Thermodynamics of Redox Reactions

Which of the following metals will react with (that is, be oxidized by) HNO3, but not with HCl Cu, Zn, Ag  

Our next step is to see how is related to thermodynamic quantities such as AG° and K. In a galvanic cell, chemical energy is converted to electrical energy to do electrical work such as running an electric motor. Electrical energy, in this case, is the product of the emf of the cell and the total electrical charge (in coulombs) that passes through the cell  

The total charge is determined by the number of electrons that pass through the cell, so we have 

In general, it is more convenient to express the total charge in molar quantities. The charge of one mole of electrons is called the Faraday constant (F), after the English chemist and physicist Michael Faraday, where 

Therefore, the total charge can now be expressed as nF, where n is the number of moles of electrons exchanged between the oxidizing agent and reducing agent in the overall redox equation for the electrochemical process. 

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Evaluating the Thermodynamics of Redox Reactions under Environmental Conditions... 

Before we proceed to evaluate the thermodynamics of redox reactions at environmental conditions, we need to make a few remarks on microbial processes that determine the redox conditions in the environment. 

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

A particular advantage of half-reaction approach is that it leads naturally to the discussion of the thermodynamics of redox reactions in terms of electrode potential. 

J. Burgess (1999) Ions in Solution Basic Principles of Chemical Interaction, 2nd edn, Horwood Publishing, Westergate - An excellent introduction to the properties of ions in aqueous solutions including treatment of the thermodynamics of redox reactions. 

We now embark on an investigation of the thermodynamics of redox reactions. Our ultimate goal is a description of electron transfer in plant photosynthesis and in the last stages of the oxidative breakdown of glucose. However, before we can understand these complex processes, we must examine a very much simpler system with a more controllable environment where precise measurements can be made. That is, we must consider electron transfer in an electrochemical cell, a device that consists of two electronic conductors (metal or graphite, for instance) dipping into an electrolyte (an ionic conductor), which may be a solution, a liquid, or a solid. 

Depending on the nature of the class, the instructor may wish to spend more time with the basics, such as the mass balance concept, chemical equilibria, and simple transport scenarios more advanced material, such as transient well dynamics, superposition, temperature dependencies, activity coefficients, the thermodynamics of redox reactions, and Monod kinetics, may be omitted. Similarly, by excluding Chapter 4, an instructor can use the text for a course focused only on the water environment. In the case of a more advanced class, the instructor is encomaged to expand on the material suggested additions include more rigorous derivation of the transport equations, discussions of chemical reaction mechanisms, introduction of quantitative models for atmospheric chemical transformations, use of computer software for more complex chemical equilibrium problems and groundwater transport simulations, and inclusion of case studies. References are provided with each chapter to assist the more advanced student in seeking additional material. 

Unlike the reactions that we have already considered, the equilibrium position of a redox reaction is rarely expressed by an equilibrium constant. Since redox reactions involve the transfer of electrons from a reducing agent to an oxidizing agent, it is convenient to consider the thermodynamics of the reaction in terms of the electron. 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

What Do We Need to Know Already This chapter extends the thermodynamic discussion presented in Chapter 7. In particular, it builds on the concept of Gibbs free energy (Section 7.12), its relation to maximum nonexpansion work (Section 7.14), and the dependence of the reaction Gibbs free energy on the reaction quotient (Section 9.3). For a review of redox reactions, see Section K. To prepare for the quantitative treatment of electrolysis, review stoichiometry in Section L. 

A detailed discussion of redox reactions must wait until Chapter 19, after we explore the nature of the atom, periodic properties of the elements, and thermodynamics. For now, we focus on only a few types of redox reactions that are common and relatively simple. 

The equilibrium model, despite its limitations, in many ways provides a useful if occasionally abstract description of the chemical states of natural waters. However, if used to predict the state of redox reactions, especially at low temperature, the model is likely to fail. This shortcoming does not result from any error in formulating the thermodynamic model. Instead, it arises from the fact that redox reactions in natural waters proceed at such slow rates that they commonly remain far from equilibrium. 

The so-called midpoint potential, Em, of protein-bound [Fe-S] clusters controls both the kinetics and thermodynamics of their reactions. Em may depend on the protein chain s polarity in the vicinity of the metal-sulfur cluster and also upon the bulk solvent accessibility at the site. It is known that nucleotide binding to nitrogenase s Fe-protein, for instance, results in a lowering of the redox potential of its [4Fe-4S] cluster by over 100 mV. This is thought to be essential for electron transfer to MoFe-protein for substrate reduction.11 3... 

The thermodynamic feasibility of redox reactions at the semiconductor-electrolyte interface can be assessed from thermodynamic considerations. Since typical redox potentials for many redox couples encountered in electrolytes of natural or technical systems often lie between the band potentials of typical semiconductors, many electron transfer reactions are (thermodynamically) feasible (Pichat and Fox, 1988). With the right choice of semiconductor material and pH the redox potential of the cb can be varied from 0.5 to 1.5 V and that of the vb from 1 to more than 3.5 V (see Fig. 10.4). 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

Owing to the allotropic forms of oxygen, to its various redox states, and related chemical species that are thermodynamically stable or exist for kinetic reasons, a lot of redox reactions are usually described. However, many of them are not really important for the common works, particularly for those in solutions consequently, only some of them have been described here. The publications cited earlier can be searched for data useful for the calculation of Gibbs energy or potentials of particular reactions. 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

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