Redox chemistry equilibrium constant

Undoubtedly one of the most used LFER in transition metal chemistry involves electron transfer rate constants and associated equilibrium constants in outer sphere redox reactions. These are an unusual class of reactions in chemistry since bonds are only stretched or contracted in the formation of the activated complex. They therefore lend themselves well to theoretical treatment. We shall have more to say about these reactions in Chap. 5. It is sufficient here to state the simple form of the LFER with an example (Fig. 2.8). For the reaction... 

Figure 8.29 Sequence of redox equilibria mediated by biologic activity. From W. Stumm and J. J. Morgan (1981), Aquatic Chemistry, copyright 1981 by John Wiley and Sons. Reprinted by permission of John Wiley Sons. The various equilibrium constants are listed in table 8.24.

Problems in this chapter include some brainbusters designed to bring together your knowledge of electrochemistry, chemical equilibrium, solubility, complex formation, and acid-base chemistry. They require you to find the equilibrium constant for a reaction that occurs in only one half-cell. The reaction of interest is not the net cell reaction and is not a redox reaction. Here is a good approach ... 

The hydrated electron, if the major reducing species in water. A number of its properties are important either in understanding or measuring its kinetic behavior in radiolysis. Such properties are the molar extinction coefficient, the charge, the equilibrium constant for interconversion with H atoms, the hydration energy, the redox potential, the reaction radius, and the diffusion constant. Measured or estimated values for these quantities can be found in the literature. The rate constants for the reaction of Bag with other products of water radiolysis are in many cases diffusion controlled. These rate constants for reactions between the transient species in aqueous radiolysis are essential for testing the "diffusion from spurs" model of aqueous radiation chemistry. 

As you know, oxidation-reduction reactions can involve molecules, ions, free atoms, or combinations of all three. To make it easier to discuss redox reactions without constantly specilying the kind of particle involved, chemists use the term species. In chemistry, a species is any kind of chemical unit involved in a process. For example, a solution of sugar in water contains two major species. In the equilibrium equation NH3 + H2O NH/ + OH , there are four species the two molecules NH3 and H2O and the two ions NH/ and OH. ... 

Equilibrium and rate constants for the hydrolysis and chloride complexation of Fe(III) and Fe(II) ions are necessary in a detailed study of iron redox chemistry. Table I lists an internally consistent set of values for the relevant equilibrium constants. Their accuracy is discussed later in the context of a brief sensitivity analysis of the data. The rates of iron hydrolysis and chloride complexation reactions are also mentioned. 

Many of the observed attributes of enzymes arise by natural selection in order to help the host organism survive and reproduce. Benner et al. have proposed that one such attribute, the stereospecificities of dehydrogenases, has functional significance based on stereochemical arguments (18, 79). The central features of their functional model can be summarized as follows. The stereospecificities of dehydrogenases acting on alcohols are correlated with the equilibrium constant for the alcohol-carbonyl redox reaction as listed in Table IV (18). Enzymes catalyzing reactions where the eq is <10 " ilf transfer the pro-S proton from NADH when is >10"" Af, the pro-R proton is transferred. Thus the more readily reduced carbonyl compounds use the pro-R proton, but the more difficult to reduce carbonyl compounds use the pro-S proton. The proposed correlation is restricted to simple aldehydes and ketones (i.e., without additional chemistry that would influence the equilibrium constant, such as cyclizations of polyols or formation of lactones). The natural substrate of the enzyme must be well... 

In the field of radioactive waste management, the hazardous material consists to a large extent of actinides and fission and activation products. Two isotopes of nickel ( i and Ni) pertain to the latter category. The scientific literature on thermodynamic data, mainly on equilibrium constants and redox potentials in aqueous solution, has been contradictory in a number of cases, especially in actinide chemistry. A critical and comprehensive review of the available literature is necessary in order to establish a reliable thermoehemical database that fulfils the requirements for rigorous modelling of the behaviour of the actinides, fission and activation products in the environment. 

Environment and Life, RSC Publishing, p. i60 As this cytoplasmic (in cell) organic chemistry was reductive of necessity, because the ingredients were made of the oxides of carbon, it was inevitable that oxidizing compounds, which became oxygen, would be released. There followed in the environment an unavoidable and predictable sequence of the oxidation of minerals and non-metal elements in solution, limited by diffusion but generally following sequentially equilibrium constants, redox potentials. ... 

The relationship between the equilibrium constant of a redox reaction and the EMF of the corresponding galvanic cell (potential cell) is the subject of Nemst s law. It is of tremendous interest in analytical chemistry from both theoretical and practical points of view. 

A converse exists to the calculation of equilibrium constants from the halfreduction potentials It is the possibility to obtain the unknown redox potentials of some couples. In order to achieve it, a redox equilibrium between two couples is investigated. The equilibrium constant is determined, if the standard redox potential of one of both couples is already known. The value of the other (unknown) is immediately deduced. This strategy is, of course, of great importance in physical and analytical chemistries. It is in this way that the standard potentials of slow electrochemical systems (see electrochemistry), in particular, those of organic redox couples, have been determined. 

This woik was prepared under the sponsorship of the Commission on Electrochemical Data of the Section of Analytical Chemistry of the International Union of Pure and Applied Chemistry. It is included in a general program of the Section of Analytical Chemistry. The Commission on Equilibrium Data, for example publishes elsewhere solubility products and stability constants of metal-ion complexes. From these constants and from the values reported here, the values of the potentials of many redox systems not reported in the present Tables can be computed. 

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