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I did some growing up in the city of New Orleans. One of the most striking landmarks of the Big Easy is the Greater New Orleans Bridge. There is a toll to cross this bridge and we can go a long way toward understanding the chemistry of batteries by examining the economics of bridge tolls. Why would anyone pay a bridge toll? Obviously because they want to get to the other side. How much of a toll would they be willing to pay? It depends on how badly they want to cross the river by car; let us call this desire the automotive force, or AMF, with units of dollars/car. The AMF depends on the location of the bridge; it will be high when the bridge connects, for example, a major commercial district with a major residential area. People will pay a toll to cross the bridge when the cost of the toll does not exceed the automotive force.
Let us consider a chemical analogue. Whenever we have manipulated alkalis in this book we have been careful to avoid the use of aluminum containers or utensils. The reason is that aluminum is easily oxidized by alkaline solutions:
When sodium hydroxide is sprinkled on wet aluminum foil a vigorous reaction takes place; the solution foams up, the foil is eaten away, and a great deal of heat is released. If we were to look closely at the foil we would find some areas in which aluminum is being oxidized to aluminum hydroxide; these areas comprise the anode, the site of oxidation. In other areas water is being reduced to hydrogen gas; those areas comprise the cathode, the site of reduction. In the example so far, half of the aluminum foil acts as the anode and half as the cathode with electrons being passed from anodic regions to adjacent cathodic ones. Such a situation is analogous to a city in which residences and businesses are mixed in and among one another; there is plenty of traffic, but since it is mostly local it would be difficult to collect a toll from people commuting to and from work.
The situation would be different if we were to connect to the aluminum foil an inert metal, one which does not react with alkali. Such a metal could not be oxidized by the alkali and so it could not serve as an anode. But electrons would be able to move from the anodic regions of the aluminum foil, through the inert metal, and into the water. The water would be reduced to hydrogen gas at the surface of the inert metal and so the inert metal would serve very well as a cathode. With reduction taking place at the inert metal surface, the entire surface of the aluminum foil would become available for oxidation. Such a situation is analogous to a city in which the business district is adjacent to the residential district; there would be an automotive force reflecting the desire of people to travel from one district to another, but no way to collect a toll.
Let us now separate the aluminum foil from the inert metal, leaving each one in contact with the alkaline solution. Such an arrangement, a voltaic cell, is analogous to a city in which the business and residential districts are separated by a river. Figure 21-1 shows a process schematic for the aluminum-alkali cell. Here the electrons consumed at the anode and produced at the cathode are shown explicitly as reactants and products. The electrons on the reactant side represent a wire dipping into the solution, or electrolyte; hydrogen gas is produced at this wire, the cathode. The electrons on the product side represent the wire leading from the aluminum anode. With the cathode at the top and the anode at the bottom of the schematic, we can imagine the electrons flowing downhill, impelled by the electromotive force, or EMF, analogous to the automotive force. Of course, in the physical cell the relative placement of the anode and cathode is irrelevant. The convention established in the process schematic simply helps us to re-meme-ber the direction of electron flow. If the wire from the anode is connected to the cathode we have, in effect, created a bridge and may "charge" the electrons a "toll" for the privilege of crossing over; we might require them to light a bulb or turn a motor. How much of a toll would they be "willing" to pay? Any amount up to the value of the electromotive force, measured in volts.
The EMF, or voltage, of a cell depends on the identity of the conductors and on the identity and concentration of the electrolyte. Equation 21-1 shows the unbalanced reactions for four voltaic cells, (a) the aluminum-alkali cell, (b) the Leclanche carbon-zinc cell, (c) the alkaline carbon-zinc cell, and (d) the lead-acid cell. The aluminum-alkali cell is the one we have been discussing so far, with an EMF of 1.0 V, more or less, depending on the concentration of the alkali. The Leclanche cell is the familiar flashlight battery, with a voltage of 1.5 V. The alkaline cell comes in the same voltage and sizes as the Leclanche cell. Six lead-acid cells, at 2.0 V each, comprise the common automobile battery. Take a moment to balance these redox reactions using the method of Chapter 11 and then sketch out the process schematics for each cell. You can identify the anodes and cathodes by re-meme-bering that electrons go in at the top, down through the cell, and out the bottom; under this convention, the positive cathode is the electrode on top.
The flow of electrons is really only half the story of the voltaic cell. Returning to the bridge analogy, the Greater New Orleans Bridge collects tolls only from cars traveling towards the business district; the return trip is free. The "return trip" in our aluminum-alkali cell is afforded by the alkaline solution. Negative hydroxide ions are produced at the inert metal cathode and consumed at the aluminum anode. Just as the electrons flow from anode to cathode, the hydroxide ions migrate through the electrolyte solution from cathode to anode. This migration of charge through the electrolyte constitutes an electric current equal and opposite to that in the wire. The unit of electric current is the ampere or amp. The electric current is proportional to the rate at which electrons flow through the wire. The more electrons cross over in a given time period, the higher the current. Thus electric current is analogous to the rate at which traffic crosses the bridge.
The amount of bridge traffic depends, of course, on the AMF; if people have little motivation to cross the bridge, traffic will be light. But it also depends on the population of the city; a large city will have more traffic than a small one. Similarly, the current that can be delivered by a voltaic cell depends on the size of the cell. For example, flashlight cells come in different sizes, D, C, A, AA, and AAA. Each of these cells has the same voltage. But the big D cell can deliver more current than the tiny AAA cell. Hence a boom box, with its large speakers and motors, requires D cells; a pocket radio, with only headphones, can get by with AAA cells.
The big payoff for the bridge comes when lots of people are willing to pay high tolls for long periods of time. For electrical devices, the payoff is called energy, with units of joules:
1 joule = (1 volt)(1 amp)(1 second)
The rate of energy delivery is called power, with units of watts:
1 watt = (1 volt)(1 amp)
Thus 1 joule = (1 watt)(1 second)
We can use these definitions to do UFA, for example:
More power is delivered by two voltaic cells than by one and there are two ways of connecting cells to form a battery. When two cells are connected in series the anode of one is connected to the cathode of the other. For example, when Leclanche or alkaline cells are loaded into a flashlight the "nose" or button end, the positive cathode, of one is placed in contact with the flat bottom, the negative anode, of the next. The voltage of this battery of two cells will be double that of either cell on its own. You can think of two process schematics stacked one atop the other; the electrons "fall" twice the normal distance. For series batteries of more than two cells, the voltage is proportional to the number of cells. Thus, if you cut open a 9-volt transistor battery, you will find six 1.5-volt cells within. Similarly, a 12-volt automobile battery consists of six 2-volt lead-acid cells. Since the same current flows through each of the cells in a series battery, the power delivered is proportional to the number of cells; two cells deliver twice the power of one.
Conversely, when two identical cells are connected in parallel, that is, anode to anode and cathode to cathode, the voltage of the pair is the same as that of an individual cell. Since the same current flows through each cell, the current of the pair is twice that of either on its own. Six 1.5-volt cells in parallel will have a voltage of 1.5 volts, but will be able to deliver more current than any of them alone. You can think of the process schematics arrayed side-by-side. In effect, the cathodes and anodes form one giant cathode and one giant anode. Since the voltage is the same as that of the individual cells but the current is proportional to the number of them, the power delivered is, again, proportional to the number of cells; two cells deliver twice the power of one. Thus more power is delivered by more cells no matter which connection scheme is used. The choice of series of parallel connection depends on the voltage required by the device to be powered.
This chapter has introduced a host of intangible concepts and their units: the volt, the amp, the joule, and the watt. It may have left you weary, feeling small. When tears are in your eyes I will dry them all. I am on your side. When times get rough and friends just can't be found, just remember that a voltaic cell is like a bridge over troubled water.
Locate MSDS's for aluminum (CAS 7429-90-5), soda ash (CAS 497-19-8), and charcoal (CAS 7440-44-0). Summarize the hazardous properties in your notebook, including the identity of the company which produced each MSDS and the NFPA diamond for each material.
Your most likely exposure will be eye or skin contact. In case of eye contact flush them with water and call an ambulance. In case of skin contact wash the affected area with plenty of water.
You should wear safety glasses while working on this project. Leftover soda can be flushed down the drain with plenty of water. Used aluminum foil should be thrown in the trash. Used charcoal may be thrown in the trash or saved for reuse.
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You should not remain ignorant if you are to proceed in the Work.
The NFPA diamond was introduced in Section 15.2. You may substitute HMIS or Saf-T-Data ratings at your convenience.