Standard reduction half-reaction

Standard potentials (reactants and products at unit activity, hydrogen ion activity of 1, i.e. pH = 0), of reduction, oxidation, net reaction, the ith redox-active species, and any half-reaction Standard reduction potential at hydrogen ion activity of 10 7 (pH = 7.0)... 

For each galvanic cell, give the balanced cell equation and determine Standard reduction potentials are found inTable18.1. Give the balanced cell equation and determine %° for the galvanic cells based on the following half-reactions. Standard reduction potentials are found in Table 18.1. 

The reaction can be written as the difference of two half-reactions whose reduction potentials are listed in Table 19-1. The nE° values can be calculated as standard reduction potentials so long as one is reversed to an oxidation. The reversal of the reaction results in the opposite sign for the potential. 

N electrode potentials potentials, E, of half-reactions as reductions-versus the standard hydrogen electrode. 

The tables of standard apparent reduction potentials produced here are different from classical tables (2) of ° in that they can be reproduced at other pHs in the range 5 to 9 and other ionic strengths in the range 0 to 0.25 M. For some half reactions, standard transformed reduction potenials can be calculated at temperatures other than 298.15 K. These values have all been calculated from the species database BasicBiochemDataS (9) that has been calculated from experimental measure-... 

The set of reactions that creates the complex redox environments of landfill leachate plumes consists of combinations of two half-reactions oxidation half-reaction and reduction halfreaction. Table 3 presents the most prominent overall redox reactions, along with their calculated Gibbs free energy change under standard conditions (AGo(W)). The lower (the more negative) the AGq(W), the more energy is gained. 

Thinking it Through Standard cell potentials for redox reactions can be calculated from the proper combination of half-cell standard reduction potentials. Looking at the target equation, the liquid bromine in changing to ... 

The emf of a cell can be calculated from the standard electrode potentials of the half-reactions. In order to find the emf, we have to look at the two halfreactions involved in the reaction. Then, set up the two half-reactions so that when they are added we will get the net reaction. Once we have set the equations properly and assigned the prpper potentials to those half-reactions, we can add the standard electrode potentials. A common mistake that students make is that they forget the fact that the standard electrode potentials are given in terms of reduction reactions. Redox reactions involve both oxidation and reduction. If one half-reaction is reduction, the other should be oxidation. So we must be careful about the signs of the half-reaction potentials, before we add the two half-reaction potentials to get the emf value. Do the next example. 

To relate this value to the standard reduction pertential in part (a), we first need to recognize that we can write the equilibrium reaction as the sum of two half-reactions. The reduction step is the reaction from part (a), and the oxidation step is the... 

Cathode (Reduction) Half-Reaction Standard Potential, °(V)... 

Half-cell reaction Standard reduction potential, E9 (in volts)... 

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... 

Standard, reduction potentials are determined by measuring the voltages generated in reaction half-cells (Figure 21.2). A half-cell consists of a solution containing 1 M concentrations of both the oxidized and reduced forms of the substance whose reduction potential is being measured, and a simple electrode. 

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

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... 

Standard Reduction Potentials for Several Biological Reduction Half-Reactions ... 

When an element can exist in several oxidation staie.s it is sometimes convenient to display the various reduction potentials diagramaltcally. the corresponding half-reactions under standard conditioas being implied. Thus, in acidic aqueous soiultons... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Any redox reaction can be split into two half-reactions, an oxidation and a reduction. It is possible to associate standard voltages x (standard oxidation voltage) and (standard reduction voltage) with the oxidation and reduction half-reactions. The standard voltage for the overall reaction, °, is the sum of these two quantities... 

There is no way to measure the standard voltage for a half-reaction only fi° can be measured directly. To obtain values for x and fi ed> the value zero is arbitrarily assigned to the standard voltage for reduction of H+ ions to H2 gas ... 

Standard half-cell voltages are ordinarily obtained from a list of standard potentials such as those in Table 18.1 (page 487). The potentials listed are the standard voltages for reduction half-reactions, that is,... 

As pointed out earlier, the standard voltage for a redox reaction is the sum of the standard voltages of the two half-reactions, reduction and oxidation that is,... 

E° values have been measured for many reactions and tabulated as standard half-cell potentials. Table 9.3 summarizes half-cell potentials as standard reduction potentials for a select set of reactions.aa In the tabulations, E° for... 

Standard potentials are also called standard electrode potentials. Because they are always written for reduction half-reactions, they are also sometimes called standard reduction potentials. 

TABLE 12.1 Standard Potentials Species i at 25°C Reduction half-reaction E° (V)... 

Use the information in Appendix 2B to determine the standard potential for the redox couple Ce4"7Ce, for which the reduction half-reaction is... 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

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. 

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. 

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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