1. Stoichiometry andtheMole.All electrochemical reactions are stoichiometric since an integral number of electrons must be transferred for each reacting atom, molecule, or ion. Many general chemistry texts include sections in which current in amperes andthe time the current flows are used to calculate the total charge that has flowed through an electrochemical cell. This is, in turn, used to calculate moles of electrons transferred.
2. Oxidation andReduction:CommonlyUsed Batteries.Dry cell batteries are a good example of the practical application of chemical principles. A standard designis to make the round "can" part of the battery of zinc metal and to serve as the anode.
A carbon rod is inserted in the center to make contact with a mixture made of graphite carbon and manganese dioxide.This mixture serves as the cathode. An electrolyte/salt "bridge" mixture of moist zinc chloride, ammonium chloride, and nonconducting filler such as starch and flour surrounds the cathode mixture and connects it electrically to the zinc can. Not all features of the half-cell reactions are understood, although, in general, zinc loses electrons to become Zn2+ andmanganese(IV)in manganese dioxide gains two electrons to become Mn2+ (see Figure 7).
Figure 7. Dry cell battery. Alkaline batteries have numerous designs. A common "button"-type battery used for camera flashes and other small applications that demand high current uses mercury(II) oxide mixed with graphite as the cathode and powdered zinc amalgam(Zn and Hg) mixed with potassium hydroxide as the anode (see Figure 8). Because this type of battery contains mercury and mercury compounds, run-down batteries should be recycled to recover the mercury or treated to block entry of mercury compounds intothe environment.Unfortunately, most people throw them unwittingly into the trash.A similar kind of battery uses silver oxide as the cathode and can be recycled for the silver.
Figure 8. Alkaline
"button"-type battery.
The general characteristics of several varieties of dry cells are given in Figure 9:
Details of lead-acid storage batteries for automobiles have been excluded from this module because explanations of this type of battery are included in most standard texts.
High-Tech and Advanced Batteries
For those who design batteries, a number of engineering performance parameters are important. Some of these are cost, performance at low or high temperatures, shelf life, lifetime in use, rechargeability, size, safety, energy or power density (quantity of current that can be delivered per unit time at a given electric potential), constant discharge electric potential (known as a "flat" discharge because when electric potential vs. time is graphed, the line is "flat" rather than negatively sloped), ease of construction, availability of materials (especially if the battery has military applications), and resistance of electrodes to corrosion and to forming an unreactive layer. Advances in batteries are typically made by improving materials from which electrodes and electrolytes (salt bridges) are constructed. One trick is to make a previously impractical material practical.
For example, sodium/sulfur batteries can potentially store about four times as much energy as the common lead-acid battery currently used in automobiles. A practical sodium/sulfur battery could make electric automobiles feasible. However, sodium/sulfur batteries operate at high temperatures and pressures. Therefore, the electrolyte material which separates them must be able to withstand those conditions while maintaining its ability to allow ions to migrate through it. Currently, researchers are attempting to develop a tough conductive ceramic by treating alumina with zirconium oxide doped with a small amount of rare earth oxide such as yttrium oxide. Other researchers are testing a sodium ion-containing glass made from oxides of silicon, sodium, aluminum, and zirconium.
Other systems under investigation as potential batteries for electric automobiles include zinc/bromine, lithium/iron disulfide, lithium/iron monosulfide, and aluminum/air batteries. Of those currently under development, only the aluminum/air battery has a theoretical energy density greater than gasoline,and is the only battery that could power an automobile for more than 300 miles without a recharge--the electrical equivalent of filling the tank. (The Ideal School Supply Company, Oak Lawn, IL 60453 sells an aluminum/air battery kit for educational purposes.) Scientists and engineers are constantly seeking new materials with interesting properties to accompany suitably high or low half-reaction reduction potentials. Other methods for improving batteries can include finding a coating for electrodes that allows them to function while protecting them from corrosion or the formation of an inert layer; redesigning the shapes of electrodes and their relationship to each other to lower internal resistance and to make recharging easier and more efficient; and spinning an electrode mechanically so that corrosion or surface layer formation is uniform.
Because batteries have so many applications and because the commercial potential of electric automobiles will be tremendous if an appropriate battery can be developed, large companies and entrepreneurial small companies invest heavily in electrochemistry research. For more information on advanced batteries see Brodd (1988) and Rawls (1985).
Fuel Cells
Fuel cells resemble voltaic cells in that electrochemical reactions occur in half cells connected by an external circuit. Batteries contain all the chemical reactants internally with only electricity flowing out (discharge) or in (during recharging). Fuel cells, however, constantly produce electricity but require reactants to be constantly fed in as "fuel." One simple fuel cell involves hydrogen fed into one electrode, oxygen fed into the other, producing electron flow through the external circuit and water.
Fuel cells are potentially much more efficient sources of electricity than are steam turbine-driven power plants. (See Links to Physics.) Several companies have built or are working on large fuel cells. Westinghouse Corporation has built a 1.5-MW fuel cell that uses phosphoric acid as the electrolyte and hydrogen and oxygen as reactants. It is called a PAFC Generator, where PAFC stands for Phosphoric Acid Fuel Cell.
Although fuel cells show promise as a system for generating electricity in the future, a number of technical problems must be solved. Most fuel cells use hydrogen as a fuel. A fuel cell that worked well with methyl alcohol would be more desirable because methyl alcohol is derived from plants more easily and cheaply than hydrogen gas can currently be produced. Many fuel cell combinations react too slowly to supply sufficient current for industrial uses. Catalysts that can speed up the electrode reactions are needed. Many noble metal (e.g., Pt, Au, Ag, etc.) compounds work as fuel cell catalysts, but are expensive. Recent research has focused on using biological molecules such as porphyrins and phthalocyanines in conjunction with noble metal catalysts to increase the efficiency and decrease the amount of noble metal required. Fuel cells work best at high temperatures, but high temperatures are not possible for many applications; a polymer that conducts hydrogen ions (protons) at room temperature would be especially attractive as a solid electrolyte for many fuel cells (see Potential Projects).
One interesting application of the fuel cell idea is to turn a chemical plant into a fuel cell; for example, plants that make ammonia and methyl alcohol can be modified to become electricity-generating fuel cells. The opinion of many scientists and engineers is that fuel-cell technology will be a very profitable enterprise in the next few decades. For more information, see the 1988-89 series of articles by Lindstrom.
Electrochemistry and Dental Fillings
The material most commonly used for filling decaying teeth is called a dental amalgam. It is made by combining mercury with other metals such as silver or tin. If a person bites a piece of aluminum foil and the foil presses against a dental filling, a sharp, momentary pain often results. This result is due to a galvanic cell being created in the mouth. The aluminum serves as the anode and the filling is the cathode. Saliva acts as the electrolyte. A weak current flows between the electrodes and is detected by the sensitive nerve of the tooth as an unpleasant sensation.
Another type of reaction can occur if a filling makes contact with a gold inlay of a neighboring tooth. In this case, the dental filling made of mercury and tin acts as the anode and the gold inlay as the cathode. The dental filling starts to corrode. Prolonged corrosion will create the eventual need for a new filling. When this reaction occurs, tin(II) ions are released into the mouth, resulting in an unpleasant metallic taste. (See Transparency 5).
3. Kinetics/Rates of Reactions. One feature of electrochemical reactions is that the rate of reaction, unlike reactions in solution, is limited by how fast reacting molecules or ions can travel through solution to get to the electrode surface. When the external circuit is first connected, reactions occur very quickly involving molecules that happen to be close to the electrode at the time. Therefore, there is essentially a "burst" of current for a voltaic cell or a "burst" of electroplating or electrolysis for an electrolysis cell. Once atoms and molecules near the electrode are used up, reacting species must travel to the electrodes. If those reacting species are charged, they will be attracted to the charged electrodes. If the reacting species are not charged, travel to the electrodes will be controlled by the rate of diffusion unless the reaction mixture is vigorously stirred. Even with vigorous stirring, the layer, just a few molecules thick, near the electrodes might not be stirred.
One way to stir the reaction at a molecular level if the species are charged is to place the electrochemical cell in a magnetic field, thereby forcing the ions to spiral toward the electrode, stirring the mixture on the way in. (This is analogous to the path taken by charged particles from the sun as they spiral into the earth at the poles under the influence of earth's magnetic field.) A suggestion for a student research project based on this principle appears in Extensions and Projects. Since the current that a voltaic cell or fuel cell can deliver depends on the rate at which reactants reach the electrodes, this is a very important effect. Once reacting species reach the electrode, their reaction rate depends on all the features of reactions in solutions. Catalysts play a role in batteries at the electrode surface, lowering the activation energy for and thereby speeding up the electron transfer reactions. The chemical trick to placing catalysts on the electrode surface is, of course, to maintain or even improve half-reaction reduction potential characteristics. For a good description of how electrochemists modify the surface of electrodes, read Faulkner (1984).
4. Periodic Properties. Although reduction potentials are more complex, they are similar in concept to the ideas of ionization potential and electronegativity, which are useful in discussions of periodicity. Where possible, periodic properties should be reinforced by comparing reduction potentials of elements within a single family.
6. Electroplating. Electroplating is another important commercial process. Steel trash cans and steel siding are protected from rusting by electroplating a thin layer of zinc on the surface. A large silver anode, serving as a source of silver ions, is used to silver plate spoons, forks, and knives hung in a plating bath of silver cyanide. The plated objects serve as the cathodes in this electrolytic cell. A typical coating of silver has a thickness of about 0.1 mm.
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