Voltaic cells operation

To understand how a voltaic cell operates, let us start with some simple cells that are readily made in the general chemistry laboratory. 

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. 

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. 

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. 

In general, if the concentrations of reactants increase relative to those of products, the emf increases. Conversely, if the concentrations of products increase relative to reactants, the emf decreases. As a voltaic cell operates, reactants are converted into products, which increases the value of Q and decreases the emf. 

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)... 

Table 9.8 summarizes some experimental results from a number of voltaic cells operating under standard conditions. These cell potentials can also be calculated from standard electrode potentials (Chapter 19). 

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. 

Liquid voltaic cells are systems composed of conducting, condensed phases in series, with a thin gap containing gas or liquid dielectric (e.g., decane) situated between two condensed phases. The liquid voltaic cells contain at least one liquid surface [2,15], Due to the presence of a dielectric, special techniques for the investigation of voltaic cells are necessary. Usually, it is the dynamic condenser method, named also the vibrating plate method, the vibrating condenser method, or Kelvin-Zisman probe. In this method, the capacity of the condenser created by the investigated surface and the plate (vibrating plate), is continuously modulated by periodical vibration of the plate. The a.c. output is then amplified, and fed back to the condenser to obtain null-balance operation [49,50]. 

Johnston, W. D. Jr. "Solar Voltaic Cells" Dekker New York, 1980 Green, M. A. "Solar Cells Operating Principles, Technology and Systems Applications" Prentice-Hall Englewood Cliffs, NJ, 1981. 

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

This type of cell essentially operates like a simple battery, with many diverse applications, and it is anticipated that such voltaic cells could be charged by the human body to provide a future power source for implanted medical devices such as heart pacemakers. 

A complete circuit is necessary for a voltaic cell to operate. 

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

The standard potential for this voltaic cell seems reasonable given the reduction potentials of the half-cells that comprise It. The mathematical operations with negative numbers are correct and the answer is correct to the thousandths place. 

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. 

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-... 

Galvanic cell An electrochemical cell that provides energy during its operation synonymous with voltaic cell. 

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. 

Animation Operation of a Voltaic Cell Online Learning Center... 

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. 

So far, we ve considered cells with all components in their standard states. But most cells don t start at those conditions, and even if they did, the concentrations change after a few moments of operation. Moreover, in all practical voltaic cells, such as batteries, reactant concentrations are far from standard-state values. Clearly, we must be able to determine Ecc h the cell potential under nonstandard conditions. 

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. 

---