Galvanic cell. EMF

We first set up the general expression for the dependence of the galvanic cell emfs on the activity of the components participating in the cell reaction. Starting with... 

As for thermodynamic measurements using liquid electrolytes, galvanic cells with solid ion conductors are widely applied to study thermodynamic properties of solids and melts. These measurements are based on the determination of galvanic cell emf (Chap. 1) when the reference electrode potential is known. In a simplest case, when A " cation-conducting electrolyte is employed and the RE comprises metal A, the cells... 

A problem with compiling a list of standard potentials is that we know only the overall emf of the cell, not the contribution of a single electrode. A voltmeter placed between the two electrodes of a galvanic cell measures the difference of their potentials, not the individual values. To provide numerical values for individual standard potentials, we arbitrarily set the standard potential of one particular electrode, the hydrogen electrode, equal to zero at all temperatures ... 

We saw in Section 9.3 that the standard reaction Gibbs free energy, AGr°, is related to the equilibrium constant of the reaction by AGr° = —RT In K. In this chapter, we have seen that the standard reaction Gibbs free energy is related to the standard emf of a galvanic cell by AGr° = —nFE°, with n a pure number. When we combine the two equations, we get... 

Each electrode compartment of a galvanic cell contains a silver electrode and 10.0 ml, of 0.10 M AgN03(aq) they are connected by a salt bridge. You now add 10.0 ml. of 0.10 M NaCl(aq) to the left-hand electrode compartment. Almost all the silver precipitates as silver chloride but a little remains in solution as a saturated solution of AgCI. The measured emf is E = +0.42 V. What is the concentration of Ag+ in the saturated solution ... 

Predict the standard emf of each of the following galvanic cells ... 

Predict the standard cell emf and calculate the standard reaction Gibbs free energy for galvanic cells having the following cell reactions ... 

C (298.15 K) and 1 bar. standard cell potential See standard emf. standard emf ( °) The emf when the concentration of each solute taking part in the cell reaction is 1 mol-L 1 (strictly, unit activity) and all the gases are at 1 bar. The standard emf of a galvanic cell is the difference between its two standard potentials E° = E°(cathode) — °(anode). 

Knowledge of the Volta potential of a metal/solution interface is relevant to the interpretation of the absolute electrode potential. According to the modem view, the relative electrode potential (i.e., the emf of a galvanic cell) measures the value of the energy of the electrons at the Fermi level of the given metal electrode relative to the metal of the reference electrode. On the other hand, considered separately, the absolute value of the electrode potential measures the work done in transferring an electron from a metal surrounded by a macroscopic layer of solution to a point in a vacuum outside the solotion. ... 

Equilibrium potentials can be calculated thermodynamically (for more details, see Chapter 3) when the corresponding electrode reaction is known precisely, even when they cannot be reached experimentally (i.e., when the electrode potential is nonequilibrium despite the fact that the current is practically zero). The open-circuit voltage of any galvanic cell where at least one of the two electrodes has an nonequilibrium open-circuit potential will also be nonequilibrium. Particularly in thermodynamic calculations, the term EMF is often used for measured or calculated equilibrium OCV values. 

This equation links the EMF of a galvanic cell to the Gibbs energy change of the overall current-producing reaction. It is one of the most important equations in the thermodynamics of electrochemical systems. It follows directly from the first law of thermodynamics, since nF% is the maximum value of useful (electrical) work of the system in which the reaction considered takes place. According to the basic laws of thermodynamics, this work is equal to -AG . 

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... 

Direct measurements of solute activity are based on studies of the equilibria in which a given substance is involved. The parameters of these equilibria (the distribution coefficients, equilibrium constants, and EMF of galvanic cells) are determined at different concentrations. Then these data are extrapolated to very low concentrations, where the activity coincides with concentration and the activity coefficient becomes unity. 

This can be accomplished by applying an electrical potential in the external circuit in such a manner that an emf occurs in opposition to that of the galvanic cell. The opposing emf is varied by means of a potentiometer until the current flow from the cell is essentially zero. Under these conditions, the cell may very well approach reversibility. This is readily tested by changing the direction of the current and allowing an infinitesimally small current flow in the opposite direction. If the cell is reversible, the cell reaction will proceed in the reverse direction with the same efficiency as in the forward direction. For a reversible reaction... 

The transition to the reversible emf of a galvanic cell is now quite straightforward. Combining the last equation with the equation AG = -n F E, the Nemst equation could be obtained ... 

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

In case (a), the galvanic cell under non-faradaic conditions, one obtains an emf of 0.34 - (-0.76) = 1.10 V across the Cu electrode ( + pole) and the Zn electrode (- pole). In case (b), the galvanic cell with internal electrolysis, the electrical current flows in the same direction as in case (a) and the electrical energy thus delivered results from the chemical conversion represented by the following half-reactions and total reaction, repsectively ... 

In case (c), a voltage opposite to and higher than the emf of the galvanic cell is imposed as a consequence, the current flow and hence also the electrochemical reactions are reversed, which means that half-reaction 1 becomes an anodic oxidation and half-reaction 2 is a cathodic reduction, so that Zn is deposited instead of Cu. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

A description of an electrolytic cell has already been given under cell features (Section 1.3.2, Fig. 1.1c). Another example is the cell with static inert electrodes (Pt) shown in Fig. 3.1 where an applied voltage (Eappl) allows a current to pass that causes the evolution of Cl2 gas at the anode and the precipitation of Zn metal on the cathode. As a consequence, a galvanic cell, (Pt)Zn 2 ZnCl2 Cl2 iPt+, occurs whose emf counteracts the voltage applied this counter- or back-emf can be calculated with the Nernst equation to be... 

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. 

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