A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 7
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The resultfor a system with constant C& is shown in Figure 1.2.11.1.3 FARADAIC PROCESSES AND FACTORS AFFECTINGRATES OF ELECTRODE REACTIONS1.3.1 Electrochemical Cells—Types and DefinitionsElectrochemical cells in which faradaic currents are flowing are classified as either galvanic or electrolytic cells. A galvanic cell is one in which reactions occur spontaneouslyat the electrodes when they are connected externally by a conductor (Figure 1.3.1a).These cells are often employed in converting chemical energy into electrical energy. Galvanic cells of commercial importance include primary (nonrechargeable) cells (e.g., theLeclanche Zn-MnO 2 cell), secondary (rechargeable) cells (e.g., a charged Pb-PbO 2 storage battery), and fuel cells (e.g., an H 2 -O 2 cell).
An electrolytic cell is one in which reactions are effected by the imposition of an external voltage greater than the open-circuitpotential of the cell (Figure 13.1b). These cells are frequently employed to carry out desired chemical reactions by expending electrical energy.
Commercial processes involvingelectrolytic cells include electrolytic syntheses (e.g., the production of chlorine and aluminum), electrorefining (e.g., copper), and electroplating (e.g., silver and gold). Thelead-acid storage cell, when it is being "recharged," is an electrolytic cell.1.3 Faradaic Processes and Factors Affecting Rates of Electrode Reactions с 19Galvanic cellElectrolytic cellPower supply2+0(Anode)Zn -> Z n2+Zn/Zn //Cu /Cu2++ 2eFigure 1.3.100(Cathode)Cu,2+_Cu/Cu2+, H2SO4/Pt(Cathode)CuCu2+(7)(Anode)+ 2e -» Cu{a) Galvanic and (b) electrolytic cells.Although it is often convenient to make a distinction between galvanic and electrolytic cells, we will most often be concerned with reactions occurring at only one of theelectrodes.
Treatment is simplified by concentrating our attention on only one-half of thecell at a time. If necessary, the behavior of a whole cell can be ascertained later by combining the characteristics of the individual half-cells. The behavior of a single electrodeand the fundamental nature of its reactions are independent of whether the electrode ispart of a galvanic or electrolytic cell. For example, consider the cells in Figure 1.3.1. Thenature of the reaction C u 2 + + 2e —» Cu is the same in both cells. If one desires to platecopper, one could accomplish this either in a galvanic cell (using a counter half-cell witha more negative potential than that of Cu/Cu 2+ ) or in an electrolytic cell (using anycounter half-cell and supplying electrons to the copper electrode with an external powersupply).
Thus, electrolysis is a term that we define broadly to include chemical changesaccompanying faradaic reactions at electrodes in contact with electrolytes. In discussingcells, one calls the electrode at which reductions occur the cathode, and the electrode atwhich oxidations occur the anode. A current in which electrons cross the interface fromthe electrode to a species in solution is a cathodic current, while electron flow from a solution species into the electrode is an anodic current.
In an electrolytic cell, the cathode isnegative with respect to the anode; but in a galvanic cell, the cathode is positive with respect to the anode.61.3.2The Electrochemical Experimentand Variables in Electrochemical CellsAn investigation of electrochemical behavior consists of holding certain variables of anelectrochemical cell constant and observing how other variables (usually current, potential, or concentration) vary with changes in the controlled variables. The parameters ofimportance in electrochemical cells are shown in Figure 1.3.2.
For example, in potentiometric experiments, / = 0 and E is determined as a function of C. Since no current flowsin this experiment, no net faradaic reaction occurs, and the potential is frequently (but notalways) governed by the thermodynamic properties of the system. Many of the variables(electrode area, mass transfer, electrode geometry) do not affect the potential directly.6Because a cathodic current and a cathodic reaction can occur at an electrode that is either positive or negativewith respect to another electrode (e.g., an auxiliary or reference electrode, see Section 1.3.4), it is poor usage toassociate the term "cathodic" or "anodic" with potentials of a particular sign. For example, one should not say,"The potential shifted in a cathodic direction," when what is meant is, "The potential shifted in a negativedirection." The terms anodic and cathodic refer to electron flow or current direction, not to potential.20Chapter 1.
Introduction and Overview of Electrode ProcessesExternal variablesElectrodevariablesTemperature (T)Pressure {P)Time (?)MaterialSurface area (A)GeometrySurface conditionElectrical variablesPotential (£)Current (i)Quantity of electricity (Q)Mass transfervariablesMode (diffusion,convection,...)Surface concentrationsAdsorptionSolution variablesBulk concentration of electroactivespecies (C o , c R )Concentrations of other species(electrolyte, pH,...)SolventFigure 1.3.2Variables affecting the rate of an electrode reaction.Another way of visualizing an electrochemical experiment is in terms of the way inwhich the system responds to a perturbation. The electrochemical cell is considered as a"black box" to which a certain excitation function (e.g., a potential step) is applied, and acertain response function (e.g., the resulting variation of current with time) is measured,with all other system variables held constant (Figure 1.3.3). The aim of the experiment isto obtain information (thermodynamic, kinetic, analytical, etc.) from observation of the(a) General conceptExcitationSystemResponse(b) Spectrophotometric experimentLamp-MonochromatorOptical cellwith samplePhototube(c) Electrochemical experimentFigure 1.3.3 (a) General principle of studying a system by application of an excitation (orperturbation) and observation of response, (b) In a spectrophotometric experiment, the excitationis light of different wavelengths (A), and the response is the absorbance (si) curve, (c) In anelectrochemical (potential step) experiment, the excitation is the application of a potential step,and the response is the observed i-t curve.1.3 Faradaic Processes and Factors Affecting Rates of Electrode Reactions2110•ЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛЛ-Q,Cu/Cd/Cd(NO3)2 (1M)//KCI(saturated)/Hg2CI2/Hg/Cu/ 0C d 2 + + 2e = Cd E° = -0.403 V vs.
NHEHg 2 CI 2 + 2e = 2Hg + 2СГ E = 0.242 V vs. NHEFigure 1.3.4 Schematic cell connectedto an external power supply. The doubleslash indicates that the KC1 solutioncontacts the Cd(NO 3 ) 2 solution in such away that there is no appreciable potentialdifference across the junction betweenthe two liquids. A "salt bridge" (Section2.3.5) is often used to achieve thatcondition.excitation and response functions and a knowledge of appropriate models for the system.This same basic idea is used in many other types of investigation, such as circuit testing orspectrophotometric analysis.
In spectrophotometry, the excitation function is light of different wavelengths; the response function is the fraction of light transmitted by the systemat these wavelengths; the system model is Beer's law or a molecular model; and the information content includes the concentrations of absorbing species, their absorptivities, ortheir transition energies.Before developing some simple models for electrochemical systems, let us considermore closely the nature of the current and potential in an electrochemical cell. Consider thecell in which a cadmium electrode immersed in 1 M Cd(NO 3 ) 2 is coupled to an SCE (Figure1.3.4).
The open-circuit potential of the cell is 0.64 V, with the copper wire attached to thecadmium electrode being negative with respect to that attached to the mercury electrode.7When the voltage applied by the external power supply, £ app i, is 0.64 V, / = 0. When £ applis made larger (i.e., £ app i > 0.64 V, such that the cadmium electrode is made even morenegative with respect to the SCE), the cell behaves as an electrolytic cell and a currentflows. At the cadmium electrode, the reaction Cd 2+ + 2e —» Cd occurs, while at the SCE,mercury is oxidized to Hg2Cl2.
A question of interest might be: "If £ appl = 0.74 V (i.e., ifthe potential of the cadmium electrode is made -0.74 V vs. the SCE), what current willflow?" Since / represents the number of electrons reacting with Cd 2+ per second, or thenumber of coulombs of electric charge flowing per second, the question "What is /?" is essentially the same as "What is the rate of the reaction, Cd 2+ + 2e —> Cd?" The following relations demonstrate the direct proportionality between faradaic current and electrolysis rate:dQi (amperes) = — (coulombs/s)(1.3.1)Q(coulombs)= N (mol electrolyzed)nF (coulombs/mol)(1.3.2)where n is the stoichiometric number of electrons consumed in the electrode reaction(e.g., 2 for reduction of Cd1 ).Rate (mol/s) = Щ- = -±=dtnF7(1.3.3)This value is calculated from the information in Figure 1.3.4. The experimental value would also include theeffects of activity coefficients and the liquid junction potential, which are neglected here.
See Chapter 2.22 I* Chapter 1. Introduction and Overview of Electrode ProcessesInterpreting the rate of an electrode reaction is often more complex than doing the samefor a reaction occurring in solution or in the gas phase. The latter is called a homogeneousreaction, because it occurs everywhere within the medium at a uniform rate. In contrast, anelectrode process is a heterogeneous reaction occurring only at the electrode-electrolyte interface.
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