Standard Reduction Potentials E

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

Here are some things to be aware of in looking at this table  

We can use this table of standard reduction potentials to write the overall cell reaction and to calculate the standard cell potential, the potential (voltage) associated with the cell at standard conditions. There are a couple of things to remember when using these standard reduction potentials to generate the cell reaction and cell potential  

Since the standard cell potential is for a galvanic cell, it must be a positive 

Since one half-reaction must involve oxidation, we must reverse one of the half-reactions shown in the table of reduction potentials in order to indicate the oxidation. If we reverse the half-reaction, we must also reverse the sign of the standard reduction potential. 

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MbFe(IV)=0 and with lipid peroxyl radicals (Castellucio et al, 1995). It may accordingly be concluded that the most relevant single parameter for predicting the antioxidative activity of a new plant phenol would be the standard reduction potential, E . 

The order of catalytic activity was Fe > Ga > Sn > Ti, which is the same order as the standard reduction potential E°Mn+/M for these metals. This illustrates that redox properties rather than acid properties are responsible for the activity. Comparison of the activation energies between the different Fe-Si-TUD-1 samples was carried out by conducting the reaction at temperatures between 40° and 80°C. For Fei, Fe2, Fes and Feio the activation energy was 47, 85, 182 and 216 kJ/mol, respectively. The large difference in activation energies between these samples may... 

Figure 2.87 Schematic of the cyclic voltammogram expected from a reversible electrochemical redox system 0 + e + R having a standard reduction potential °. E is the potential of the working electrode, and I the current.

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

The complete kinetic expression in eq 16 relates the experimental rate constant ke with the forward rate constant ki, as a direct function of the decomposition rate constant k2 and the standard reduction potential E °. Since an independent measurement of k2f = 1.2 cm s"1 (or k2 = 105 s"1) is available for Me2Co(M)+, it can be used in conjunction with E° = 0.53 V to convert ke to ki, shown in Figure 11 (14). 

If the sign of the standard reduction potential, E°, of a half-reaction is positive, the half-reaction is the cathodic (reduction) reaction when connected to the standard hydrogen electrode (SHE). Half-reactions with more positive E° values have greater tendencies to occur in the forward direction. Hence, the magnitude of a halfcell potential measures the spontaneity of the forward reaction. 

Reduction always occurs at the cathode. Note that H°ed for silver is +0.7991 volt, according to the Table of Standard Reduction Potentials. E°ed for copper is +0.337. This means that the copper metal is higher in the activity series than the silver metal, so copper metal will reduce the silver ion. The equation that describes reduction (or the cathode reaction) is therefore... 

The more the two half-reactions are separated in the table, the greater is the tendency for the net reaction to occur. This tendency for an overall redox reaction to occur, whether by direct contact or in an electrochemical cell, is determined from the standard reduction potentials, E° values, of the half-reactions involved, and the value of this potential are indications of the tendency of the overall redox reaction to occur. We will now present a scheme for determining this potential, which is symbolized E"d. ... 

An LDH with the approximate stoichiometry Mgo.3Co(II)o.6Co(III)o.2(OH)2 (N03)o.2 H2O has been synthesized by oxidation of Co(ll) using an am-moniacal solution and hydrothermal treatments vmder various O2 N2 atmospheres [176]. The ammoniacal solution plays a number of roles in the synthesis. Firstly, it provides a basic medium. Secondly, it acts as hgand by forming a complex [CoCNHsle] ", which facihtates oxidation of Co to [CoCNHsle] because of the low standard reduction potential (E°) ... 

Bright, silvery-white metal face-centered cubic crystal structure (a = 0.5582 nm) at ordinary temperatures, transforming to body-centered cubic form (a= 0.4407) at 430°C density 1.54 g/cm at 20°C hardness 2 Mohs, 17 Brinnel (500 kg load) melts at 851°C vaporizes at 1,482°C electrical resistivity 3.43 and 4.60 microhm-cm at 0° and 20°C, respectively modulus of elasticity 3-4x10 psi mass magnetic susceptibility -i-1.10x10 cgs surface tension 255 dynes/cm brick-red color when introduced to flame (flame test) standard reduction potential E° = -2.87V... 

When two conjugate redox pairs are together in solution, electron transfer from the electron donor of one pair to the electron acceptor of the other may proceed spontaneously. The tendency for such a reaction depends on the relative affinity of the electron acceptor of each redox pair for electrons. The standard reduction potential, E°, a measure (in volts) of this affinity, can be determined in an experiment such as that described in Figure 13-14. Electrochemists have chosen as a standard of reference the half-reaction... 

Biological oxidation-reduction reactions can be described in terms of two half-reactions, each with a characteristic standard reduction potential, E °. 

Standard Reduction Potentials The standard reduction potential, E °, of any redox pair is defined for the half-cell reaction ... 

To predict the voltage that will be observed when different half-cells are connected to each other, the standard reduction potential, E°, for each half-cell is measured by an experiment shown in an idealized form in Figure 14-7. The half-reaction of interest in this diagram is... 

Multiplying a half-reaction by any number does not change the standard reduction potential, E°. The potential difference between two points is the work done per coulomb of charge carried through that potential difference (E = worklq). Work per coulomb is the same whether 0.1, 2.3, or 104 coulombs have been transferred. The total work is different in each case, but work per coulomb is constant. Therefore, we do not double E° if we multiply a half-reaction by 2. 

A Latimer diagram displays standard reduction potentials E°, connecting various oxidation states of an element.11 For example, in acid solution, the following standard reduction potentials are observed ... 

Table 3.1 Biological Standard Reduction Potentials (E ) for a Series of Antioxidant Compounds and Flavonoids and Biological Standard Free Energy Variation (AG" ) for the Reactions between These Compounds and Oxygen Free Radicals... 

Table 9.1 Standard Reduction Potentials E° at 298.15 K, 1 bar. and Zero Ionic Strength...

The numerical value of an electrode potential depends on the nature of the particular chemicals, the temperature, and on the concentrations of the various members of the couple. For the purposes of reference, half-cell potentials are taken at the standard states of all chemicals. Standard state is defined as 1 atm pressure of each gas (the difference between 1 bar and 1 atm is insignificant for the purposes of this chapter), the pure substance of each liquid or solid, and 1 molar concentrations for every nongaseous solute appearing in the balanced half-cell reaction. Reference potentials determined with these parameters are called standard electrode potentials and, since they are represented as reduction reactions (Table 19-1), they are more often than not referred to as standard reduction potentials (E°). E° is also used to represent the standard potential, calculated from the standard reduction potentials, for the whole cell. Some values in Table 19-1 may not be in complete agreement with some sources, but are used for the calculations in this book. 

All the substrates have the same nucleofugal group, and there is a correlation between the relative reactivity and the standard reduction potential (E° of ArBr or Ei/2 of the unsub-... 

Because oxidation is simply the opposite of reduction, it is only necessary to create a table of one of the values. By convention, the reduction potential is used in tables, and the values are typically given for the standard reduction potential, E°, also written E°red, in units of volts, V. Because oxidation takes place at the anode, this is the value that will need to be reversed (since oxidation potential = -reduction potential). Therefore, we can rearrange Equation 18.1 so we can use values from the reduction potential tables in Equation 18.2 ... 

Very electropositive metals have low Pauling electronegativities (Xp < 1.4) and standard reduction potentials (E° < —1.6V), and include the very active metals such as Na whose cations are hard acids. 

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