Solid zinc activation

Solid zinc activation has been used with success for the formation of stable organozinc compounds from functionalized benzylic bromides [8] and has been applied to the condensation of activated halides with nitriles [9] (Blaise reaction) or carbonyl compounds (10] (Reformatsky reaction). In this case, solid zinc is activated by catalytic generated zinc formed by electroreduction of anhydrous ZnBr2 solution in acetonitrile as solvent. Results and practical approaches have been reported [llj. 

The activity of a solid The activity of a pure solid in its standard state is unity, so the activity of pure copper or of zinc metal electrodes is one. We write this as a(cU) or a,zn, = 1. 

Zm (aq) + 2e = Zn(s) a zinc activity in pure zinc solid phase is equal to a unit. It follows from the text above that for a hydrogen electrode, the electrode potential can be written as in Eq. (8) ... 

Preactivated and Water-Activated Types. A typical preactivated industrial-type zinc/air cell, the Edison Carbonaire cell, is manufactured in a 1100-Ah size and is available in two-and three-eell configurations, as illustrated in Fig. 38.12. The cell case and cover are molded from a tinted transparent acrylic plastic. The construction features are shown in Fig. 38.13 identifying the wax-impregnated carbon cathode block, the solid zinc anodes, and the lime-filled reservoir. These cells normally have a bed of lime to absorb carbon dioxide and to remove soluble zinc compounds from solution and precipitate them as calcium zincate. They are made with transparent cases so that the electrolyte level and the state of charge can be monitored visually. The state of charge can be monitored by observing the condition of the zinc plates and the condition of the lime bed. The bed turns darker as it is converted to zincate. 

Data on the phase equilibria have been measured over the complete composition range and temperatures from the liquidus down into the subsolidus regions. Furthermore there are numerous studies of the zinc activity in the liquid as well as in the solid solution phases and enthalpies of formation have been determined at several temperatures. 

Trifluoromethylzinc compounds prepared by electrolysis of CFjBr allowed the identification of a new activation method of solid zinc by electroscoring. Faradaic yields are higher than 100%, thus indicating the occurrence of a chemical route at the surface of the anode along with the electrochemical process. It was found, in keeping with these results, that this chemical process only takes place during electrolysis. 

Greater deviations which are occasionally observed between two reference electrodes in a medium are mostly due to stray electric fields or colloid chemical dielectric polarization effects of solid constituents of the medium (e.g., sand [3]) (see Section 3.3.1). Major changes in composition (e.g., in soils) do not lead to noticeable differences of diffusion potentials with reference electrodes in concentrated salt solutions. On the other hand, with simple metal electrodes which are sometimes used as probes for potential controlled rectifiers, certain changes are to be expected through the medium. In these cases the concern is not with reference electrodes, in principle, but metals that have a rest potential which is as constant as possible in the medium concerned. This is usually more constant the more active the metal is, which is the case, for example, for zinc but not stainless steel. 

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

This chapter presents detailed and thorough studies of chemical synthesis in three quite different chemical systems zinc ferrite, intermetallic, and metal oxide. In addition to different reaction types (oxide-oxide, metal-metal, and metal oxide), the systems have quite different heats of reaction. The oxide-oxide system has no heat of reaction, while the intermetallic has a significant, but modest, heat of reaction. The metal oxide system has a very large heat of reaction. The various observations appear to be consistent with the proposed conceptual models involving configuration, activation, mixing, and heating required to describe the mechanisms of shock-induced solid state chemistry. 

A simplified Pourbaix diagram for the zinc electrode at pH between 0 and 14 is shown as an example in Fig. 3.1. The vertical axis is that of the values of electrode potential on the SHE scale for activities of the Zn + and HZnO ions of 1 mol/kg. The segments of solid lines correspond to the equilibrium potentials of the following electrode reactions ... 

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