Electromotive Force of Electrochemical Cells

As shown in Fig. 6-3, it is also in the same TUPAC convention that a positive electric charge flows from the left hand electrode through the electrolyte to the right hand electrode, as the cell reaction proceeds in the direction as written in Eqn. 6-3. This defines the sign of the electromotive force of electrochemical cells. 

The chaise number, n, which represents the number of electrons involved in a imit advancement of the cell reaction, is also important in the cell reactions of Eqns. 6-1 and 6-3 n = 2. 

The electromotive force of an electrochemical cell is the difference in electrode potential between the two electrodes in the cell. According to the TUPAC convention, the electromotive force is the potential of the right hand electrode referred to the potential of the left hand electrode. We consider, for example, a hydrogen-oxygen cell shown in Fig. 6—4 the cell reaction is given by Eqn. 6-1 and the cell diagram is given by Eqn. 6-5  

From the electrode reactions in equilibrium at the left hand electrode (anode) and at the right hand electrode (cathode), we obtain the real potential, a., of electrons in the two electrodes as shown in Eqns. 6-6 and 6-7  

The effect of temperature on the electromotive force can be derived from Eqn. 6-10 to obtain Eqn. 6-11  

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Karpachev SV, Pal guyev SF (1960) Electromotive force of electrochemical cell with solid electrolytes. Tr Inst Electrokhim, Ural Fil, Akad Nauk SSSR Sverdlovsk (Rus) 1 79-89... 

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 ... 

What is wrong with the following argument If the terminals of an electrochemical cell are constructed from the same metal, the chemical potential of electrons [species i in Eq. (36)] at the terminals, which depends only on T, P and concentrations, are the same. From Eq. (36), the electromotive force of the cell is therefore zero ... 

In order to make the cell current zero we need to put an electrostatic voltage Ep in the cell circuit Fig. 9.6. This electrostatic voltage is called the electromotive force of the cell. The electrochemical cell is often described by a cell diagram such as shown in Eq. 9.16 ... 

To make absolutely clear in which direction the electrochemical reaction proceeds and which is the polarity of the electrodes, diagrams of electrode systems are represented according to the following rule The electrode at which oxidation takes place is written always on the left side of the diagram, while the electrode at which reduction occurs is on the right side the electromotive force of the cell has then a positive sign. 

Guggenheim has expressed1 the electromotive force of the cell (C. 1) in terms of the thermodynamic electrochemical potentials . The final result... 

The term R0I is called the ohmic drop.2 Figure 3.3 shows the schematic potential distribution in an electrochemical cell. At the anode a potential drop Ua occurs, which is given by the sum of the equilibrium potential U and the anodic over potential r]a. An analogous situation takes place at the cathode. Note that the electromotive force of the cell is U0 = Uejj a + Uej/ C. In practical applications of Equation (3.19), one has to pay attention to the sign convention attributed to the various quantities. 

For the electrochemical reactions of charge to proceed at the two electrodes, the external voltage applied to the cell, Uch, should be higher than the electromotive force of the cell, AE, at the given H2SO4 concentration. The difference between the two voltages is called polarization of the cell (battery), A(7p. 

The voltage of an electrochemical cell is a thermodynamic order of magnitude it is the difference in potential which is established, with a current of strictly zero, between the two electrodes. The absolute value (i.e. regardless of the sign) is called the electromotive force of the cell (eml) and... 

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)... 

Electrochemical Method.—In this the value of the equilibrium constant K is calculated from the maximum work measured by means of the electromotive force of a voltaic cell (cf. Chap. XVI.). 

The electrode potential of an electrode reaction at equilibrium can be measured as the electromotive force of an electrochemical cell composed of both the reaction electrode and the normed hydrogen electrode. The potential of the reaction electrode thus measured is taken as the equilibrium potential of the electrode reaction relative to the normal hydrogen electrode. 

As shown in Figure 18, the potential is almost proportional to the logarithm of H2 concentration diluted in air. When H2 is diluted in N2, the observed potential corresponds to the electromotive force of a H2-02 fuel cell, and in fact the EMF was as large as about 1.0 V with a theoretical slope of 30 mV/decade, as shown in the same figure. It has been shown that in the case of H2 diluted in air, the following electrode reaction, i.e., electrochemical oxidation of hydrogen (2) and electrochemical reduction of oxygen (3), are important. 

The Gibbs energy change is related to some other important physical quantities, such as the equilibrium constant for a chemical reaction and the electromotive force of an electrochemical cell, by the Nemst and van t Hoff equations ... 

In corrosion the dynamic electrochemical processes are of importance and hence considerations of the consequences of perturbation of a system at equilibrium are considered. Let us consider the familiar Daniel cell consisting of copper metal in copper sulfate, and zinc metal in zinc sulfate solution. This, as depicted in Figure 1.18 gives an electromotive force of 1.1 V when there is no current flow. When a small current flows through the resistance R, the potential decreases below 1.1 V. On continued flow of current, the potential difference between the electrodes approaches a value near zero, and... 

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] ... 

The Nemst equation applies (if we neglect the activity coefficients of the ions, in keeping with PB theory) to the emf (electromotive force) of an electrochemical cell. The emf of such a cell and the surface potential of a colloidal particle are quantities of quite different kinds. It is not possible to measure colloidal particle with a potentiometer (where would we place the electrodes ), and even if we could, we have no reason to expect that it would obey the Nemst equation. We have been at pains to point out that all the experimental evidence on the n-butylam-monium vermiculite system is consistent with the surface potential being roughly constant over two decades of salt concentration. This is clearly incompatible with the Nemst equation, and so are results on the smectite clays [28], Furthermore, if the zeta potential can be related to the electrical potential difference deviations from Nemst behavior, as discussed by Hunter... 

The differences in the hydration of a solnte in H2O and D2O have been extensively stndied by measnring their thermodynamic properties, the change of free energy (AG°t), enthalpy (A//°t), and entropy (AY°t) at the transfer of 1 mol of solnte from a highly dilute solution in H2O to the same concentration in D2O under reversible conditions (mostly 25 °C and atmospheric pressure). Greyson measured the electromotive force (emf) of electrochemical cells of several alkali halides containing heavy and normal water solutions. The cell potentials had been combined with available heat of solution data to determine the entropy of transfer of the salts between the isotopic solvents. The thermodynamic properties for the transfer from H2O to D2O and the solubilities of alkali halides at 25° in H2O and D2O are shown in Table 4. 

Potentiometric sensors are based on the measurement of the voltage of a cell under equilibrium-like conditions, the measured voltage being a known function of the concentration of the analyte. Potentiometric measurements involve, in general, Nernstian responses under zero-current conditions that is, the measurement of the electromotive force of the electrochemical cell. 

This principle is used every time one measures the electromotive force of an electrochemical cell. In this case the potential measuring device determines the difference between the electrochemical potentials of electrons in two pieces of the same metal, for example, in two copper wires. The classical device for doing this measurement is a Poggendorf compensation potentiometer but a modern in-... 

The quantity U0 is known as the electromotive force of the electrochemical cell. 

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