Voltage, of electrochemical cell

The usefulness of Table 12-1 is clear. Qualitative predictions of reactions can be made with the aid of the ordered list of half-reactions. Think how the value of the list would be magnified if we had a quantitative measure of electron losing tendencies. The voltages of electrochemical cells furnish such a quantitative measure. 

Other measurements of AfG involve measuring AG for equilibrium processes, such as the measurement of equilibrium constants, reversible voltages of electrochemical cells, and phase equilibrium measurements. These methods especially come into play in the measurement of Afand AfG for ions in solution, which are processes that we will now consider. 

Combine half-cell reactions and their standard reduction potentials to obtain the overall reactions and standard voltages of electrochemical cells (Section 17.2, problems 13-16). 

Generally, the available cell voltage of electrochemical cells depends on the thermodynamics of the two electrode reactions in the prevailing electrolyte, hence the difference in the electrode potentials, and is confined, according to the series of electrochemical potentials, to a few volts [9]. According to the individual electrode potentials of the reaction (by lUPAC standard zero volt in the series of... 

The Nernst equation is very useful for estimating the voltage of electrochemical cells at nonstandard conditions of concentration or pressure. But despite the fact that the Nernst equation contains temperature, T, as a variable, it has limited use at temperatures other than 25°G, the common reference temperature. That s because E° itself varies with temperature. We can estimate how varies with temperature by considering the following two expressions ... 

By applying a specific voltage on the reference electrode it is possible to compensate non-linearities during exposure to higher gas concentrations and/or to increase the sensitivity and selectivity to different gas species. The fifetime of this kind of electrochemical cell is limited by the consumption of electrode material. 

Thus in practise, we can recognise two types of electrochemical cell. These are called electrolytic cells and galvanic cells. An electrolytic cell uses an external power source (i. e. a voltage source) to move the electrons and perform the electrolysis. The aim of the electrolysis may be to generate a species in solution, produce a precipitate, produce... 

The potential of this electrode is defined (Section 5.2) as the voltage of the cell Pt H2(l atm) H (a = 1) M M, where the left-hand electrode, = 0, is the normal hydrogen reference electrode (described in Section 5.6). In Chapter 6, we derive the Nemst equation on the basis of the electrochemical kinetics. Here we use a simplified approach and consider that Eq. (5.9) can be used to determine the potential E of the M/M electrode as a function of the activity of the products and reactants in the equilibrium equation (5.10). Since in reaction (5.10) there are two reactants, and e, and only one product of reaction, M, Eq. (5.9) yields... 

Figure 19.18. Data of electrochemical fuel cells, (a) Processes in a fuel cell based on the reaction between hydrogen and oxygen, (b) Voltage-current characteristic of a hydrogen-air fuel cell operating at 125°C with phosphoric acid electrolyte [Adlharl, in Energy Technology Handbook (Considine, Ed.), 1977, p. 4.61). (c) Theoretical voltages of fuel cell reactions over a range of temperatures, (d) Major electrochemical systems for fuel cells (Adlharl, in Considine, loc. cit., 1977, p. 4.62).

Example 10 illustrates how thermochemical data for aqueous ions may be obtained from measurements in electrochemical cells. The problem of measuring cell potentials in the standard state, which is a hypothetical state, will be discussed in section 10.12. The temperature variation of the voltage of such cells would provide AHJ of aqueous ions, through the use of Eq. (48). 

In other words, E is the electromotive force (EMF) of the reaction cell, where the voltage of the cell is unique for each reaction couple. Spontaneous processes have a negative free energy consequently, an electrochemical process will have a positive EMF. 

Standard Cell. The electrochemical cell used as a voltage standard is the Weston cell, shown in Fig. 13. The voltage of this cell changes only slightly with temperature and is given in absolute volts by... 

Apply the Nernst equation to calculate the voltage of a cell in which reactants and products are not in their standard states and to calculate the value of an equilibrium constant from the voltage of an electrochemical cell (Section 17.3, problems 27-38). 

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. 

All electrochemical cells involve two half-reactions an oxidation half-reaction in which electrons are released, and a reduction half-reaction in which electrons are taken up. The net voltage of the cell, the only quantity that can be measured experimentally, is the algebraic sum of the potentials for the two half-reactions. Each potential is a measure of the relative ability of a given half-reaction to occur. However, since the potential for a half-reaction cannot be measured directly, numerical values for half-reaction potentials are arbitrary and must be based on a reference potential. The hydrogen half-reaction serves as the reference for all electrochemical potentials. 

As in conventional chemical reactors, similarity criteria are also employed in the scale-up of electrochemical reactors. Apart from similarities such as geometric similarity, kinematic similarity, and chemical similarity, electrical similarity is a unique criterion in the scale-up of electrochemical cells. It is defined as the condition where geometrically, kinematically, and chemically similar cells have identical cell voltages and current distributions inside the cells. It is also important to note that, although the principles of similarity criteria are the same for chemical and electrochemical reactors, suitable modifications have to be made to obtain electrochemical similarity. 

Electrode potential denotes the open-circuit voltage of a cell that consists of an electrode under study and the reference electrode. The potential of this electrode is compared with that of the reference electrode, which is identical for all electrodes to be studied. The term electrode potential will always be understood as electrode potential relative to a reference electrode. The values of electrode potential are reported as positive or negative, based on the polarity of this electrode relative to the reference electrode. In some cases when electrochemical equilibrium is not established at the electrode surface the value of the cell potential will not be the same as the equilibrium value. 

A second method to determine partial molar Gibbs energies and activities of alloy components in an alloy is the measurement of the cell voltage of the cell shown in Figure 3.7. The alloy can be formed by metallurgical alloying or, as is shown in Figure 3.7, be deposited on a substrate either electrochemically or by vapor deposition methods. 

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