Electromotive Forces of Galvanic Cells

The maximum of energy obtainable during the galvanic cell operation is expressed by the product of the quantity of electricity passed through the cell and the voltage across the electrodes, called the electromotive force of a cell (EMF). 

If the electromotive forces of all the cells are to be determined precisely they must be measured when no current is passing through them. For this purpose 

Recently voltage has been measured by an electronic voltmeter the main 

The Weston cell is the most suitable standard cell for measuring electromotive forces because of its excellent reproducibility it is shown in diagrammatical form in Fig. 9. 

An H shaped glass container has electrodes at the bottom of the lower arms connected to the external wires by sealed-in platinum wires. The negative electrode is of cadmium amalgam containing 12.5 % cadmium, while the positive electrode is formed by mercury with a layer of paste, which consists of mercurous sulphate, dispersed mercury and small crystals of cadmium sulphate. 

---

The existence of a contact potential between two different metals was recognized over a century ago by Volta, who ascribed the origin of the electromotive force of galvanic cells to it. This point of view receded somewhat into the background in the later decades of last century, but is now re-established, as will be seen in 5. It is not very easy to demonstrate the existence of this contact potential and its actual value depends very much on the cleanliness of the surface indeed without very careful cleaning of the surface, and removal of surface films, which requires a high standard of vacuum technique, the true value for the clean metal can scarcely be obtained at all. 

The relationship between chemical equilibrium and the electromotive force of galvanic cells was first recognised by van t Hoff in 1886. It was not until much later, however, that a cell in chemical equilibrium was investigated experimentally, as it is not easy to find cells in which the equilibrium is not entirely to the one side or the other. In most cells the reaction and the production of current proceed until one of the reacting substances has disappeared almost entirely (e.g. the precipitation of copper by zinc in the Daniel cell). At the instigation of Bredig, Kniipffer investigated a cell made up as follows ... 

The Effect of Pressure Changes on the Electromotive Force of Galvanic Cells. If we apply equation (23) to the reacting substances and the products of a chemical reaction and subtract one from the other we obtain... 

The Effects of Gravity and Centrifugal Force on the Electromotive Force of Galvanic Cells... 

The standard molar entropies of aqueous electrolytes, 2°°, are preferably obtained from the temperature coefficients of the electromotive forces of galvanic cells. The absolute values for individual ions are based on 5°°(H", aq) = —22.2 1.4 J K moP at 298.15 K, from data for thermocells [1]. The S °° values increase with the masses of the ions but are small or negative for multi-charged ions. 

As has already been mentioned, the EMF the electromotive force) of a cell is given by the potential difference between leads of identical metallic material. In view of this, a galvanic cell is represented schematically as having identical metallic phases at either end. 

However, electrochemical cells are most conveniently considered as two individual half reactions, whereby each is written as a reduction in the form indicated by Equations2.ll and 2.12. When this is done and values of the appropriate quantities are inserted, a potential can be calculated for each half cell of the electrode system. Then the reaction corresponding to the half cell with the more positive potential will be the positive terminal in a galvanic cell, and the electromotive force of that cell will be represented by the algebraic difference between the potential of the more-positive half cell and the potential of the less-positive half cell ... 

In general the electromotive force of a cell operating under reversible conditions is referred to as emf ( ), while that observed when conditions are irreversible is termed a voltage. In other words, emf, 8, is the maximum possible voltage that a galvanic cell can produce. 

The decrease in free energy of the system in a spontaneous redox reaction is equal to the electrical work done by the system on the surroundings, or AG = nFE. The equilibrium constant for a redox reaction can be found from the standard electromotive force of a cell. 10. The Nernst equation gives the relationship between the cell emf and the concentrations of the reactants and products under non-standard-state conditions. Batteries, which consist of one or more galvanic cells, are used widely as self-contained power sources. Some of the better-known batteries are the dry cell, such as the Leclanche cell, the mercury battery, and the lead storage battery used in automobiles. Fuel cells produce electrical energy from a continuous supply of reactants. 

This is also the reason why the reversible potential difference of a cell is mentioned with the system composition. Formerly, the reversible potential difference was named the electromotive force of the cell. (For the meaning of the word reversible, see immediately below). For example, the following galvanic cell (see Chap. 13 for the electrochemical cell representations)... 

Seat of Electromotive Force of a Cell.— When we have a galvanic cell inserted in a drcuit, it is, in general, possible for sudden changes of potential to occur at different points of the drcuit. Thus, suppose we have the cell—... 

A second source of standard free energies comes from the measurement of the electromotive force of a galvanic cell. Electrochemistry is the subject of other articles (A2.4 and B1.28). so only the basics of a reversible chemical cell will be presented here. For example, consider the cell conventionally written as... 

This effect appears to be of importance in the case of normal galvanic cells, the electromotive forces of which depend on the concentration of solutions in equilibrium with depolarising solids such as calomel or mercurous sulphate. The exact relationships are, unfortunately, not yet wholly elucidated. 

Because, as we have already seen, the standard potential of hydrogen is zero, the electromotive force of the galvanic cell (eq. 8.161) directly gives the value of the standard potential for the Zn,Zn redox couple. Table 8.14 lists the standard potentials for various aqueous ions. The listed values are arranged in decreasing order and are consistent with the standard partial molal Gibbs free energies of table 8.13. 

From this example it can be seen that electrolysis will start only when the external voltage will exceed the value Ep of the electromotive force of the galvanic cell which acts in a direction opposite to the electrolyzing current. The value Ep can also be considered as the expression of the effort of the system to maintain the initial... 

Wagner equation — denotes usually one of two equations derived by -> Wagner for the flux of charged species Bz under an -> electrochemical potential gradient, and for the - electromotive force of a -> galvanic cell with a mixed ionic-electronic -> conductor [i-v] ... 

E = EMF, or Electromotive Force, or Cell Potential (In the context of galvanic cells) also see below... 

---