Half-redox reactions

Redox half-reactions are often written for brevity as, for example, Li Li. with the state symbols omitted. The electrode... 

When either hydrogen ions or hydroxide ions participate in a redox half-reaction, then clearly the redox potential is alTected by change of pH. Manganate(Vir) ions are usually used in well-acidified solution, where (as we shall see in detail later) they oxidise chlorine ions. If the pH is increased to make the solution only mildly acidic (pH = 3-6), the redox potential changes from 1.52 V to about 1.1 V, and chloride is not oxidised. This fact is of practical use in a mixture of iodide and chloride ions in mildly acid solution. manganate(VII) oxidises only iodide addition of acid causes oxidation of chloride to proceed. 

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

The redox half-reaction when 1 is used as a titrant is... 

A metallic electrode whose potential is a function of the concentration of M"+ in an M"+/M redox half-reaction. 

Table 16-6 Redox half reactions, pg°, pe, and calculated pe values from the redox species distributions psc for the zone 125-135 m in Saanich Inlet...

A redox half-reaction at an active electrode also may convert one metal salt into another. For example, the cathode In a nickel-cadmium battery is NiO(OH), which is reduced to nickel(II) hydroxide. The half-reaction reduces... 

Defining a reference value for the SHE makes it possible to determine E ° values of all other redox half-reactions. As an example. Figure 19-14 shows a cell in which a standard hydrogen electrode is connected to a copper electrode in contact with a 1.00 M solution of C U . Measurements on this cell show that the SHE is at higher electrical potential than the copper electrode, indicating that electrons flow from the SHE to the Cu... 

They are the basis of many products and processes, from batteries to photosynthesis and respiration. You know redox reactions involve an oxidation half-reaction in which electrons are lost and a reduction half-reaction in which electrons are gained. In order to use the chemistry of redox reactions, we need to know about the tendency of the ions involved in the half-reactions to gain electrons. This tendency is called the reduction potential. Tables of standard reduction potentials exist that provide quantitative information on electron movement in redox half-reactions. In this lab, you will use reduction potentials combined with gravimetric analysis to determine oxidation numbers of the involved substances. 

The classes of indicators to be described are all organic dyestuffs, exhibiting reversible redox reactions. If a redox half reaction is represented by... 

Different redox half-reactions have different tendencies to occur. Consider the three metals sodium, magnesium, and aluminum, all in the same row of the periodic table. A sodium atom, as well as atoms of other alkali metals, has a very strong tendency to give up one electron to become a sodium ion. The tendency is so strong that the reaction of sodium with ordinary water borders on explosive ... 

If an inert electrode is used because one or both redox half-reactions do not have a suitable conducting electrode material associated with the reaction, the inert electrode is shown with its phase boundary. If the electrode components are in the same phase, they are separated by commas if not, a vertical phase boundary line. For example, consider the following redox reaction ... 

Metal indicator electrodes develop a potential which is usually determined by the equilibrium position of a redox half-reaction at the electrode surface. These are further classified into the following three types, namely ... 

Redox half-reactions are often written for brevity as, for example, Li+ + e - Li. with the state symbols omitted. The electrode system represented by the half-reaction may also be written as Li+ /Li. The standard redox potentials for ion-ion redox systems can be determined by setting up the relevant half-cell and measuring the potential at 298 K relative to a standard hydrogen electrode. For example, the standard redox potential for the half-reactions... 

The transfer coefficient a is generally an index indicative of the symmetry of the energy barrier for a redox half-reaction. The significance of this definition is the following. 

Table 7.4 Electrochemical Energies of Aquatic Redox Half Reactions. ...

The Pourbaix diagram for the O2-H2O couple is presented in Figure 7.7, along with the (,J. -pH conditions characteristic of various natural and polluted waters. The equations for the boundary lines are calculated as follows. The redox half reaction that defines the upper boundary is given by Eq. 7.3T Its equilibrium constant is... 

Since water dissociates into H" and OH, the redox half reaction that defines the lower set of boundary lines is... 

In a similar feshion, fi. n-pH diagrams can be constructed for other redox half reactions. Some examples are given in Figure 7.8. These diagrams suggest that in oxic seawater = +0.4 V and pH = 8), the stable form of iron is Fe(OH)3, nitrogen is stable as nitrate, sulfur as sulfete, and carbon as bicarbonate, if each of these species reaches redox equilibrivun. 

The values of pe° and AG° for any particular redox half reaction can be calculated from the values for complete redox reactions or combinations of reactions having the half reaction in common. For example, the oxidation of glucose... 

The values of pe and AG can also be calculated from the standard free energies of formation, AGj, of each of the reactants and products in a redox half reaction. For example, for the reduction of ferrihydrite [amorphous Fe(OH)3] the half reaction is... 

Most redox reactions consume or produce protons and the stoichiometry is often such that pe is very sensitive to pH, as the examples in the previous section show. A simple method for determining which species will predominate under particular conditions of pe and pH in an unknown redox system is to construct pe-pH diagrams . This is done as follows. Consider the following redox half reaction involving H+ ... 

A reference electrode composed of metallic mercury and solid mercurous chloride. The redox half-reaction is ... 

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 ... 

Occasionally, the monocation and electron generated in a redox half-reaction are also collectively referred to as an ion pair. 

Problem 37 Some silver mines dump shredded iron cans into ponds containing dissolved silver salts. Write the two redox half-reactions and the overall balanced reaction that explains the deposition of silver from the solution. 

The reaction of an electrochemical cell always involves a combination of two redox half reactions such that one species oxidises a second species to give the respective redox products. Thus, the overall cell reaction can be expressed by a balanced chemical equation ... 

For simplicity, we have written and balanced these equations for acidic conditions, but the speciation of iron and some contaminants, as well as the thermodynamic potentials for the associated redox half-reactions, will vary with pH. The most efficient way to represent the effects of pH is in an Eh-pH diagram, such as Fig. 6. This particular diagram shows that reduction of three contaminants (CC14, ArN02, and Cr(VI)) by Fe° is thermodynamically favorable over a wide range of pH, even though the speciation of the Fe(II)... 

This equation represents the exchange of electrons between OXi and RED2 to produce REDi and OX2. In this, OXi is considered to be the oxidant because it oxidizes RED2 to OX2 and RED is the reductant because it reduces OXj to RED). A typical redox half-reaction that occurs in many estuarine waters is the reduction of Fe3+ to Fe2+ ... 

Two electrons are removed from iron by the copper ion. The iron, now an ion, moves into the solution, while copper, now a solid, forms on the surface of the nail. This process can be summarized using redox half-reactions (Chapter 11), such as the one shown here ... 

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