Potential of galvanic cell

Predict whether the cell potentials of galvanic cells depend on the electrodes and electrolytes in the half-cells. Give reasons for your prediction. 

In this section, you learned how to identify the different components of a galvanic cell. Also, you found out how galvanic cells convert chemical energy into electrical energy. You were introduced to several common primary batteries that contain galvanic cells. In the next section, you will learn more about the cell potentials of galvanic cells. 

Measurements of the potentials of galvanic cells at open circuit give information about the thermodynamics of cells and cell reactions. For example, the potential of the cell in Figure 1, when the solution concentrations are 1 molar (1 M) at 25°C, is 1.10 V. This is called the standard potential of the cell and is represented by E°. The available energy (the Gibb s free energy AG°) of the cell reaction given in equation (3) is related to E° by... 

This equation in not only of considerable value in electrochemistry proper, when calculating the reversible potential of galvanic cells, but is also of great service in thermodynamics for ascertaining various thermodynamic constants. 

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... 

The electrical potentials of galvanic cells resulting from differences in chemical potentials are already familiar from numerous examples discussed in this book In particular the potential of cells of the general type... 

The Standard Potential of Chlorine. Measurements of the potentials of galvanic cells without liquid junctions from which the standard potential of chlorine may be deduced have been made by Lewis and Ruppert40 who used, as one electrode, platinum over which a mixture of chlorine and nitrogen was bubbled, and, as reference, a calomel electrode and hydrochloric acid as electrolyte. The arrangement may be represented by... 

The Variation of the Standard Potentials of Some Electrodes with the Temperature. In a number of cases the standard potentials of galvanic cells without liquid junctions have been determined over a range of temperatures. From these determinations it has been possible to prepare Table V, which gives the standard potentials of a number of electrodes at intervals of 12.5° from 0° to 50°. Some slight adjustments, of the order of 0.2 millivolt, of the original data have been necessary to bring the figures into accord with the Ho values at 25° adopted in this book. A more complete table of standard potentials of the elements at 25° will be found at the end of Chapter 14. 

The Potentials of the Normal and Decinormal Calomel Electrodes. A large portion of the studies on the potentials of galvanic cells has been made using calomel electrodes containing normal or decinormal potassium chloride. Such cells, in general, involve liquid junctions. It is therefore important for the interpretation of these results to decide upon the potentials of the combinations ... 

Table Vll. A Comparison of Ionization Constants, Kq, at 25°, as Determined from Conductance Measurements and from the Potentials of Galvanic Cells without Liquid Junctions...

An interesting application of electrode potentials is to the calculation of the e.m.f. of a voltaic cell. One of the simplest of galvanic cells is the Daniell cell. It consists of a rod of zinc dipping into zinc sulphate solution and a strip of copper in copper sulphate solution the two solutions are generally separated by placing one inside a porous pot and the other in the surrounding vessel. The cell may be represented as ... 

A galvanic cell can be constmcted from a zinc electrode immersed in a solution of zinc sulfate and an iron electrode immersed in a solution of iron(II) sulfate. What is the standard potential of this cell, and what is its spontaneous direction under standard conditions ... 

C19-0128. A galvanic cell is constructed using a silver wire coated with silver chloride and a nickel wire immersed in a beaker containing 1.50 X 10 M NiCl2 (a) Determine the balanced cell reaction, (b) Calculate the potential of the cell, (c) Draw a sketch showing the electron transfer reaction occurring at each electrode. 

Electrode potentials (as well as values of the EMF of galvanic cells) depend on the composition of the electrolyte and other phases of variable composition. The electrode potential corresponds to the Galvani potential of the electrode-electrolyte interface, up to a constant term f =(Po + const. Introducing the concendation dependence of the chemical potential p into Eq. (3.21), we find that... 

The EMF values of galvanic cells and the electrode potentials are usually determined isothermally, when all parts of the cell, particularly the two electrode-electrolyte interfaces, are at the same temperature. The EMF values will change when this temperature is varied. According to the well-known thermodynamic Gibbs-Helmholtz equation, which for electrochemical systems can be written as... 

It will be assumed that the interactions between each of metals (1) and (2) and the corresponding surface layers of the electrolyte solution are approximately identical, and also that specific adsorption of ions does not occur in the system being considered. In this case the values of the expressions in the last two sets of brackets in Eq. (9.10) become zero, and from (9.10) and (9.11) an important relation is obtained which links the OCV of galvanic cells with the Volta potential ... 

This expression explains the qualitative agreement found to exist between the OCV values of galvanic cells and the Volta potentials of the corresponding metal pairs. But through terms and it also explains why OCV values depend on solution composition. All parameters of this equation can be measured experimentally. 

The ionic potentials can be experimentally determined either with the use of galvanic cells containing interfaces of the type in Scheme 7 or electroanalytically, using for instance, polarography, voltammetry, or chronopotentiometry. The values of and Aj f, obtained with the use of electrochemical methods for the water-1,2-dichloroethane, water-dichloromethane, water-acetophenone, water-methyl-isobutyl ketone, o-nitrotol-uene, and chloroform systems, and recently for 2-heptanone and 2-octanone [43] systems, have been published. These data are listed in many papers [1-10,14,37]. The most probable values for a few ions in water-nitrobenzene and water-1,2-dichloroethane systems are presented in Table 1. 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

The EMF of a galvanic cell is a thermodynamic equilibrium quatity. Thus, the potential of a cell must be measured under equilibrium conditions, i.e. without current flow. The measured EMF must be compensated by a known external potential difference. The measurement of the EMF of a cell is thus based on determination of a potential difference that exactly compensates the measured potential difference so that no current passes. This is easily achieved by the Poggendorf compensation method (see Fig. 3.13). 

In this section, you learned that you can calculate cell potentials by using tables of half-cell potentials. The half-cell potential for a reduction half-reaction is called a reduction potential. The half-cell potential for an oxidation half-reaction is called an oxidation potential. Standard half-cell potentials are written as reduction potentials. The values of standard reduction potentials for half-reactions are relative to the reduction potential of the standard hydrogen electrode. You used standard reduction potentials to calculate standard cell potentials for galvanic cells. You learned two methods of calculating standard cell potentials. One method is to subtract the standard reduction potential of the anode from the standard reduction potential of the cathode. The other method is to add the standard reduction potential of the cathode and the standard oxidation potential of the anode. In the next section, you will learn about a different type of cell, called an electrolytic cell. 

The voltage we measure is characteristic of the metals we use. As an additional example, unit activity solutions of CuCE and AgCl with copper and silver electrodes, respectively, give a potential difference of about 0.45 V. We could continue with this type of measurement for aU the different anode-cathode combinations, but the number of galvanic cells needed would be very large. Fortunately, the half-reactions for most metals have been calculated relative to a standard reference electrode, which is arbitrarily selected as the reduction of hydrogen ... 

Predict the standard cell potential and calculate the standard free energy for the following galvanic cells (the standard potentials of these cells were obtained in Exercise 12.19). 

Electrochemical cells may be one of two types. Should a current spontaneously flow on connecting the electrodes via a conductor, the cell is a galvanic cell. An electrolytic cell is one in which reactions occur when an external voltage greater than the reversible potential of the cell is applied. Simple examples involving copper are given in Figure 1. It is the electrolytic cell which is of interest in the electrodeposition of metals. 

Here we investigate some of the properties of galvanic cells, cells used to produce an electric potential. Luigi Galvani discovered the first such cell by accident in 1791. Following Galvani s discovery, Alessandro Volta developed a practical cell in 1800, and it was Volta s cell that led to the work of Davy and Faraday. 

Consider a galvanic cell with two Cu electrodes in contact with 0.100 and 0.00100 M solutions of Cu (N03)2. The standard potential of the cell is E° = 0.000 V. The half-cell and overall cell reactions are ... 

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. 

It follows that the contact potential between the two metals (difference between the work functions) is the principal factor determining the e.m.f. of galvanic cells consisting of two metals it is generally decidedly larger than the difference between the two electrode-electrolyte potentials, V -Vs, and VP— Vs. 

The relationship between the molar - Gibbs energy change (AG) of - cell reaction and the - potential of the cell reaction ( Ceii) is given under the entry - galvanic cell. 

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