electrode Potentials and Electromotive Forces 378... 

Electrode potential - The electromotive force of a cell in which the electrode on the left is the standard hydrogen electrode and that on the right is the electrode in question. [2]... 

Fig. 6-1. Electrochemical cell, electric charge flow in a closed cell circuit, and electron levels of two electrodes in an open cell circuit M = electrode S = electrolyte solution a, = real potential of electrons in electrode, e.Ji -electromotive force.

To recall the basic concepts of the thermodynamics of cell operation, such as the electrode potential E, the standard electrode potential and the electromotive force (emf). 

In a similar though less diabolical manner, the electrons produced at the anode of a voltaic cell have a natural tendency to flow along the circuit to a location with lower potential the cathode. This potential difference between the two electrodes causes the electromotive force, or EMF, of the cell. EMF is also often referred to as the cell potential and is denoted fj.g,. The cell potential varies with temperature and concentration of products and reactants and is measured in volts (V). The standard cell potential, or E° gn, is the that occurs when concentrations of solutions ire all at 1 M and the cell is at standard temperature and pressure (STP). 

It has already been mentioned that the electromotive force of concentration cells with transference is the sum of the both electrode potentials and the liquid junction potential which arises, when two solutions of the same substance but of different concentrations are brought into contact the value of the mentioned potential is finally given by the equations (VI-28) and (VI-29). 

Conventions concerning the signs of electric potential differences, electromotive forces, and electrode potentials1... 

The potential of an electrochemical cell, also known as the cell potential or electromotive force (emf) is the sum of the potential drops at the cathode and anode, where the reduction and oxidation reactions occur. With the introduction of a reference electrode the potentials of these two electrodes can be measured independently, allowing the independent investigation of the reactions that are taking place at each electrode (working or counter). These redox reactions are called half-cell reactions or simply half-reactions. The halfreaction potential can be measured with a SHE electrode at standard conditions, i.e., at electrolyte concentrations of 1 M, gas pressures of 1 atm., and... 

An electrochemical power source comprises two electrodes of different materials immersed in electrolyte, whereby electrode systems with different potentials are formed at the two electrodes. Electrochemical reactions proceed at the two interfaces which involve transfer of electrons between the electrode surface and ions from the solution. The difference between the potentials of the two electrodes generates the electromotive force of the electrochemical power source. When the two electrodes (anode and cathode) are connected to a conductor with a load, electric current which can do work flows between them, i.e., the chemical energy can be converted into an electrical one. Electric current flows due to changes of the valences of the materials at the two electrodes. Michael Faraday established that, when one gram equivalent of any substance takes part in an electrochemical reaction, the quantity of electricity that flows is always equal to 96,487 coulombs (C). This value is called Faraday constant, after the name of M. Faraday, and is denoted by the symbol F. The value of the constant is generally rounded to 96,5(X) C. 

The electrode reactions taking place at the electrodes of direct methanol fuel cells, the overall current-producing reactions, and the corresponding thermodynamic values of equilibrium electrode potentials EP and electromotive force (EMF) of the direct methanol fuel cell are given as follows ... 

The potential difference is closely related to the difference of the electrochemical potential based on the electrochemical affinity. If we could measure A(p directly, we could organize the table of electromotive forces based on the Galvani potential difference. However, A

reference electrode to measure the half cell potential at an electrode. When a certain electrcxle is coupled with a reference electrode, then the electromotive force can be measured. Since we usually use some reference electrodes as standards, the electromotive force is defined as the equilibrium potential of the reaction. The table was made in such a way and the hydrogen reference electrode was used to measure and calculate potentials for the half cell reactions. 

The concepts of EP and related open circuit potential and voltage (OCP and OCV), and electromotive force (emf) are well described in many textbooks on physical chemistry or electrochemistry. However, proving that the measured OCV (OCP) is indeed EP requires an approach that is specific to the particular electrochemical process being analyzed. For the EASP electrodes, a good confirmation of the OCP = EP relationship is the stability of the OCV versus lithium-metal (Li I Li+) reference electrode over time and independence upon lithium ion s concentration. According to the lUPAC [25] rule, the electrochemical circuit for such measurements has the form... 

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