Electromotive force electrochemical potential difference

To measure the electrode potential of a test electrodes, M, we usually use an electrochemical cell consisting of test electrode M and reference electrode both of which are coimected by a metal lead of A and A" of the same metallic conductor to a potentiometer outside the cell as shown in Fig. 4-23. The difference in the electrode potential, E, measured between the test electrode and the reference electrode, conventionally called the electromotive force, equals the difference in the Fermi level of electrons between the two electrodes E = - 8j(M) - EjtM )... 

Alternatively, a potentiometer connected across the open circuit is found to register a potential difference that renders the Cu electrode positive relative to the Zn electrode the magnitude of the open—circuit electrochemical potential difference, which later will be shown to be identical with the electromotive force (emf), symbolized by fS, is approximately 1.1 volts. 

An electrical potential difference between the electrodes of an electrochemical cell (called the cell potential) causes a flow of electrons in the circuit that connects those electrodes and therefore produces electrical work. If the cell operates under reversible conditions and at constant composition, the work produced reaches a maximum value and, at constant temperature and pressure, can be identified with the Gibbs energy change of the net chemical process that occurs at the electrodes [180,316]. This is only achieved when the cell potential is balanced by the potential of an external source, so that the net current is zero. The value of this potential is known as the zero-current cell potential or the electromotive force (emf) of the cell, and it is represented by E. The relationship between E and the reaction Gibbs energy is given by... 

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 ... 

Let us consider the general electrochemical cell shown in Figure 5.2. The potential difference across the electrochemical cell, denoted , is a measurable quantity called the electromotive force (EMF) of the cell. The potential difference in Figure 5.2 is made up of four contributions since there are four phase boundaries in this cell two metal-solution interphases and two metal-metal interfaces. The cell in Figure 5.2 can be represented schematically as Pt/M7S/M/Pt. 

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 ... 

An electrochemical cell generates a potential difference E. (The symbol E, commonly used in electrochemistry, refers to electromotive force, an archaic term for potential difference.) The electrical work done when n moles of electrons is passed by the cell can be found using Eq. (15-1), w = -nFE. It can be shown that the electrical work done by an electrochemical cell, at constant temperature and pressure, is equal to the change in Gibbs free energy of the cell components,... 

This process leads to the development of a voltage difference between anode and cathode and when the external circuit is open , this potential difference is the electromotive force (the e.m.f.) of the cell. When there is an electrical load in the external circuit, for example a motor, then a current will flow and electrochemical energy is converted into mechanical energy. 

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... 

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... 

One now defines the electromotive force (emf) fl by Ffl - 2(f2 — f ), where F is the Faradav (96,487 coulombs, the numerical value of the charge associated with one mole of electrons). More generally, the emf is related to the difference in the electrochemical potential for electrons that develops under open circuit conditions between the two electrodes of the cell under study. For the example considered here Ffl - (f2" - " ) + This may be revamped by noting that the... 

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. 

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-... 

In the context of electrochemical cells, this potential difference is generally referred to as the electromotive force, E, or emf. This term will be used in the following discussion of cells. 

The potential difference between the electrodes in a working electrochemical cell is called the cell potential. The cell potential is not a constant and changes with time as the cell reaction proceeds. Thus the cell potential is a potential difference measured under non-equilibrium conditions as electric current is drawn from the cell. Electromotive force is the zero-current cel potential and corresponds to the potential difference of the cell when the cell (not the cell reaction) is at equilibrium. Infinitesimally small changes from this equilibrium are reversible with constant concentration and, consequently, it is possible to relate emf to thermodynamic properties. 

During his Leipzig period, Nernst performed a series of electrochemical studies from which, at the age of twenty-five, he arrived at his well-known equations. These equations described the concentration dependence of the potential difference of galvanic cells, such as batteries, and were of both great theoretical and practical importance. Nernst started with the investigation of the diffusion of electrolytes in one solution. Then he turned to the diffusion at the boundary between two solutions with different electrolyte concentrations he determined that the osmotic pressure difference would result in an electric potential difference or electromotive force (emf). Next he divided both solutions into two concentration half-cells, connected to each other by a liquid junction, and measured the emf via electrodes dipped into both solutions. The data supported his first equation where the... 

The combination of electrode compartments into an electrochemical cell makes it possible to measure the series of the Galvani potential differences as the electromotive force (emf) of the cell. 

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