Chemical equilibrium, oxidation-reduction

Reactions that take place consecutive to the electrode process can be studied polarographioally only in those cases in which the electrode process is reversible. In these cases the wave-heights and the wave-shape remain unaffected by the chemical processes. However, the half-wave potentials are shifted relative to the equilibrium oxidation-reduction potential, determined e.g. potentiometrically. Hence, whereas in all above examples, limiting currents were measured to determine the rate constant, it is the shifts of half-wave potentials which are measured here. First- and second-order chemical reactions will be discussed in the following. 

Like any chemical system, oxidation-reduction reactions will progress toward equilibrium. Because of this fact, the cell potential provides a way to measure equilibrium constants or free energy changes. 

Quantitative structure-chemical reactivity relationships (QSRR). Chemical reactivities involve the formation and/or cleavage of chemical bonds. Examples of chemical reactivity data are equilibrium constants, rate constants, polarographic half wave potentials and oxidation-reduction potentials. 

REDOX HALF-REACTIONS. Electron transfer reactions involve oxidation (or loss of electrons) of one component and reduction (or gain of electrons) by a second component. Therefore, a complete redox reaction can be treated as the sum of two half-reactions such that the stoichiometry and electric charge is balanced across a chemical equilibrium. For each such half-reaction, there is an associated standard potential E°. The hydrogen ion-hydrogen gas couple is ... 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

The selectivity inherent in the chemical affinity of one element or compound for another, together wiLli lliedi known sLoicldoiiieliic and llieiiiiodyiiamic behavior, permits positive identification aud analysis under many circumstances. In a somewhat opposite sense, the apparent dissociation of substances at equilibrium in chemical solution gives rise to electrically measurable valence potentials, called oxidation-reduction potentials, whose magnitude is indicative of the conceuliaiiou and composition of llie substance. Wlnle individually all llie above ellecls are unique for eaeli element or compound, many are readily masked by the presence of more reactive substances so they can be applied only to systems of known composition limits. 

Since natural waters are generally in a dynamic rather than an equilibrium condition, even the concept of a single oxidation-reduction potential characteristic of the aqueous system cannot be maintained. At best, measurement can reveal an Eh value applicable to a particular system or systems in partial chemical equilibrium and then only if the systems are electrochemically reversible at the electrode surface at a rate that is rapid compared with the electron drain or supply by way of the measuring electrode. Electrochemical reversibility can be characterized... 

A third chemical weathering mechanism that is of importance is oxidation/ reduction that involves mainly the elements carbon, iron, manganese and, of course, oxygen. An equilibrium reaction between dissolved C02 and bicarbonate ions can lead to the precipitation of ferrous iron, giving a hematite (ferric oxide) precipitate ... 

Eh is a measure of oxidation-reduction potential in the solution. The chemical reactions in the aqueous system depend on both the pH and the Eh. While pH measures the activity (or concentration) of hydrogen ions in the solution, Eh is a measure of the activity of all dissolved species. Aqueous solutions contain both oxidized and reduced species. For example, if iron is present in the solution, there is a thermodynamic equilibrium between its oxidized and reduced forms. Thus, at the redox equilibrium, the reaction is as follows ... 

The measurements of water quality parameters (oxidation-reduction potential, pH, temperature, conductivity, dissolved oxygen, and turbidity) and the collection of field screening data with field portable instruments and test kits constitute a substantial portion of field work. Field measurements, such as pH, stand on their own as definitive data used for the calculations of solubility of chemical species and chemical equilibrium in water, whereas others serve as indicators of well stabilization or guide our decision-making in the field. Table 3.8 shows the diversity of field measurement... 

As a chemical phenomenon, weathering can be viewed as the result of the tendency of the rock-water-mineral system to attain equilibrium. This occurs through the usual chemical mechanisms of dissolution and precipitation, acid-base reactions, complexation, hydrolysis, and oxidation-reduction. 

If a clear separation of time scales exists for Eqs. 4.52a and 4.52b, as compared to Eqs. 4.52c-4.52e, then the kinetics of surface oxidation-reduction can be decoupled from those of surface species detachment. For example, if Mn(II) oxidation is negligibly slow, Kb - 0 and Eqs. 4.52a and 4.52b can be solved approximately, as is described in either Section 3.4 in connection with Eqs. 3.48 and 3.49, or Section 4.3 in connection with Eq. 4.30. The first approach applies to constant [HSeO ], whereas the second one requires small deviations of the concentrations of the four chemical species in Eqs. 4.52a and 4.52b from their equilibrium values. 

Depending on the velocity of fluid flow, the thickness varies from 10 to 100 pm, and it may cover from less than 20% to more than 90% of the metal surface. Biofilms or macrofouling in seawater can cause redox reactions that initiate or accelerate corrosion. Biofilms accumulate ions, manganese and iron, in concentrations far above those in the surrounding bulk water. They can also act as a diffusion barrier. Finally, some bacteria are capable of being directly involved in the oxidation or reduction of metal ions, particularly iron and manganese. Such bacteria can shift the chemical equilibrium between Fe, Fe2+, and Fe3+, which often influences the corrosion rate. (Dexter)5... 

Another method of evaluating standard oxidation-reduction potentials is to make use of chemical determinations of equilibrium constants. ... 

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. 

At an intermediate temperature a state of chemical equilibrium would be achieved in a closed container. If a sample of iron oxide were to be heated to 700° C with hydrogen gas in a container for a long time, the reaction would proceed, with the formation of water vapor, until a certain amount of tvater vapor had been formed. Similarly if metallic iron and water vapor were to be heated in the container some hydrogen and iron oxide would be formed. Ultimately in either case a steady state would be achieved, in which the rate of reduction of iron oxide by hydrogen is just equal to the rate of oxidation of iron by water vapor. This steady state (chemical equilibrium) is analogous to the steady state between a crystal or a liquid and its vapor, discussed in Chapter 3 (physical equilibrium). It has been found possible to develop a quantitative theory of chemical equilibrium, tvhich will be discussed in a later chapter (Chap. 19). 

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. ... 

---