Sodium standard reduction potentials

Explain why you cannot use the table of standard reduction potentials in Appendix E to calculate the external voltage required to electrolyze molten sodium chloride. 

Although reduction potentials may be estimated for half-reactions, there are limits for their values that correspond to both members of a couple having stability in an aqueous system with respect to reaction with water. For example, the Na+/Na couple has a standard reduction potential of -2.71 V, but metallic sodium reduces water to dihydrogen. The reduced form of the couple (Na) is not stable in water. The standard reduction potential for the Co3 + / Co2 + couple is +1.92 V, but a solution of Co3+ slowly oxidizes water to dioxygen. In this case the oxidized form of the couple is not stable in water. The standard reduction potential for the Fe3T/Fe2+ couple is +0.771 V, and neither oxidized form or reduced form react chemically with water. They are subject to hydrolysis, but are otherwise both stable in the aqueous system. The limits for the stability of both oxidized and reduced forms of a couple are pH dependent,... 

The trend shown by the standard reduction potentials lor the reductions of sodium, magnesium and aluminium ions to their respective metals... 

Standard [reduction] potentials for hundreds of electrodes have been determined (mostly in the period 1925-45, during which time they were referred to as oxidation potentials ) and are usually tabulated in order of increasing tendency to accept electrons. This ordering is also known as the electromotive series of the elements. As can be seen in the abbreviated version in Table 1, sodium is the most active of the metallic elements in the sense that its oxidation product Na+ shows the smallest tendency (as indicated by the highly negative voltage) to undergo reduction. 

It is necessary to know the thermodynamic reduction potentials of the active metals in chloroaluminate melts. Scordilis-Kelley et al. [451,467] have studied standard reduction potentials in ambient temperature chloroaluminate melts for lithium and sodium, and they have calculated those of K, Rb, Cs. The values are, respectively, -2.066 V, -2.097 V, -2.71 V, -2.77 V and -2.87 V [versus A1(III)/A1 in a 1.5/1.0 A1C13/MEIC reference melt]. 

Electrolysis of aqueous sodium chloride (brine) is an important industrial process for the production of chlorine and sodium hydroxide. In fact, this process is second only to the production of aluminum as a consumer cf electricity in the United States. Sodium is not produced in this process under normal circumstances because H20 is more easily reduced than Na+, as the standard reduction potentials show ... 

This reaction has been assigned a potential of 0.000 volts under standard conditions. The standard reduction potential for fluorine gas is 2.890 volts while that for sodium metal is -2.714 volts. 

In order to measure the emf of a given half-cell, it is necessary to connect it with a second half-cell and measure the voltage produced by the complete cell. In general, the second halfcell serves as a reference cell and should be one with a known, stable electrode potential. Although the standard hydrogen electrode serves to define the standard reduction potential, in practice it is not always convenient to use an SHE as a reference electrode. It is difficult to set up and control. Other, more convenient reference electrodes have been developed. In principle, any metal-ion half-cell could be used under controlled conditions as a reference electrode, but in practice, many metals are unsatisfactory materials. Active metals, such as sodium and potassium, are subject to chemical attack by the electrolyte. Other metals, such as iron, are difficult to obtain in the pure form. With some metals, the ionic forms are unstable to heat or to exposure to the air. Also, it is frequently difficult to control the concentration of the electrolytes accurately. As a result, only a few systems provide satisfactory stable potentials. 

Look up the standard reduction potentials for sodium, magnesium, and aluminum. What do these potentials indicate in terms of their relative strengths as reducing agents Should aluminum liberate hydrogen readily from water Indicate clearly your reasoning. 

Electrolytic methods using molten salts are important for obtaining the more active metals, such as sodium, magnesium, and aluminum. These metals cannot be obtained from aqueous solution because water is more easily reduced than the metal ions. The standard reduction potentials of water under both acidic (B°ed O-OO V) and basic (B°ed —0.83 V) conditions are more positive than those of Na" (B°ed = 2.71 V), Mg (EJj = -2.37 V), and Al +(B°ed = -1.66 V). 

The oxidative stabilities of xanthate collectors are tested by standard reduction potential. The oxidative stability of xanthate and hexyl xanthate are —69 and -155 mV, respectively. It was reported that, xanthate concentration could be tested using mercuric acetate by amperometric titration. The testing range of xanthate concentration was 100-500 mg/L. In addition, xanthate concentration could also be tested using HgCl2 and mercury electrode by potentiometric titration. According to M. Oktarvik, xanthate concentration was tested by specific conductance. For example, the concentration of sodium ethyl xanthate can be expressed as follows ... 

Based on an analysis of standard reduction potentials, should it be more or less difficult to electrolyze lithium chloride to lithium metal or sodium chloride to sodium metal Provide a brief rationale for your answer. 

Using standard reduction potentials, decide whether sodium metal could be used to reduce Al " to aluminum metal. 

Could hydrogen peroxide be used to oxidize sodium metal Use standard reduction potentials to back up your answer. 

Magnesium and its alloys are definitely anodic to the A1 alloys and, thus, contact with aluminum increases the corrosion rate of magnesium. For example, in sodium chloride solutions (3-6%), the potential of Mg alloys is -1.67 V/SHE while that of Al-12%Si and pure aluminum are -0.83 to -0.85, respectively. However, such contact is also likely to be harmful to aluminum, since magnesium may send sufficient current to the aluminum to cause cathodic corrosion in alkaline medium. Aluminum oxide is amphoteric and so it is soluble in acid as well as in alkaline solutions. The standard reduction potentials of these two half-reduction reactions are (-1.66 V/SHE) and (-2.35 V/SHE), respectively. Alkaline reaction of the possible existence of aluminum phase in sacrificial Mg anodes is ... 

Table A. 13 in the appendix gives values for standard reduction potentials for a number of half-cells. Longer versions of such tables are available in handbooks. Unfortunately, some older works use values that are the negative of the standard reduction potentials (they are called oxidation potentials). If you are not certain whether an old table gives reduction potentials or oxidation potentials, look for an active metal electrode such as sodium or potassium. If the table contains reduction potentials, the half-cell potential of such a metal electrode will be negative.

Use standard reduction potentials to predict which metal in each of the following pairs is the stronger reducing agent under standard conditions (a) zinc or magnesium (b) sodium or tin. 

Although the entire discussion of electrochemistry thus far has been in terms of aqueous solutions, the same principles apply equaly well to nonaqueous solvents. As a result of differences in solvation energies, electrode potentials may vary considerably from those found in aqueous solution. In addition the oxidation and reduction potentials characteristic of the solvent vary with the chemical behavior of the solvent. as a result of these two effects, it is often possible to carry out reactions in a nonaqueous solvent that would be impossible in water. For example, both sodium and beryllium are too reactive to be electroplated from aqueous solution, but beryllium can be electroplated from liquid ammonia and sodium from solutions in pyridine. 0 Unfortunately, the thermodynamic data necessary to construct complete tables of standard potential values are lacking for most solvents other than water. Jolly 1 has compiled such a table for liquid ammonia. The hydrogen electrode is used as the reference point to establish the scale as in water ... 

Chloride ions will be discharged at a platinum, graphite or magnetite anode from saturated neutral solutions of sodium or potassium chloride rather than hydroxyl ions although at equilibrium conditions it should be the very opposite, as the reversible deposition potential (reduction potential) of oxygen in neutral solution is much lower ) (7Eoh- i o2. Pt = 0.815 V at 25 °C) than the standard... 

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