Standard Electrode Reduction Potentials in Aqueous Solution at

TABLE 4.2 Some Standard Electrode (Reduction) Potentials in Aqueous Solution at 25°C... 

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions for 1 molar solute concentration. The standard electrode potential values are set under ideal and standard-state conditions (latm pressure and 25°C temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25°C. 

In aqueous solution, primary interest centers on the production of CO, formic acid, methanol and alcohols, and methane and hydrocarbons. The standard redox potentials (versus the saturated calomel electrode, SCE) for the common C02 reduction products of formic acid, CO, formaldehyde, methanol, and methane in aqueous solution at pH 7.0 are given as [42] ... 

Standard Reduction Electrode Potentials for Inorganic Systems in Aqueous Solutions at 25°C... 

The oxygen reduction reaction (ORR) is the major process at fuel cell cathodes. Typically the cathode electrocatalyst is Pt dispersed on high surface area carbon however, the performance of Pt is not ideal. The standard potential for the oxygen electrode (H2O/O2) in acid solution at 298 K is 1.23 V (RHE). The cathode of the Pt-activated acid fuel cell operating at a moderate current density and an O2 pressure of 1.0 atm is in the region of 0.8 V—clearly there is considerable scope for improved performance and cost reduction in the fuel cell cathode area. The catalysis of the oxygen reduction reaction, largely at metal surfaces in aqueous media, was surveyed recently by Adzic [57]. 

By international agreement, a standard electrode potential, E", measures the tendency for a reduction process to occur at an electrode. In all cases, the ionic species are present in aqueous solution at unit activity (approximately 1M), and gases are at 1 bar pressure. Where no metallic substance is indicated, the potential is established on an inert metallic electrode, such as platinum. 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

In any chemical reaction involving the transfer of electrons there will be two couples involved, one of which undergoes oxidation and the other reduction, so that it will not be possible to study the above reaction (equation 20) in the absence of a second redox couple. To overcome this difficulty of not being able to measure the absolute value of AG° or E° for equation (20), a scale of relative values of E° can be obtained by measuring the potential of a redox couple relative to a common redox couple which is assigned an arbitrary value. In aqueous solution this common redox couple is the standard hydrogen electrode (equation 22), in which the H+(aq) and H2(g) are at unit activity and fugacity, respectively. 

All voltages are standard reduction potentials (relative to the standard hydrogen electrode) at 25°C and 1 atm pressure. All species are in aqueous solution unless otherwise indicated. 

As we noted before, this equation is valid for a pure valence-band process. This has only been found experimentally for redox systems where the standard potential occurs very close to the valence band ( f,redox -E < 0.2 eV) as proven, for instance, for the reduction and oxidation of Cu VCu system at GaAs electrodes in aqueous solutions (Reineke and Memming, 1992). In other cases where the standard potential of the redox system occurs closer to the middle of the bandgap so that the reduction of a redox system is expected to occur via the conduction band. Using eq. 2.42 we have then... 

The chemical reactivity of CO2 is low. However, the equilibrium potentials of CO2 reduction are not very negative as compared with that of the hydrogen evolution reaction (HER) in aqueous electrolyte solutions. For example, electrochemical reduction of CO2 to HCOO in aqueous solution is given below together with the standard electrode potential at pH 7.0 at 25°C with respect to the standard hydrogen electrode (SHE). 

Table 7.2 Standard electrode potentials for the single- and multi-electron reduction of carbon dioxide (in an aqueous solution at pH 7) [126]...

Standard emf Values for the Cell H2/HCl/AgCl, Ag in Various Aqueous Solutions of Organic Solvents at Various Temperatures Temperature Dependence of the Standard Potential of the Silver Chloride Electrode Standard Electrode Potentials of Electrodes of the First Kind Standard Electrode Potentials of Electrodes of the Second Kind Polarographic Half-Wave Potentials (E1/2) of Inorganic Cations Polarographic E1/2 Ranges (in V vs. SCE) for the Reduction of Benzene Derivatives Vapor Pressure of Mercury... 

Determining Ehaif-ceii The Standard Hydrogen Electrode What portion of ceii for the zinc-copper reaction is contributed by the anode half-cell (oxidation of Zn) and what portion by the cathode half-cell (reduction of Cu ) That is, how can we know half-cell potentials if we can only measure the potential of the complete cell Half-cell potentials, such as Ezine and °opper. are not absolute quantities, but rather are values relative to that of a standard. This standard reference halfcell has its standard electrode potential defined as zero (E fereiice — 0.00 V). The standard reference half-cell is a standard hydrogen electrode, which consists of a specially prepared platinum electrode immersed in a 1 M aqueous solution of a strong acid, H (fl ) [or H30 (a )], through which H2 gas at 1 atm is bubbled. Thus, the reference half-reaction is... 

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