Electrolytic Cell Example

Electrolytic cell is a device containing two electrodes in contact with electrolyte that brings about chemical reaction when connected to an outside source of electricity. Electrolytic cells have many practical applications, that is, recovery of pure metal from alloys, plating of one metal with another known as electroplating, etc. 

In Eigure 6.2, the part to be plated is the cathode of the circuit. The electrolyte solution contains one or more dissolved salts. The application of direct current leads to dissolution of metal atoms from the anode and deposition on the cathode surface. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated. Thus, the ions in the electrolyte bath are continuously replenished by the anode. The cations with positive charge coming from the anode associate with the ions in the solution. At the cathode, the cations are reduced to the metal form by gaining two electrons. 

Eigure 6.3 shows the schematic of an electrolytic cell. Two electrodes are submerged inside a cupric sulfate solution connected to an electric potential source. Electrode is a metal immersed in an electrolyte solution so that it makes contact with it. Here, copper in a solution of cupric sulfate is an example of an electrode. Electrochemical cell is a system consisting of two 

At the cathode, there is a deposition of copper, that is, electrons are consumed  

We have derived before the molar flux of ith species as 

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Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

Manganate(VI) formed in the initial oxidation process must first be dissolved in a dilute solution of potassium hydroxide. The concentrations depend on the type of electrolytic cell employed. For example, the continuous Cams cell uses 120 150 g/L KOH and 50 60 g/L K MnO the batch-operated Bitterfeld cell starts out with KOH concentrations of 150 160 g/L KOH and 200 220 g/L K MnO. These concentration parameters minimize the disproportionation of the K MnO and control the solubiUty of the KMnO formed in the course of electrolysis. 

A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

In the previous example of an electrolytic cell the two electrodes were immersed in the same solution of silver nitrate, and the system was therefore thermodynamically at equilibrium. However, if the activities of Ag at the electrodes differ, the system is unstable, and charge transfer will occur in a direction that tends to equalise the activities, and equilibrium is achieved only when they are equal. 

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

Equations (5.18) and (5.19), particularly the latter, have only recently been reported and are quite important for solid state electrochemistry. Some of then-consequences are not so obvious. For example consider a solid electrolyte cell Pt/YSZ/Ag with both electrodes exposed to the same P02, so that Uwr = 0. Equation (5.19) implies that, although the work functions of a clean Pt and a clean Ag surface are quite different (roughly 5.3 eV vs 4.7 eV respectively) ion backspillover from the solid electrolyte onto the gas exposed electrode surfaces will take place in such a way as to equalize the work functions on the two surfaces. This was already shown in Figs. 5.14 and 5.15. 

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. 

When an electrolytic cell is designed, care must be taken in the selection of the cell components. For example, consider what happens when an aqueous solution of sodium chloride is electrolyzed using platinum electrodes. Platinum is used for passive electrodes, because this metal is resistant to oxidation and does not participate in the redox chemistry of the cell. There are three major species in the solution H2 O, Na, and Cl. Chloride ions... 

An example of the electrolytic cell with an LM used for the voltammetric investigation is... 

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. 

A description of an electrolytic cell has already been given under cell features (Section 1.3.2, Fig. 1.1c). Another example is the cell with static inert electrodes (Pt) shown in Fig. 3.1 where an applied voltage (Eappl) allows a current to pass that causes the evolution of Cl2 gas at the anode and the precipitation of Zn metal on the cathode. As a consequence, a galvanic cell, (Pt)Zn 2 ZnCl2 Cl2 iPt+, occurs whose emf counteracts the voltage applied this counter- or back-emf can be calculated with the Nernst equation to be... 

What can be learnt from XPS about electrochemical processes will be demonstrated and discussed in the main part of this chapter by means of specific examples. Thereby a survey of new XPS and UPS results on relevant electrode materials will be given. Those electrode materials, which have some potential for a technical application, are understood as practical and will be discussed with respect to the relevant electrochemical process. The choice of electrode materials discussed is of course limited. Emphasis will be put on those materials which are relevant for technical solid polymer electrolyte cells being developed in the author s laboratory. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

Electrolytic cells use electricity from an external source to produce a desired redox reaction. Electroplating and the recharging of an automobile battery are examples of electrolytic cells. 

In the operation of both galvanic and electrolytic cells, there is a reaction occurring on the surface of each electrode. For example, the following reaction takes... 

It consists in a deposition of ions from an electrolyte onto the cathode in an electrolytic cell, under the influence of an applied potential. Usually the process is accompanied by material dissolution from the anode. The electrowinning from aqueous solutions is an important commercial method for the production (and/or refinement) of many metals, including, for instance, chromium, nickel, copper, zinc. As for the electrodeposition from non-aqueous solutions, the primary production of aluminium, electrodeposited from a solution of A1203 in molten cryolite, is a typical example. Other metals which may be regularly reduced in a similar way are Li, Na, K, Mg, Ca, Nb, Ta, etc. 

The information you have just learned permits a very precise control of electrolysis. For example, suppose you modify a Daniell cell to operate as an electrolytic cell. You want to plate 0.1 mol of zinc onto the zinc electrode. The coefficients in the half-reaction for the reduction represent stoichiometric relationships. Figure 11.23 shows that two moles of electrons are needed for each mole of zinc deposited. Therefore, to deposit 0.1 mol of zinc, you need to use 0.2 mol of electrons. 

Represent one example of a galvanic cell, and one example of an electrolytic cell, using chemical equations, half-reactions, and diagrams. 

Having identified the main features of electrochemistry, the remainder of this chapter will focus on the use of electrolytic cells and will use as examples the electrodeposition (or electroplating) of metals such as copper, zinc, iron, chromium, nickel and silver. The chapter will also consider the electrochemistry of some organic molecules. Electroanalysis will not be considered since a full description is not within the scope of this chapter. Eor those interested readers, there is a review on the topic [2],... 

Combustion products can affect sensitive electronic equipment. For example, hydrogen chloride (HCI) is formed by the combustion of PVC cables. Corrosion due to combusted PVC cable can be a substantial problem. This may result in increased contact resistance of electronic components. Condensed acids may result in the formation of electrolytic cells on surfaces. Certain wire and cable insulation, particularly silicone rubber, can be degraded on exposure to HCI. A methodology for classifying contamination levels and ease of restoration is presented in the SFPE Handbook... 

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