Operation of a Voltaic Cell

Animation Operation of a Voltaic Cell Online Learning Center 

The oxidation half-cell. In this case, the anode compartment consists of a zinc bar (the anode) immersed in a Zn electrolyte (such as a solution of zinc sulfate, ZnS04). The zinc bar is the reactant in the oxidation half-reaction, and it conducts the released electrons out of its half-cell. 

CHAPTER 21 Electrochemistry Chemical Change and Electrical Work 

For many redox reactions, there are no reactants or products capable of serving as electrodes, so inactive electrodes are used. Most commonly, inactive electrodes are rods of graphite or platinum they conduct electrons into or out of the half-cells but cannot take part in the half-reactions. In a voltaic cell based on the following half-reactions, for instance, the reacting species cannot act as electrodes  

Relative charges on the electrodes. The electrode charges are determined by the source of electrons and the direction of electron flow through the circuit. In this cell, zinc atoms are oxidized at the anode to Zn ions and electrons. The 

Zn ions enter the solution, while the electrons enter the bar and then the wire. 

The electrons flow left to right through the wire to the cathode, where Cu in the solution accept them and are reduced to Cu atoms. As the cell operates, electrons are continuously generated at the anode and consumed at the cathode. Therefore, the anode has an excess of electrons and a negative charge relative to the cathode. In any voltaic cell, the anode is negative and the cathode is positive. 

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Figure 21.11B summarizes these four key stages in the operation of a voltaic cell. Let s find/f for the zinc-copper cell. At equilibrium. Equation 21.10 becomes... 

By now, you may be thinking that spontaneous electrochemical processes are always beneficial, but consider the problem of corrosion, the natural redox process that oxidizes metals to their oxides and sulfides. In chemical teims, coiTOsion is the reverse of isolating a metal from its oxide or sulfide ore in electrochemical terms, the process shares many similarities with the operation of a voltaic cell. Damage from corrosion to cars, ships, buildings, and bridges runs into tens of billions of dollars annually, so it is a major problem in much of the world. We focus here on the corrosion of iron, but many other metals, such as copper and silver, also conode. 

Corrosion is a natural, spontaneous, electrochemical process with similarities to the operation of a voltaic cell. It is a major economic problem because the anode is typically a metal tool or structure. (Section 21.6)... 

Figure 21.11B summarizes these four key stages during operation of a voltaic cell. 

The meaning of a reversible process was illustrated by Figure 7-12 on page 262. The reversible operation of a voltaic cell requires that electric current be drawn from the cell only very, very slowly. 

Thus the Volta potential may be operationally defined as the compensating voltage of the cell. Very often the terms Volta potential and compensation voltage are used interchangeably. It should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. 

Identify the parts of a voltaic cell and explain how each part operates. 

What are the components of a voltaic cell What is the role of each component in the operation of the cell ... 

In general, the work that can be obtained in an isothermal change is a maximum when the process is performed in a reversible manner. This is true, for example, in the production of electrical work by means of a voltaic cell. Cells of this type can be made to operate isothermally and reversibly by withdrawing current extremely slowly ( 331) the e.m.f. of a given cell then has virtually its maximum value. On the other hand, if large currents are taken from the cell, so that it functions in an irreversible manner, the E.M.F. is less. Since the electrical work done by the cell is equal to the product of the e.m.f. and the quantity of electricity passing, it is clear that the same extent of chemical reaction in the cell will yield more work in the reversible than in the irreversible operation. 

I Identify the parts of a voltaic cell, and explain how each part operates. I Calculate cell potentials, and determine the spontaneity of redox reactions. 

The Example Problems showed you how to use the data from Table 20.1 to calculate the standard potential (voltage) of voltaic cells. Another important use of standard reduction potentials is to determine if a proposed reaction under standard conditions will be spontaneous. How can standard reduction potentials indicate spontaneity Electrons in a voltaic cell always flow from the half-cell with the lower standard reduction potential to the half-cell with the higher reduction potential, giving a positive cell voltage. To predict whether any proposed redox reaction will occur spontaneously, simply write the process in the form of half-reactions and look up the reduction potential of each. Use the values to calculate the potential of a voltaic cell operating with these two half-cell reactions. If the calculated potential is positive, the reaction is spontaneous. If the value is negative, the reaction is not spontaneous. However, the reverse of a nonspontaneous reaction will occur because it will have a positive cell voltage, which means that the reverse reaction is spontaneous. 

A voltaic cell is constructed from a half-cell in which a cadmium rod dips into a solution of cadmium nitrate, Cd(N03)2, and another half-cell in which a silver rod dips into a solution of silver nitrate, AgN03. The two half-cells are connected by a salt bridge. Silver ion is reduced during operation of the voltaic cell. Draw a sketch of the cell. Label the anode and cathode, showing the corresponding half-reactions at these electrodes. Indicate the electron flow in the external circuit, the signs of the electrodes, and the direction of cation migration in the half-cells. 

The emf of a voltaic cell depends on the concentrations of substances and the temperature of the cell. For purposes of tabulating electrochemical data, it is usual to choose thermodynamic standard-state conditions for voltaic cells. The standard emf, eii> is the emf of a voltaic cell operating under standard-state conditions (solute concentrations are each 1 M, gas pressures are each 1 atm, and the temperature has a specified value—usually 25°C). Note the superscript degree sign (°), which signifies standard-state conditions. ... 

Standard emf ( eii) the emf of a voltaic cell operating under standard-state conditions (solute concentrations are 1 M, gas pressures are 1 atm, and the temperature has a specified value— usually 25°C). (20.5)... 

Operation of a singular cell is often defined by the current-voltage curve, as with voltaic batteries. Seeing as there are many differences between fuel cells and... 

When a voltaic cell operates, supplying electrical energy, the concentration of reactants decreases and that of the products increases. As time passes, the voltage drops steadily. Eventually it becomes zero, and we say that the cell is dead. At that point, the redox reaction taking place within the cell is at equilibrium, and there is no driving force to produce a voltage. 

A voltaic cell was operated under almost ideally reversible conditions at a current of 10 16 A. (a) At this current,... 

An electrolytic cell, in contrast to a voltaic cell, requires an external source of electrical energy for operation. The cell just considered can be operated electrolyt-... 

Ceil potential (Eceii) changes during operation of the cell. The Nemst equation shows that Eceii depends on Ecew and a term for the potential at nonstandard-state concentrations. During the operation of a typical voltaic cell, reactant concentration starts out higher than product concentration, gradually becomes equal to it, and then less than it, until Q = K and the cell can do no more work. 

A voltaic cell consists of oxidation (anode) and reduction (cathode) half-cells, connected by a wire to conduct electrons and a salt bridge to maintain charge neutrality as the cell operates. Electrons move from anode (left) to cathode (right), while cations move from the salt bridge into the cathode half-cell and anions from the salt bridge into the anode half-cell. The cell notation shows the species and their phases in each half-cell, as well as the direction of current flow. 

Let s examine the operation of an electrolytic cell by constructing one from a voltaie eell. Consider the tin-copper voltaic cell in Figure 21.23A. The Sn anode... 

For each half-cell in a voltaic cell, the standard reduction potential provides a measure of the tendency for reduction to occur The more positive the value ofE° y the greater the tendency for reduction under standard conditions. In any voltaic cell operating under standard conditions, the value for the reaction at the cathode is more positive than the E°eelectrons flow spontaneously through the external circuit from the electrode with the more negative value of to the electrode with the more positive value of E°ej. 

Analyze We are given a chemical equation for a voltaic cell and the concentrations of reactants and products imder which it operates. We are asked to calculate the emf of the cell under these nonstandard conditions. 

A voltaic cell is constructed with two Zn —Zn electrodes. The two half-cells have [Zn ] = 1.8 M and [Zn ] = 1.00 X 10 M, respectively, (a) Which electrode is the anode of the cell (b) What is the standard emf of the cell (c) What is the cell emf for the concentrations given (d) For each electrode, predict whether [Zn " ] will increase, decrease, or stay the same as the cell operates. 

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