Cell reactions nickel zinc

The actual cell voltage is about 1.5 V, it does not depend on the actual pH-value of the electrolyte solution as obvious from the absence of protons and hydroxide ions in the cell reaction equation. It slightly depends on the source of the used manganese dioxide. Initially naturally occurring manganese dioxide was used. The battery required a quality of less than 0.5% copper, nickel, cobalt, and arsenic to avoid undue corrosion of the zinc electrode. Currently synthetic manganese dioxide is prepared either by chemical (CMD) or electrochemical (EMD) procedures. For improved electrical conductivity graphite or acetylene black are added. Upon deep... 

As well as having electrical conductivity, the transition elements can be used in the production of electrical energy through their chemical reactivity. Perhaps the most immediately familiar example is the dry cell battery. Any of a number of chemical reactions may be exploited in this context. As a consequence, manganese, nickel, zinc, silver, cadmium or mercury may be found in dry cells. 

The overall reactions in the nickel—zinc cell can be represented by... 

Iron—Air Cells. The iron—air system is a potentially low cost, high energy system being considered mainly for mobile applications. The iron electrode, similar to that employed in the nickel—iron cell, exhibits long life and therefore this system could be more cost effective than the zinc—air cell. Reactions include ... 

Furthermore, 2,2-difluoro-3-hydroxyesters are readily obtained from ClCFjCOOMe and carbonyl compounds by electrolysis in a one-compartment cell using a sacrificial zinc anode and a nickel complex as catalyst. The catalytic cycle for this reaction is shown in Scheme 9 with nickel zinc exchange being a key step. In this process, the CHjCyDMF solvent (9 1) system leads to suppression of undesired Claisen condensation and an increase in the yield of 2,2-difluoro-3-hydroxyester formation. It is notable that high yields are obtained even with ketones and enolizable aldehydes, which are not good participants in the Reformatsky reaction alternative for producing these substances. 

In some apphcations nickel-cadmium batteries have been replaced by nickel-zinc batteries.The overall cell reaction for this relatively new battery is ... 

These batteries are designed to be reehaiged and used multiple times. That is, they ean have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition. Thus these are also called rechargeable batteries. Cells of this type include nickel-cadmimn (NiCd), nickel-zinc (NiZn), and lithium-ion (Li-ion) cells. 

Corrosion reactions may be minimized by essentially two means. Firstly, by covering the surfaces of metals with protective films and secondly, by exploiting inhibition processes. Steel, for example, may be protected by surface layers of chromium, nickel, zinc or tin. Cracks in a surface film of a more noble metal than the one being offered protection, can give rise to local cells in which the exposed base metal becomes an anode and the protective layer a cathode. Local corrosion then sets in. 

Table 12-11 predicts the cell will operate so as to dissolve metallic zinc and deposit metallic nickel, and its voltage will be +0.51 volt This is exactly what occurs in such a cell. Predicting is fun— let s try it again Another cell we studied is based on reaction (52) ... 

You might wonder what we would have learned if we had assumed that either of these two cells operates with the reverse reaction. Suppose we had proposed a cell based on oxidation of nickel and reduction of zinc ... 

Self-Test 12.9A Which metal, zinc or nickel, is the stronger reducing agent in aqueous solution under standard conditions Evaluate the standard emf of the appropriate cell, specify the cell with a cell diagram, and write the net ionic equation for the spontaneous reaction. 

The massive zinc (rod or plate) reacts spontaneously with activated bromides provided the preliminary electroreduction of a catalytic amount of zinc salt (ZnBr2 or ZnCl2) occurs. Reactions are carried out in nitrile solvents (CH3CN, PhCN,. ..) or their mixture with dichloromethane. An undivided cell fitted with a zinc anode and an indifferent cathode (gold, nickel, carbon, zinc,. ..) is used. As observed with benzylic bromides, the activation leads to an organozinc compound able to react with either the nitrile solvent or an electrophile reagent. The process is depicted in equation 12. 

This last electrochemical process is carried out in an undivided electrolysis cell fitted with a sacrificial magnesium anode and a nickel foam as cathode. The reaction is conducted in dimethylformamide in the presence of both NiBr2(bpy) as the catalyst and dried ZnBr2 (1.1 molar equivalents with respect to bromothiophene), which is used both as supporting electrolyte and as a zinc(II) ion source. The other conditions are the same as those described in the section concerning the aromatic halides. The yield of 3-thienylzinc bromide was roughly 80%, as determined by GC analysis after treatment with iodide (equation 34). 

The ion-exchange reaction of the synthetic zeolites NaX and NaY with cobalt, zinc and nickel ions is shown to be non-stoichiometric at low bivalent-ion occupancy, the hydrolytic sodium loss being about twice as large for NaX ( 5 ions/unit cell) as for NaY. The effect is more pronounced at high temperatures and disappears at high occupancies. Reversibility tests in NaX toward zinc and cobalt ions, as studied by a temperature-variation method, show the temperature history to be an important factor in the irreversibility characteristics. The low-temperature partial irreversibility, induced by a high-temperature treatment (45°C) is interpreted in terms of a temperature-dependent occupancy of the small-cage sites by divalent cations, which become irreversibly blocked at low temperature (5°C). 

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