Half-reaction.

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Deposition of Si02 has been achieved by exposing the substrate to the binary reaction sequence SiCL-l-2H20 —> Si02+ 4HC1. This is divided into the following half-reactions in which species at the surface are inaicated by asterisks [95] ... 

Figure C2.18.7. The integrated absorbance of tire Si-Cl stretching vibration at 625 cm and tire SiO-H stretching vibration at 3740 cm as a function of time during tire (A) SiCl and (B) H2O half-reactions at 600 K and 10 Torr. Reproduced from [95].

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

Half-reaction (i) means that Co(II) in aqueous solution cannot be oxidised to Co(III) by adding ammonia to obtain the complexes in (ii), oxidation is readily achieved by, for example, air. Similarly, by adding cyanide, the hexacyanocobaltate(II) complex becomes a sufficiently strong reducing agent to produce hydrogen from water ... 

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. 

We can determine the energetic feasibility for this reaction from the two half-reactions ... 

At equilibrium at 298 K the electrode potential of the half-reaction for copper, given approximately by... 

Discuss the factors which influence the redox potential of a half-reaction, illustrating your answer by as many examples as possible. 

Hydrogen peroxide has both oxidising properties (when it is converted to water) and reducing properties (when it is converted to oxygen) the half-reactions are (acid solution) ... 

In the absence of acid, the half-reaction will approximate to ... 

In the presence of excess iodide ions, copper(II) salts produce the white insoluble copper(I) iodide and free iodine, because copper(II) oxidises iodide under these conditions. The redox potential for the half-reaction ... 

Half-reaction Standard or formal potential Solution composition... 

A summary of oxidation-reduction half-reactions arranged in order of decreasing oxidation strength and useful for selecting reagent systems. 

TABLE 8.28 Potentials of Selected Half-Reactions at 25°C Continued)... 

Redox reactions, such as that shown in equation 6.22, can be divided into separate half-reactions that individually describe the oxidation and the reduction processes. 

Separating a redox reaction into its half-reactions is useful if you need to balance the reaction. One method for balancing redox reactions is reviewed in Appendix 4. 

Standard-state potentials are generally not tabulated for chemical reactions, but are calculated using the standard-state potentials for the oxidation, E°o, and reduction half-reactions, fi°red- By convention, standard-state potentials are only listed for reduction half-reactions, and E° for a reaction is calculated as... 

Since the potential for a single half-reaction cannot be measured, a reference halfreaction is arbitrarily assigned a standard-state potential of zero. All other reduction potentials are reported relative to this reference. The standard half-reaction is... 

Ladder diagrams can also be used to evaluate equilibrium reactions in redox systems. Figure 6.9 shows a typical ladder diagram for two half-reactions in which the scale is the electrochemical potential, E. Areas of predominance are defined by the Nernst equation. Using the Fe +/Fe + half-reaction as an example, we write... 

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