Redox reactions reduction potentials

Calculate the half reaction reduction potentials of the following redox couples in aqueous solution at 25°C under the conditions indicated using (i) and/or (ii) Eh(W) as starting point (see Tables 14.2 and 14.3). Compare the calculated Eu values with the corresponding (W) values. 

Since in a redox reaction electrons are transferred, and since electrons have charge, there is an electric potential E associated with any redox reaction. The potentials for the oxidation component and reduction component of a reaction can be approximated separately based upon a standard hydrogen electrode (SHE) discussed later in this lecture. Each component is called a hall reaction. Of course, no half reaction will occur by itself any reduction half reaction must be accompanied by an oxidation half reaction. There is only one possible potential for any given half reaction. Since tire reverse of a reduction half reaction is an oxidation half reaction, it would be redundant to list potentials for both the oxidation and reduction half reactions. Therefore, half reaction potentials are usually listed as reduction potentials To find the oxidation potential for the reverse half reaction, the sign of the reduction potential is reversed. Below is a list of some common reduction potentials. 

As in the above section, the reader is referred to Part III where details of the biologically oriented reactions of dioxygen are described. Several reviews - have been published relating to the inorganic and bio-inorganic aspects of oxygenation and redox reactions. Recent studies indicate that the primary electron-transfer step for the reduction of O2 is a one-electron process which is followed under acidic conditions by chemical disproportionation to yield HgOa (an overall 2e process) and under basic conditions by production of OH (a 4e process overall). The primary reaction reduction potential is —0.33 V and is independent of pH. 

Redoxpaar redox couple Redoxpotential redox potential Redoxreaktion redox reaction, reduction-oxidation reaction, oxidation-reduction reaction Reduktion reduction Reduktionsmittel reducing agent Redundanz redundancy reduzieren reduce... 

The cationic monomeric complexes Re(CO)4(diimine) formed in the reaction of Eq. (6,39) when L=CO show excited state redox reactions. Reductive quenching experiments of the excited state with substituted methoxybenzenes and with chloride ion leads to an sq)proximate value of 1.5 V being found for the Re(CO)4(di-imine) /Re(CO)4(diimine) excited state potential. Photosubstitution is also observed with these complexes, although a definitive assignment of the photoactive states that lead to these different reactions has not yet been achieved. 

Figure 3. Half-reaction reduction potentials of selected organic redox couples (left side), iron(III)/iron(II) couples (middle), and of some biogeochemically important redox couples (right side). Indicated are standard reduction potentials, EJ(w), at environmentally relevant conditions, i.e., T = 25°C, pH = 7.0, [Cl ] = [HCO3] = lO M, [Br ] = 10 M. Note that the standard free-energy change, AG°(w), for a given redox reaction is obtained from the difference between the EJ(w) values of the corresponding half-reactions (see also example given by Eqs. 3-1 and 3-2) AG°(w) = - n F AEj(w) where n is the number of electrons transferred, and F = 96.48 kJ mol V is the Faraday s constant. Data from Stumm (72), Schwarzenbach et al. (63), and references cited therein (am = amorphous aq = aqueous phen = phenanthroline sal = salicylate s = solid porph = porphyrin).

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Determining Equilibrium Constants for Coupled Chemical Reactions Another important application of voltammetry is the determination of equilibrium constants for solution reactions that are coupled to a redox reaction occurring at the electrode. The presence of the solution reaction affects the ease of electron transfer, shifting the potential to more negative or more positive potentials. Consider, for example, the reduction of O to R... 

Reduction Potentials—An Accounting Device for Free Energy Changes in Redox Reactions... 

Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials... 

The half-reactions and reduction potentials in Table 21.1 can be used to analyze energy changes in redox reactions. The oxidation of NADH to NAD can be coupled with the reduction of a-ketoglutarate to isocitrate ... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

The aqueous solution chemistiy of nitrous acid and nitrites has been extensively studied. Some reduction potentials involving these species are given in Table 11.4 (p. 434) and these form a useful summaiy of their redox reactions. Nitrites are quantitatively oxidized to nitrate by permanganate and this reaction is used in titrimetric analysis. Nitrites (and HNO2) are readily reduced to NO and N2O with SO2, to H2N2O2 with Sn(II), and to NH3 with H2S. Hydrazinium salts yield azides (p. 432) which can then react with further HNO2 ... 

It is clear from what has already been stated that standard reduction potentials may be employed to determine whether redox reactions are sufficiently complete... 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

One of the most useful applications of standard potentials is in the calculation of equilibrium constants from electrochemical data. The techniques that we develop here can be applied to any kind of reaction, including neutralization and precipitation reactions as well as redox reactions, provided that they can be expressed as the difference of two reduction half-reactions. 

The elemental reaction used to describe a redox reaction is the half reaction, usually written as a reduction, as in the following case for the reduction of oxygen atoms in O2 (oxidation state 0) to H2O (oxidation state —2). The half-cell potential, E°, is given in volts after the reaction ... 

A table giving the cell potentials of all possible redox reactions would be immense. Instead, chemists use the fact that any redox reaction can be broken into two distinct half-reactions, an oxidation and a reduction. They assign a potential to every half-reaction and tabulate E ° values for all half-reactions. The standard cell potential for any redox reaction can then be obtained by combining the potentials for its two half-reactions. 

The calculation o E° for this cell illustrates an important feature of cell potentials. A standard cell potential is the difference between two standard reduction potentials. This difference does not change when one half-reaction is multiplied by 2 to cancel electrons in the overall redox reaction. 

Use tabulated standard reduction potentials to determine for the following redox reaction ... 

This is a quantitative calculation, so it is appropriate to use the seven-step problem-solving strategy. We are asked to determine an equilibrium constant from standard reduction potentials. Visualizing the problem involves breaking the redox reaction into its two half-reactions ... 

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