Half-reaction Either the oxidation

Half-reaction Either the oxidation part or the reduction part of a redox reaction. 

A half-reaction describes one portion of an overall oxidation-reduction reaction, either the oxidation or the reduction. 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

Redox reactions are more complicated than precipitation or proton transfer reactions because the electrons transferred in redox chemishy do not appear in the balanced chemical equation. Instead, they are hidden among the starting materials and products. However, we can keep track of electrons by writing two half-reactions that describe the oxidation and the reduction separately. A half-reaction is a balanced chemical equation that includes electrons and describes either the oxidation or reduction but not both. Thus, a half-reaction describes half of a redox reaction. Here are the half-reactions for the redox reaction of magnesium and hydronium ions ... 

A half-reaction is either the oxidation reaction or the reduction reaction of a cell. On the other hand, the net reaction is an oxidation-reduction reaction, the combination of two half-reactions. 

As stated previously, the iodine titrant is generated electrochemically in the coulometric method. Electrochemical generation refers to the fact that a needed chemical is a product of either the oxidation halfreaction at an anode or the reduction half-reaction at a cathode. In the Karl Fischer coulometric method, iodine is generated at an anode via the oxidation of the iodide ion ... 

Even without deposition of a metal island, such powders often maintain photoactivity. The requirement for effective photoelectrochemical conversion on untreated surfaces is that either the oxidation or reduction half reaction occur readily on the dark material upon application of an appropriate potential, so that one of the photogenerated charge carries can be efficiently scavenged. Thus, for some photoinduced redox reactions, metallization of the semiconductor photocatalyst will be essential, whereas for others platinization will have nearly no effect. 

Redox reactions can be balanced using either the oxidation-number method or the half-reaction method. The concentration of an oxidizing agent or a reducing agent in solution can be determined by a redox titration. 

When E° values are established for a number of substances, these values are placed in a table that shows an equation for the half reaction associated with each E° value. Either the oxidation half reactions at the anodes or the reduction half reactions at the cathodes may be listed. For the zinc/hydrogen electrochemical cell described above, the half reactions would be as follows ... 

The rate of electron transfer that occurs to/from the metal center is high. Structure based modeling of the active site of human MnSOD [40], which includes calculating the energies of both the oxidized and reduced states with either water or hydroxide bound to the metal, suggests the rate of this internal electron transfer is enhanced by electron-relaxation effects. In addition, a 0.17 V redox potential is calculated, a value that is low compared with the experimental values of 0.31 V fori . coli and 0.26 V for B. stearothermophilus, respectively. A potential of —0.30 V seems to be optimal as it lies midway between the redox potentials of the two half reactions of the dismutation process [41],... 

For adverse conditions (in terms of either temperature or vibration) the antimony electrode has proved useful, particularly for industrial processes with extreme environmental problems. The electrode is not particularly reliable for precise measurements, but its simple form (consisting of antimony metal embedded in an insulating material) allows pH measurements under such adverse conditions. The principle of the electrode is based on a half-reaction whereby the metal and its metal oxide are both insoluble and the electrode s response is dependent on hydronium ion activity ... 

Online mass spectrometry data presented and discussed in the previous sections suggest that catalytic hypophosphite oxidation on nickel in D2O solutions proceeds via the coupling of anodic (19.11) and cathodic (19.12) half-reactions at the catalyst surface. The classical mixed-potential theory for simultaneously occurring electrochemical partial reactions [14] presupposes the catalyst surface to be equally accessible for both anodic (19.11) and cathodic (19.12) half-reactions. Equilibrium mixtures of H2, HD, and D2 should be formed in this case due to the statistical recombination of Hahalf-reactions (19.11) and (19.12) for example, the catalytic oxidation of hypophosphite on nickel in D20 solution under open-circuit conditions should result in the formation of gas containing equal amounts of hydrogen and deuterium (H/D=l) with the distribution H2 HD D2= 1 2 1 (the probability of HD molecule formation is twice as high as for either H2 or D2 formation [75]). Therefore, to get further mechanistic insight, the distribution of H2, HD, and D2 species in the evolved gas was compared to the equilibrium values at the respective deuterium content [54]. 

Given that these half-reactions are the only ones of importance and that no Cl or HOCl was present before the Cla was added, we obtain the electron balance by establishing an electron lelerence level (ERL) as the species that are either oxidized or reduced and with which the solution was prepared. In this example, the solution was prepared with HgO and... 

Equations that show either oxidation or reduction alone, such as Equations 20.4 and 20.5, are called half-reactions. In the overall redox reaction, the number of electrons lost in the oxidation half-reaction must equal the number of electrons gained in the reduction half-reaction. When this condition is met and each half-reaction is balanced, the electrons on the two sides cancel when the two half-reactions are added to give the balanced oxidation-reduction equation. 

The chemical reaction at an electrode can be an oxidation (anodic) half-reaction or a reduction (cathodic) half-reaction. Either of these half-reactions if occurring alone... 

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. 

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