Generalized cell reaction

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

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 equilibrium constant of the more general cell reaction ... 

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... 

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 ... 

For general cell reaction, the free energy is expressed as ... 

As pointed out previously, the value of the standard cell voltage, E°, is a measure of the spontaneity of a cell reaction. In Chapter 17, we showed that the standard free energy change, AG°, is a general criterion for reaction spontaneity. As you might suppose, these two quantities have a simple relation to one another and to the equilibrium constant, K, for the cell reaction. 

The approach used in Example 18.5 to find the number of moles of electrons transferred, n, is generally useful. What you do is to break down the equation for the cell reaction into two half-equations, oxidation and reduction. The quantity n is the number of electrons appearing in either half-equation. 

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 ... 

Before dealing with various important applications of the electrochemical series with some practical examples, a break is given here to present a more detailed elaboration on the hydrogen electrode, reference electrodes, and some of the theoretical and general aspects pertaining to electrode potentials and free energy changes involved with cell reactions. 

The right-hand side of this equation expresses the affinity of reaction (3.1.42). In general, for a cell reaction proceeding according to a given stoichiometric equation with stoichiometric factors v, and with transfer of charge equal to nF, that... 

It should be noted that all terms concerning the electrons in the metals as well as those connected with the metals not directly participating in the cell reaction (Pt) have disappeared from the final Eq. (3.1.49). This result is of general significance, i.e. the EMFs of cell reactions involving oxidation-reduction processes do not depend on the nature of the metals where those reactions take place. The situation is, of course, different in the case of a metal directly participating in the cell reaction (for example, silver in the above case). 

For cell reactions in general, both approaches yield the equation for the EMF ... 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

In a series of papers, Jin and Bethke (2002 2003 2005 2007) and Jin (2007) derived a generalized rate expression describing microbial respiration and fermentation. They account in their rate model for an electron-donating half-cell reaction,... 

Electrode and therefore cell potentials are very important analytically as their magnitudes are determined by the activities of the reactants and products involved in the electrode reactions. The relation between such activities and the electrode potential is given by the Nernst equation. For a general half-cell reaction written as a reduction, i.e. aA + bB +. .. ne = xX + yY +. . ., the equation is of the form... 

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. 

In general, biotransformation reactions are beneficial in that they facilitate the elimination of xenobiotics from pulmonary tissues. Sometimes, however, the enzymes convert a harmless substance into a reactive form. For example, CYP-mediated oxidation often results in the generation of more reactive intermediates. Thus, many compounds that elicit toxic injury to the lung are not intrinsically pneumotoxic but cause damage to target cells following metabolic activation. A classic example of this is the activation of benzo(a)pyrene, which is a constituent of tobacco smoke and combustion products, and is... 

In general, then, reaction and cell multiplication will slow down either by depletion of A (famine) or by build up of R (environmental pollution). 

Practically in every general chemistry textbook, one can find a table presenting the Standard (Reduction) Potentials in aqueous solution at 25 °C, sometimes in two parts, indicating the reaction condition acidic solution and basic solution. In most cases, there is another table titled Standard Chemical Thermodynamic Properties (or Selected Thermodynamic Values). The former table is referred to in a chapter devoted to Electrochemistry (or Oxidation - Reduction Reactions), while a reference to the latter one can be found in a chapter dealing with Chemical Thermodynamics (or Chemical Equilibria). It is seldom indicated that the two types of tables contain redundant information since the standard potential values of a cell reaction ( n) can be calculated from the standard molar free (Gibbs) energy change (AG" for the same reaction with a simple relationship... 

The Nernst equation allows calculation of the cell em/under nonstandard conditions, E. If the cell reaction is given by a general equation aA+bB+... +/ e=AX+jY+... 

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