The Generalized Cell Reaction

It is useful to establish a more generalized representation for the electrochemical cell reaction as follows  

The abbreviated cell representation for the generalized reaction is shown in Fig. 2.4. The reduced species on the left side of the overall reaction (M) and its associated ion (Mm+) are identified as the left-hand 

If reaction 2.54 occurs spontaneously from left to right, then  

In this relationship, n is the number of moles of electrons transferred per unit of the reaction (i.e., per x moles of M etc.). 

This convention and additional terminology and relationships are summarized in Table 2.2. 

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This form of cell is shown schematically in Fig. 9.24. The anolyte and catholyte are different redox solutions which flow or are pumped past inert electrodes. The cell is constructed of two compartments separated by an anion-selective semi-permeable membrane. The spent solutions are retained in storage tanks and the whole process is reversed during charge. The general cell reaction is thus... 

The reversible cell voltage is concentration dependent, just as the redox potential is. We will consider the general cell reaction... 

Tn modern notation, the Nernst equation for the general cell reaction rR vpP, in which n moles of electrons are transferred, is w ritten... 

Cell growth and metabolic activities are similarly described as a simple chemical reaction. It is also necessary to establish a definite formula for dry cell matter. The elemental composition of certain strains of microorganism is defined by an empirical formula CHaO/3Ns. The general biochemical reaction for biomass production is based on consumption of organic substrate, as shown below. Substrate oxidation is simplified in the following biochemical oxidation ... 

Because the overall cell reaction is exothermic, the value of the cell emf decreases with increasing temperature, so the temperature is generally kept relatively low at about 200 °C. The cell emf is 1.23 V at this temperature. 

The Nemst equation can be derived by considering a general cell reaction with A, B,. .. reactants and M,N,. .. products ... 

The standard potential applies to a half-cell when all the reactants are present at unit activity that is, when the solution species are near a concentration of 1 molar. The actual half-cell potential is a function of the solution concentrations and is related to these and to the standard potential E° by the Nemst equation. The Nemst equation for the generalized half-reaction... 

Let n be the number of faradays of charge or mole of a substance participating in a reaction, and let the general reaction be represented by the half-cell reaction of Zn as follows ... 

The equilibrium constant of the more general cell reaction ... 

To determine the potential of a half-cell reaction when the reactants and products are not at unit activity, the Nemst equation is used. Consider again the general half-cell reaction given by Equation (26.13). When this reaction is combined with the standard hydrogen electrode reaction (Equation (26.20)), the overall cell reaction is... 

However, concerns about the toxicity of cadmium have accelerated the replacement of these batteries by nickel-metal hydride batteries, described in Section 9.3.5. In nickel-cadmium (nicad) batteries, the anode is cadmium and the cathode is an unstable nickel oxyhydroxide, formed in the unusual conditions found in the cell, and written variously as Ni(OH)3 or NiO(OH). It is generally formed together with stable nickel hydroxide, Ni(OH)2. The electrolyte is NaOH or KOH. The anode and cathode are assembled in a roll separated by a cellulose separator containing the electrolyte. The cathode/separator/anode roll is contained in a nickel-plated stainless steel can (Figure 9.10). The cell voltage is 1.3 V but the working voltage is usually nearer to 1.2 V. The schematic cell reactions are as follows. 

The positive electrode, the cathode, is similar to that in nicad cells and consists of a mixture of NiO(OH)/Ni(OH)3 and Ni(OH)2. An alloy that supports hydride formation replaces the cadmium as the negative anode. The alloy most commonly used is derived from LaNis, in which a mixture of other lanthanides replaces the lanthanum, and a nickel-rich alloy replaces the nickel, to give a general formula LnA/5. The anode is composed of an agglomeration of alloy powder. A small amount of potassium hydroxide is added as an electrolyte. The cell voltage is 1.3 V, making these cells suitable for the direct replacement of nicad batteries. The cell construction is identical to that of the nicad cell (Figure 9.10), with the cadmium replaced by metal hydride. The approximate cell reactions are as follows. 

Potentiometric sensors can be classified based on whether the electrode is inert or active. An inert electrode does not participate in the half-cell reaction and merely provides the surface for the electron transfer or provides a catalytic surface for the reaction. However, an active electrode is either an ion donor or acceptor in the reaction. In general, there are three types of active electrodes the metal/metal ion, the metal/insoluble salt or oxide, and metal/metal chelate electrodes. 

The potential difference is closely related to the difference of the electrochemical potential based on the electrochemical affinity. If we could measure A(p directly, we could organize the table of electromotive forces based on the Galvani potential difference. However, A

reference electrode to measure the half cell potential at an electrode. When a certain electrcxle is coupled with a reference electrode, then the electromotive force can be measured. Since we usually use some reference electrodes as standards, the electromotive force is defined as the equilibrium potential of the reaction. The table was made in such a way and the hydrogen reference electrode was used to measure and calculate potentials for the half cell reactions. 

Some Results for More General and Realistic Situations. Next let us consider the removal of some of the approximations inherent in the foregoing model. For the small-signal flat-band case, it turns out that the half-cell reaction resistance... 

Although thiolate-surface complexes form at electrodes [58] and electrode potential measurements under reversible conditions are not usually possible, a great deal of useful information has been obtained by various electrochemical methods [35,36]. However, thermochemical properties of sulfur-centered radicals in aqueous solution have generally been obtained from studies of redox equilibria, such as those by BonifSeic and Asmus [52] and Surdhar and Armstrong [59], by methods described in Wardman [60]. In this section it will be convenient to consider sulhdes, disulfides, and sufhydryl compounds in that order. Afso alkyl and aryl compounds will be treated separately. Reduction potentials will be identified by placing the reactants and products of the half cell reaction in parenthesis [60], for example, (RS, H /RSH) is the reduction potential for ... 

It is generally accepted that the basic cell reaction for the zinc/mercuric oxide cell is... 

The Nernst equation expresses the emf of a cell in terms of activities of products and reactants taking place in the cell reaction. Consider a general cell reaction ... 

In general the spontaneous cell reaction that occurs when the two electrodes of any cell are shortcircuited may be established using the protocol set out at the beginning of the present section, and tables of electrode potentials, to decide which electrode is positively charged and which negatively charged. Electron flow in the external current will always be from the negative to the positive electrode so that an oxidation process will occur at the former and a reduction at the latter. 

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