Electrolysis thermodynamic stability

Only two processes for the manufacture of Be are of industrial importance (i). the thermal reduction of BeF2 using Mg, and (ii) the electrolysis of BeCl2 in a molten chloride electrolyte. Direct reduction of the oxide is ineffective because of its thermodynamic stability only Ca reduces BeO to the metal unfortunately, Ca cannot be used since it forms a stable intermetallic compound with Be, BejjCa. 

As the heats of formation of minerals become more exothermic, i.e., more negative, their thermodynamic stability increases. And so the difficulty by which free metals can be extracted from the minerals also increases. In other words, the more active is the metal, the easier it is to form compounds and the more difficult it is to retrieve the metal from its compounds. This relationship is obvious in the methods by which the metals are removed from their mineral matrix as shown in the third column of the above table heating is a less severe metallurgic process, whereas electrolysis is a more severe method. 

A very simple potential/pH diagram, showing only these three equilibria, appears in Fig. 13M. The shaded area represents the region of thermodynamic stability of water. Water electrolysis cannot occur inside this region. Above and below it, oxygen and hydrogen evolution are thermodynamically possible. Whether these reactions will in fact occur at a measurable rale depends on their kinetic parameters. 

Fig. 13M PotentiallpH diagrams for water. Only the equilibria for water electrolysis and for self ionization are shown. The partial pressures of oxygen and hydrogen are taken as unity. The shaded area is the region of thermodynamic stability of water. Data from Pourbaix in "Atlas of Electrochemical Equilibria in Aqueous Solutions", Pergamon Press, 1966.

Chemistry. There are many parts of mainline chemistry that originated in electrochemistry. The third law of thermodynamics grew out of observations on the temperature variations of the potential of electrochemical reactions occurring in cells. The concepts of pH and dissociation constant were formerly studied as part of the electrochemistry of solutions. Ionic reaction kinetics in solution is expressed in terms of the electrochemical theory developed to explain the activity of ions in solution. Electrolysis, metal deposition, syntheses at electrodes, plus half of the modem methods of analysis in solution depend on electrochemical phenomena. Many biomolecules in living systems exist in the colloidal state, and the stability of colloids is dependent on the electrochemistry at their contact with the surrounding solution. 

Electrolysis. Klectrowinning of zirconium has long been considered as an alternative to the l CroU process, and at one time zirconium was produced electrolytically in a prototype production cell (70). Electrolysis of an all-chloride molten-salt system is inefficient because of the stability of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of Zr 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. 

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