Aqueous solution oxidation-reduction reactions

The only oxidation state of importance In aqueous solution Is the +2 state. Compounds of the +4 plumbic Ion are well known but are Invariably unstable or Insoluble in aqueous solution. Due to the dominant stability of the +2 state In aqueous solution oxidation -reduction reactions are of minor Importance In lead separations and determinations. The notable exceptions are anodic oj ldatlon to the dioxide and reduction to the metal either cathodloally or by Internal electrodeposltlon. 111000 reactions have been used for the separation and determination of lead and are dlsoussed In detail In section IV-10 of this report. 

Mn, Fe, and Co in the +2 and +3 oxidation states all form hexaaquacomplexes in acidic aqueous solution. The reduction reactions of the three species are represented schematically below, where the water ligands are not shown for simplicity. It is an experimental fact from electrochemistry that Mn and Co are more easily reduced than Fe that is, they will more readily accept an electron. Based on the electron configurations of the ions involved, explain why Fe is harder to reduce than Mn and Co. ... 

The most traditional approach for the electrodeposition of metal oxides and hydroxides is based on the cathodic electrogeneration of base [6]. Depending upon the deposition potential, choice of the anion and the pH of the solution, various reactions take place at the cathode, such as the reduction of nitrate ions in aqueous solutions. These reduction reactions cause a local pH increase in vicinity of the cathode either by the consumption of protons or by the generation of hydroxide ions as in the case of nitrate reduction, which subsequently leads to the precipitation of metal hydroxides. The best known application of this technique is the synthesis of nickel hydroxide by the electroreduction of aqueous nickel nitrate solutions. Depending on the metal and deposition conditions, this cathodic technique often yields hydroxides rather than oxides, making a post-deposition calcination for the transformation of the hydroxide to the oxide necessary. Unfortunately, the deposited amorphous hydroxides are often metastable. Other cathodic electrodeposition methods involve the direct reduction of the oxidation state of the metal cation and deposition of the oxide onto the electrode [9]. 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

The optical absorption spectra of Pu ions in aqueous solution show sharp bands in the wavelength region 400—1100 nm (Fig. 4). The maxima of some of these bands can be used to determine the concentration of Pu ions in each oxidation state (III—VI), thus quantitative deterrninations of oxidation—reduction equiUbria and kinetics are possible. A comprehensive summary of kinetic data of oxidation—reduction reactions is available (101) as are the reduction kinetics of Pu + (aq) (84). 

NH2OH can exist as 2 configurational isomers (cis and trans) and in numerous intermediate gauche conformations as shown in Fig. 11.7. In the crystalline form, H bonding appears to favour packing in the trans conformation. The N-O distance is 147 pm consistent with its formulation as a single bond. Above room temperature the compound decomposes (sometimes explosively) by internal oxidation-reduction reactions into a complex mixture of N2, NH3, N2O and H2O. Aqueous solutions are much more stable, particularly acid solutions in which the compound... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Oxidation-reduction reactions involving perchlorates have been mentioned in several of the preceding sections and the reactivity of aqueous solutions is similar to that of aqueous solutions of perchloric acid. 

Many oxidation-reduction reactions (nicknamed redox reactions) take place in aqueous solution. One of these was mentioned in Section 11-2.1 when we characterized acids ... 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

Enthalpies, Entropies, and Gibb s Energies of Transition Metal Ion Oxidation-Reduction Reactions with Hydrogen Peroxide in Aqueous Solution (T = 298 K) [23]... 

Figure 10. Cyclic voltammetric response at the NPyeCME for the oxidation/ reduction reaction of benzyl alcohol (32 mM)/C10 in aqueous 4.1 mol NaOCl (A) and nonaqueous CH2CI2 (B) solutions at a scan rate of 50 mV/s. (C) Cartoon for the NPyeCME. Inset (A) corresponds to an enlarged version of the oxidation part without (a) and with (b) benzyl alcohol. In order to marntam the electrical conductivity, 0.1 M tetrabutylammonium bromide (TBAB) is added into the CH2CI2 solution.

The electrochemical treatment of contaminated groundwater technology uses direct electrical current applied between two immersed electrodes to produce oxidation-reduction reactions in aqueous solutions. Positively charged metal ions are attracted to the negatively charged electrode (the cathode), where they are reduced. 

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... 

Today, many reactions in aqueous solutions can be described as oxidation-reduction reactions (redox reactions). Oxidation is the process in which the oxidation number of atoms increases. Reduction is the process in which the oxidation number of atoms is decreased or made more negative. In another definition, oxidation is the loss of electrons by an atom, and reduction is the gain of electrons. Let us look at the following reaction ... 

T. W. Newton, The Kinetics of the Oxidation Reduction Reactions of Cranium, Neptunium, Plutonium, and Americium in Aqueous Solution, TID-26506, U.S. Energy, Research, and Development Administration (ERDA) Technical Information Center, Washington, D.C., 1975. 

Many organic compounds undergo reduction or oxidation at a DME. Consequently, polarographic techniques have been used extensively for determinations of organic compounds and for studying the mechanisms of their electrode reactions. In aqueous solution, the reduction of organic compounds is frequently a 2e process accompanied by protonation as in Equation 3.32 ... 

One can draw a useful analogy between acid-base and oxidation-reduction reactions. Both involve the transfer of a species from a source, the donor, to a sink, the acceptor. The source and sink nomenclature implies that the tendency of the proton (in the case of acids) or of the electron (for reducing agents) to undergo transfer is proportional to the fall in free energy. From the relation AG° = RTIn Ka. you can see that the acid dissociation constant is a measure of the fall in free energy of the proton when it is transferred from a donor HA to the solvent H2O, which represents the reference (zero) free energy level of the proton in aqueous solution. 

In both of these reactions, the metal nitrate and the organic component are prepared as an aqueous solution. The solution is boiled, dried, and the residue is heated, typically to 500°C. At this point the reaction begins and converts to a self-sustained reaction. Both of these examples are oxidation—reduction reactions, which derive their large exothermic character from the formation of molecular nitrogen as a product. 

For oxidation-reduction reactions in aqueous solution under an externally applied electrical potential, or in its absence, one can write O for the oxidized species and R for the reduced species, and the half-cell reaction can be written as... 

The bleaching process also proceeded on steady-state irradiation with visible light in the presence of moderately reductive compounds.33 Since this oxidation-reduction reaction is not possible in the dark, the reactions have been well studied with a variety of combinations of reductants such as the Fe(II>—Fe(III) system in aqueous solutions.1 This section discusses reversible spectral changes using a combination of two functions, i.e., oxidative (thionine) and reductive ones in a polymer matrix.34,35... 

Most aqueous reaction equations can be balanced by trial and error. Oxidation-reduction reactions require a more systematic approach to balancing equations using either an acidic or basic solution. 

For oxidation-reduction reactions that occur in aqueous solution, it is often useful to separate the reaction into two half-reactions one involving oxidation and the other involving reduction. For example, consider the unbalanced equation for the oxidation-reduction reaction between cerium(IV) ion and tin(II) ion ... 

The majority of titanium (III) compounds are rather easily oxidized and must be handled in an inert atmosphere. However, the titanium(III) urea derivative is relatively stable and can be kept in dry air for several weeks without suffering any apparent oxidation. In moist air or aqueous solution oxidation is much more rapid. Hexaureatita-nium(III) perchlorate has been prepared by the reduction of titanium (IV) compounds and subsequent reaction of the titanium (III) ion with urea in the presence of sodium perchlorate. The following procedure is a modification of this method. 

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