Reduction-oxidation diagrams

Formulating Models Sketch a diagram of the flow of electrons for one of the voltaic cells made in the lab. Be sure to label the electrons, reduction, oxidation, cathode, and anode. 

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

Recall from Example 11.10 that disproportionation is the process in which a single substance is both oxidized and reduced. Reduction potential diagrams enable us to determine which species are stable with respect to disproportionation. 

Use reduction potential diagrams to determine strengths of oxidizing and reducing agents and stability toward disproportionation (Section 17.2, problems 25-26). 

In water, many of the oxyacids and their anions are unstable with respect to disproportionation (self-oxidation and reduction). The tendency for this kind of instability may be conveniently determined from the reduction potential diagrams shown in Fig. 1. A large, positive standard reduction potential (the number over the arrows) indicates a strong tendency for the particular reaction indicated by the arrow (a reduction). A large, negative standard reduction potential indicates a strong tendency for change in the opposite direction (an oxidation). A particular oxyacid or its anion will not be stable with respect to self oxidation and reduction if a reduction... 

FIGURE 2.4 Potential energy diagram for reduction-oxidation process taking place at an electrode. 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

The oxoacids of P are dearly very different structurally from those of N (p. 459) and this difference is accentuated when the standard reduction potentials (p. 434) and oxidation-stale diagrams (p. 437) for the two sets of compounds are compared. Some reduction potentials ( /V) in acid solution are in Table 12.8 (p. 513) and these are shown schematically below, together with the corresponding data for alkaline solutions. 

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

A cell diagram corresponds to a specific cell reaction in which the right-hand electrode in the cell diagram is treated as the site of reduction and the left-hand electrode is treated as the site of oxidation. The sign of the emf then distinguishes whether the resulting reaction is spontaneous in the direction written ( > 0) or whether the reverse reaction is spontaneous ( < 0). 

Note that, because the right side of the cell diagram corresponds to reduction, E° = °(for reduction) — E°(for oxidation) where both values of E° are the standard reduction potentials. 

STRATEGY Find the standard potentials of the two reduction half-reactions in Appendix 2B. The couple with the more positive potential will act as an oxidizing agent (and be the site of reduction). That couple will be the right-hand electrode in the cell diagram corresponding to the spontaneous cell reaction. To calculate the standard emf of the cell, subtract the standard potential of the oxidation half-reaction (the one with the less-positive standard potential) from that of the reduction half-reaction. To write the cell reaction, follow the procedure in Toolbox 12.2. 

Step 3 To obtain °, subtract the standard potential of the half-reaction that was reversed (oxidation) from the standard potential of the half-reaction that was left as a reduction E° = °(for reduction) — °(for oxidation). Alternatively, write a cell diagram for the reaction in that case, ° = R° — L°. 

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