In electrolytic cells

Hexafluorozirconic acid is used ia metal finishing and cleaning of metal surfaces, whereas the fluorozirconates are used in the manufacture of abrasive grinding wheels, in aluminum metallurgy, ceramics industry, glass manufacturing, in electrolytic cells, in the preparation of fluxes, and as a fire retardant (see Abrasives Metal surface treati nts). 

Direct Current (DC). This current is transmitted for industrial uses only in exceptional situations. The most common sources of direct current are storage batteries and industrial devices called rectifiers, in which alternating current is changed (rectified) to direct current, as is used in electrolytic cells for the manufacture of chlorine gas, magnesium, aluminum, and a few other chemicals. The direct current is flowing from the source through the user application and back to the source, in one direction. The motor is primarily used for speed control of selected equipment. 

The principles discussed in this chapter have a host of practical applications. Whenever you start your car, turn on your cell phone, or use a remote control for your television or other devices, you are making use of a voltaic cell. Many of our most important elements, including hydrogen and chlorine, are made in electrolytic cells. These applications, among others, are discussed in Section 18.6. ... 

An electrochemical cell in which electrolysis takes place is called an electrolytic cell. The arrangement of components in electrolytic cells is different from that in galvanic cells. Typically, the two electrodes share the same compartment, there is only one electrolyte, and concentrations and pressures are far front standard. As in all electrochemical cells, the current is carried through the electrolyte by the ions present. For example, when copper metal is refined electrolytically, the anode is impure copper, the cathode is pure copper, and the electrolyte is an aqueous solution of CuS04. As the Cu2f ions in solution are reduced and deposited as Cu atoms at the cathode, more Cu2+ ions migrate toward the cathode to take their place, and in turn their concentration is restored by Cu2+ produced by oxidation of copper metal at the anode. 

The various possible electrode reactions at the cathode and at the anode in electrolytic cells have been shown in Table 6.2. It has been pointed before that the outcome of an electrolytic process can be made on the basis of knowledge of electrode potentials and of overvoltages. The selection of the ion discharged depends on the following factors (i) the position of the metal or group in the electrochemical series (ii) the concentration and (iii) the nature of the electrode. Examples provided hereunder deliberate on these aspects. 

The electrolysis of alumina is carried out in electrolyte cells made of mild steel which are lined inside with an insulating refractory and carbon (either carbon bricks or carbon and coal tar pitch). The cell bottom is connected to the cathode terminal and serves as the cathode. Carbon electrodes introduced from the top serve as anodes. A more detailed description is given below. 

Electrolytic reduction of compounds In electrolytic cells the reduction of an element to low oxidation states may be performed, and compounds such as sulphides, phosphides, antimonides, etc. may be prepared. 

In this section, you learned about electrolytic cells, which convert electrical energy into chemical energy. You compared the spontaneous reactions in galvanic cells, which have positive cell potentials, with the non-spontaneous reactions in electrolytic cells, which have negative cell potentials. You then considered cells that act as both galvanic cells and electrolytic cells in some common rechargeable batteries. These batteries are an important application of electrochemistry. In the next two sections, you will learn about many more electrochemical applications. 

Some metals are extracted in electrolytic cells. In section 11.3, you saw the extraction of sodium from molten sodium chloride in a Downs cell. Other reactive metals, including lithium, beryllium, magnesium, calcium, and radium, are also extracted industrially by the electrolysis of their molten chlorides. 

In industry, the process of purifying a material is known as refining. After the extraction stage, some metals are refined in electrolytic cells. 

Electrolytes are ubiquitous and indispensable in all electrochemical devices, and their basic function is independent of the much diversified chemistries and applications of these devices. In this sense, the role of electrolytes in electrolytic cells, capacitors, fuel cells, or batteries would remain the same to serve as the medium for the transfer of charges, which are in the form of ions, between a pair of electrodes. The vast majority of the electrolytes are electrolytic solution-types that consist of salts (also called electrolyte solutes ) dissolved in solvents, either water (aqueous) or organic molecules (nonaqueous), and are in a liquid state in the service-temperature range. [Although nonaqueous has been used overwhelmingly in the literature, aprotic would be a more precise term. Either anhydrous ammonia or ethanol qualifies as a nonaqueous solvent but is unstable with lithium because of the active protons. Nevertheless, this review will conform to the convention and use nonaqueous in place of aprotic .]... 

Such reactions take place in electrolytic cells, at the boundary of the fluids which carry the ions M and N. 

Electrolysis is the process of driving a reaction in a nonspontaneous direction by using an electric current. An electrolytic cell is an electrochemical cell in which electrolysis takes place. The arrangement of components in electrolytic cells is different from that in galvanic cells. Specifically, the two electrodes usually share the same compartment, there is usually only one electrolyte, and concentrations and pressures are usually far from standard. 

FIGURE 18.19 Electrorefining of copper metal, (a) Alternating slabs of impure copper and pure copper serve as the electrodes in electrolytic cells for the refining of copper, (b) Copper is transferred through the CuS04 solution from the impure Cu anode to the pure Cu cathode. More easily oxidized impurities (Zn, Fe) remain in solution as cations, but noble metal impurities (Ag, Au, Pt) are not oxidized and collect as anode mud. 

The main advantages of bipolar membranes are no formation of gases at their surfaces or within the bipolar membranes themselves, a power consumption to dissociate water into 02 and H2 about half that used in electrolytic cells, a minimum formation of by-product or waste streams in the case of dilute (< 1 kmol/m3) acids or bases, and reduced downstream purification steps. 

With few exceptions the electrodes in electrolytic cells are arranged either vertically of horizontally. There is no exact rule as to which particular arrangement should be used as the position of the electrodes depends on the specific nature of the electrolytic process involved. Nevertheless, certain general rules may be laid down. 

Since this is an oxidation, it must be the anode. In electrolytic cells, the anode is positively charged and will therefore attract anions. 

Electrolytic cells can be created to separate materials. Unlike the reactions in voltaic cells, the reactions in electrolytic cells are nonspontaneous. In an electrolytic cell, a voltage is applied using a power supply. 

When aqueous solutions are placed in electrolytic cells, the collection of the solute at the cathode and anode may be affected by the presence of hydrogen and hydroxide ions in the solution. 

Ionomers are used to prepare membranes for a variety of applications including dialysis, reverse osmosis, and in electrolytic cells for the chlor-alkali industry. This latter application needs materials that show good chemical resistance and ionomers based on perfluorinated backbones with minor amounts of sulfonic or carboxylic acids are ideal. They also show good ion-exchange properties. 

In electrochemistry the same phenomenon (essentially related to charge conservation) occurs, yet the reduction of the acceptor A occurs at one electrode (the cathode in electrolytic cells) and the oxidation of the donor D at the other (anode). Thus the kinetics of the overall cell reaction depends on both half-reactions, in a similar way as the kinetics of a homogeneous electron transfer depends on the acceptor and the donor. However,... 

C. Wagner, "Theoretical Analysis of the Current Density Distribution in Electrolytic Cells," Journal of The Electrochemical Society, 98 (1951) 116-128. 

The 02 evolution reaction which is of great importance in electrolytic cells, proceeds in general via the reverse of the above reactions. Owing to the principle of microscopic reversibility, it is obvious that active 02 reduction electrocatalysts are equally active for the 02 evolution reaction. 

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