Electrolytic cells aqueous salt solutions, electrolysis

In an electrolytic cell, an external energy source makes a nonspontaneous redox reaction (AG > 0) occur. In electrolysis of a molten binary ionic compound (salt), the cation is reduced to the metal and the anion is oxidized to the nonmetal. For an aqueous salt solution, the products depend on whether water or one of the ions of the salt requires less energy to be reduced or oxidized. 

Electrolysis of Water and Nonstandard Half-Cell Potentials Before we can analyze the electrolysis products of aqueous salt solutions, we must examine the electrolysis of water itself. Extremely pure water is difficult to electrolyze because very few ions are present to conduct a current. If we add a small amount of a salt that cannot be electrolyzed in water (such as Na2S04), however, electrolysis proceeds rapidly. A glass electrolytic cell with separated gas compartments is used to keep the H2 and O2 gases from mixing (Figure 21.25). At the anode, water is oxidized as the O.N. of O changes from —2 to 0 ... 

Understand the basis of an electrolytic cell describe the Downs cell for the production of Na, the chlor-alkali process and the importance of overvoltage for the production of CF, the electrorefining of Cu, and the use of cryolite in the production of Al know how water influences the products at the electrodes during electrolysis of aqueous salt solutions ( 21.7) (SP 21.8) (EPs 21.63-21.75,21.82)... 

Practically all of the sodium produced in the United States involves the use of the Downs electrolytic cell, which consists of a massive graphite anode surrounded by two or more iron cathodes. The electrolyte is an aqueous solution of sodium choloride. The NaCl salt is continuously added. As electric current is passed between the electrodes, chlorine gas is collected in a hood over the graphite anode and piped off to further processing for marketing. The electrolysis of sodium chloride proceeds as follows ... 

Electrolysis, the splitting (lysing) of a substance by the input of electrical energy, is often used to decompose a compound into its elements. Electrolytic cells are involved in key industrial production steps for some of the most commercially important elements, including chlorine, copper, and aluminum. The first laboratory electrolysis of H2O to H2 and O2 was performed in 1800, and the process is still used to produce these gases in ultrahigh purity. The electrolyte in an electrolytic cell can be the pure compound (such as H2O or a molten salt), a mixture of molten salts, or an aqueous solution of a salt. The products obtained depend on several factors, so let s examine some actual cases. 

Electrolysis cell, (a) Molten salt electrolyte, (b) Aqueous electrolytes solution. 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

Sodium chloride is plentiful as rock salt, but the solid does not conduct electricity, because the ions are locked into place. Sodium chloride must be molten for electrolysis to occur. The electrodes in the cell are made of inert materials like carbon, and the cell is designed to keep the sodium and chlorine produced by the electrolysis out of contact with each other and away from air. In a modification of the Downs process, the electrolyte is an aqueous solution of sodium chloride. The products of this chloralkali process are chlorine and aqueous sodium hydroxide. 

All the electrochemical devices that will be introduced in this chapter are constituted by a central membrane, the electrolyte, and they involve an electrochemical circuit. The role of fuel cells will be detailed because, under this name, different systems are involved with varied features and scientific technical aspects, for example, according to the temperature and the electrolyte, different kinds of electrochemistry can be seen solid-state, molten salt, ionic liquids and more common aqueous solutions. Furthermore, fuel cells have reached a state of maturity and are excellent examples for understanding the behaviour of membranes in electrochemical devices. As electrolysis is constituted of similar elements to fuel cells, we will be much more synthetic with respect to this thematic. 

By the mid 1970s it was clear that the hydrodimcrizalion of acrylonitrile could be run very effectively with only a low concentration ortctraalkylammoniurn ion and that a saturated solution of acrylonitrile in an aqueous buffer was an appropriate medium. In such circumstances, it seemed likely that the electrolysis could be run jn an undivided cell without the additional complication of an anode depolarizer. Such a system was first reported by Phillips Petroleum. They ran a pilot plant with an undivided cell consisting of a lead cathode and a steel anode a very simple electrolyte, 6% acrylonitrile and 0,03% tetrabutylaminon-ium salt in aqueous dipotassium hydrogen phosphate was employed and the anode reaction was oxygen evolution. The yield of adiponitrile remained above 90% and the chief by-products were propioniirile and trimer. No base-initiated chemistry was observ due to the use of an effective buffer. 

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