Reactions standard reduction

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. 

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

Calculate %° values for the following cells. Which reactions are spontaneous as written (under standard conditions) Balance the reactions. Standard reduction potentials are found in Table 17.1. 

The metal anodic oxidation reaction, Fe Fe + 2e, can be written in tlie standard (reduction) notation as ... 

Although it is only slowly oxidized in moist air at ambient temperature, cadmium forms a fume of brown-colored cadmium oxide [1306-19-0] CdO, when heated in air. Other elements which react readily with cadmium metal upon heating include the halogens, phosphoms, selenium, sulfur, and tellurium. The standard reduction potential for the reaction... 

The standard reduction potential of Cr " (Table 2) shows that this ion is a strong reducing agent, and Cr(II) compounds have been used as reagents in analytical chemistry procedures (26). The reduction potential also explains why Cr(II) compounds are unstable in aqueous solutions. In the presence of air, the oxidation to Cr(III) occurs by reaction with oxygen. However, Cr(II) also reacts with water in deoxygenated solutions, depending on acidity and the anion present, to produce H2 and Cr(III) (27,28). 

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 ... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

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,... 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

Spirapril (37) is a clinically active antihypertensive agent closely related structurally and mechanistically to enalapril. Various syntheses are reported with the synthesis of the substituted proline portion being the key to the methods. This is prepared fkim l-carbobenzyloxy-4-oxopro-line methyl ester (33) by reaction with ethanedithiol and catalytic tosic acid. The product (34) is deprotected with 20% HBr to methyl l,4-dithia-7-azospiro[4.4 nonane-8-carboxylate (35), Condensation of this with N-carbobenzyloxy-L-alanyl-N-hydroxysuccinate leads to the dipeptide ester which is deblocked to 36 by hydrolysis with NaOH and then treatment with 20% HBr. The conclusion of the synthesis of spirapril (37) follows with the standard reductive alkylation [11]. 

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... 

It is clear from what has already been stated that standard reduction potentials may be employed to determine whether redox reactions are sufficiently complete... 

Alkali and alkaline-earth metals have the most negative standard reduction potentials these potentials are (at least in ammonia, amines, and ethers) more negative than that of the solvated-electron electrode. As a result, alkali metals (M) dissolve in these highly purified solvents [9, 12] following reactions (1) and (2) to give the well-known blue solutions of solvated electrons. 

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. 

In addition to defined standard conditions and a reference potential, tabulated half-reactions have a defined reference direction. As the double arrow in the previous equation indicates, E ° values for half-reactions refer to electrode equilibria. Just as the value of an equilibrium constant depends on the direction in which the equilibrium reaction is written, the values of S ° depend on whether electrons are reactants or products. For half-reactions, the conventional reference direction is reduction, with electrons always appearing as reactants. Thus, each tabulated E ° value for a half-reaction is a standard reduction potential. 

In any galvanic cell that is under standard conditions, electrons are produced by the half-reaction with the more negative standard reduction potential and consumed by the half-reaction with the more positive standard reduction potential. In other words, the half-reaction with the more negative E ° value occurs as the oxidation, and the half-reaction with the more positive E ° value occurs as the reduction. Figure 19-15 summarizes the conventions used to describe galvanic cells. 

First, identify the half-reactions. Then look up the standard reduction potentials in a table, and subtract the more negative value from the more positive value. 

The calculation o E° for this cell illustrates an important feature of cell potentials. A standard cell potential is the difference between two standard reduction potentials. This difference does not change when one half-reaction is multiplied by 2 to cancel electrons in the overall redox reaction. 

C19-0020. Use standard reduction potentials to determine the net reaction and standard cell potential for cells of two compartments, each containing a 1.00 M solution of the indicated cation in contact with an... 

Use tabulated standard reduction potentials to determine for the following redox reaction ... 

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