We have become quite proficient by now at balancing redox reactions. So far we have used the electrons merely as a bookeeping device to ensure that the reaction is balanced. There is a deeper meaning to these electrons, however, and a whole new world of chemical possibility is opened up when we learn to harness them.
Let us take the simple example of the reaction of zinc with sulfuric acid.
Zn(s) + H2SO4(aq) -----> ZnSO4(aq) + H2(g)
This is a really easy reaction to balance (in fact it is already balanced, but try it anyway). The fact is that if you dip a strip of zinc metal in dilute sulfuric acid, the zinc will be oxidized to zinc sulfate and the sulfuric acid will be reduced to hydrogen gas.
Recall that ionic substances fall apart when they dissolve in water. For example,
H2SO4(aq) -----> 2 H+(aq) + SO42-(aq)
ZnSO4(aq) -----> Zn2+(aq) + SO42-(aq)
So another way of writing our zinc-acid reaction is
Zn(s) + 2 H+(aq) + SO42-(aq) -----> Zn2+(aq) + SO42-(aq) + H2(g)
Notice that the sulfate ion appears on both the right and left hand sides of the reaction. In other words the sulfate ion is unchanged during the reaction. We call such an ion a spectator ion. Since it is the same on both sides, it makes sense to cancel it out, just as we would for water, hydrogen ions, or electrons. If we do so, we get the net ionic equation.
Zn(s) + 2 H+(aq) -----> Zn2+(aq) + H2(g)
When a chemist writes such a reaction, he knows that the H+ is not in solution by itself; it must be accompanied by some negative ion which is not participating in the reaction. Similarly, Zn2+ must have its charge balanced by some negative ion--the same ion that accompanied the H+. Looked at this way, it matters very little which specator ions are present. The reaction would be the same for nitrate, sulfate, acetate--any ion which forms a soluble compound with both hydrogen and zinc.
Now lets look at the half reactions:
Zn(s) -----> Zn2+(aq) + 2 e-
2 H+ + 2 e- -----> H2(g)
In the first half-reaction, zinc is losing electrons, that is, it is being oxidized. In the second half-reaction, hydrogen is gaining electrons, that is, it is being reduced. Remember LEO (Lose Electrons Oxidation) says GER (gain electrons reduction). In all of the examples we have discussed so far, this oxidation and reduction takes place directly because the two reactants are in contact with each other. The lead sulfide was in contact with the hot charcoal when we smelt lead. The saltpeter, sulfur, and charcoal are all mixed up when we burn gunpowder. The energy of these reactions comes out in the form of heat. The same is true if we dip zinc in acid: the reaction takes place where the zinc and acid come into contact and the energy is released as heat.
But what about those electrons? What if there were a way to separate the oxidation from the reduction, preventing the oxidant from directly giving its electrons to the reductant. Instead of releasing the energy of the reaction in the form of heat as the electrons are transferred, perhaps we could set up a pathway for the electrons to be tranferred by an indirect route and require them to do some electrical work along the way.
Think about traffic in New Orleans. New Orleans is divided by the Mississippi River into the East Bank and the West Bank. If you live on the East Bank and work on the East Bank, you don't have to pay any tolls. If someone were to charge a toll for the road you usually take to work, you would just choose a different road. But if you live on the West Bank and work on the East Bank, you don't have the luxury of choosing among many roads -- there are only a few ways to cross the river. Consequently if someone puts a toll on the bridge, you have to pay to get across.
We can do the same thing electrochemically. If we separate the oxidation from the reduction, we can provide a wire for the electrons to travel through and we can have them do some work (light a light bulb, power a motor, etc.) as they go through the wire. In a sense, we charge them a toll to cross the bridge from oxidant to reductant.
So lets imagine two jars, one filled with water and zinc, the other filled with sulfuric acid. The zinc and acid can't react directly because they are not in contact with each other. If we connect a wire from the zinc to the acid, this allows electrons to flow from the zinc to the acid. And since they can't get across any other way, we can require them to do work for us. But very shortly, the acid will get a net negative charge and the zinc will get a net positive charge. The electrons will stop flowing because like chages repel and unlike charges attract. We need some way to complete the circuit and preventing these charges from building up. If we connect a tube of water between the two beakers, this will allow the spectator ions (remember the spectator ions?) to flow from the acid to the zinc and complete the circuit. This solution of soluble ions is called an electrolyte. Any of the soluble compounds discussed in the metathesis project are examples of electrolytes.
Now, you don't really have to go to all this trouble. After all, there are toll roads in Richmond that don't necessarily cross the James River. As in traffic, all we have to do is make sure that it is easier for the electrons to go through the wire than to transfer from the oxidant to the reductant directly. So, if we have a single jar of sulfuric acid and dip a zinc wire into the acid, the acid that comes into contact with the zinc will react directly. But most of the acid is not in contact with the zinc. If we simply put our acid in a copper jar (copper is not oxidized by dilute acid), and put the zinc strip in the middle of the jar, the acid in contact with the copper cannot receive electrons from the zinc because they are not in contact. But if we pass a wire from the copper to the zinc, the electrons can pass from the zinc through the wire, through the copper, to the acid. The sulfate ions left over from the acid will diffuse toward the zinc electrode where they will balance the positive charge of the zinc ions as they come off the electrode. As long as the wire is a good conductor it will represent the path of least resistance for electrons to get from the acid to the zinc.
Some reactions have a large driving force. They want to go really bad and will release a lot of energy when they do. For example charcoal, sulfur, and saltpeter release a lot of energy when they react as burning gunpowder. Other reactions are more timid, for example, the oxidation of glucose to ethanol. We can quantify this driving force the electromotive force, measured in Volts. The EMF for each half-reaction is called the standard reduction potential. Here is a list of common half-reactions:
We can get the EMF of any cell containing these half-reactions by turning around
the half-reaction which appears lower in the list and subtracting its
standard reduction potential from the one which is higher in the list.
For example, in our zinc acid cell, zinc is lower in the table and the reaction will
+0.00 V: 2 H+ + 2 e- -----> H2(g)
+0.76 V: Zn(s) -----> Zn2+(aq) + 2 e-
+0.76 V: 2 H+ + Zn(s) -----> H2(g) + Zn2+(aq)
The EMF of this cell will turn out to be close to +0.76 V. (It may be slightly different depending on the concentration of the acid, but that's a more advanced topic.)
The electrode where the oxidation is occurring is called the anode while the electrode where the reduction is occurring is called the cathode. In our example, the zinc is the anode and the inert copper electrode is the cathode.
Notice that the highest voltage possible from this short list is only 3.07 V! How do we get higher voltages? We connect these cells in series to form a battery. That is we connect the cathode from one cell to the anode of the next cell. This is why flashlight batteries are placed end to end to produce higher voltages. In our example two zinc acid cells in series would give an EMF of 1.52 V. 10 of them would provide 7.6 volts.
There are two electrical quantities that must be distinguished: voltage and current. You can think about these by analogy to the flow of water in a waterfall. With a tall waterfall, each drop hits very hard when it hits the bottom. With a short waterfall, the drops don't hit very hard. This is analogous to voltage. Some waterfalls have a lot of water flowing, others only a little. This is analogous to current. Current is defined as the rate at which electrons flow from anode to cathode. Thu unit of current is the Ampere (or Amp for short).
Consider a very tall waterfall with very little water flowing over it. If you stand under such a waterfall, each water drop will hit you pretty hard. They may even sting you a little. But there are so few drops that you probably would not be hurt by such a waterfall. And you certainly wouldn't be able to use it to run a mill (for grinding corn, or making paper, or some such). Now consider a short waterfall with a lot of water flowing over it. While each drop might not hit you very hard, the sheer amount of water might knock you down. It's the same with electrical power. High voltage may hurt, but it's current that kills.
In galvanic cells, the voltage is determined entirely by the kinds of materials in the cell: the metals used for the electrodes and the compound used for the electrolyte solution. Size does not determine voltage. In everyday life, AAA, AA, C, and D cells all have a voltage of 1.5 Volts.
But size does matter as far as current is concerned. Big batteries provide more current than little ones. Thus a small radio may operate on 2 AAA batteries. A larger cassette player may need 4 C batteries and a large boom-box may need 8 D batteries. And a battery-operated kiddy car may use a 20 pound lead-acid battery. As the amount of current drawn by the device increases, so must the size of the batteries.
We have seen that to make a high-voltage battery, we connect the cells in series: the anode of one cell to the cathode of the next. When two identical cells are connected in series, the voltage doubles, but the current is about the same as for each cell. To make a high current battery, we connect the cells in parallel: all the anodes together and all the cathodes together. In effect this makes one big cell with the same voltage as the individual cells but with much more capacity to provide current. With two identical cells in parallel, the current doubles, but the voltage is about the same as for each cell.
And what about lots of big batteries connected to produce high voltage? This is like a tall waterfall with lots of water going over it. Such a waterfall is very powerful. EMF times current gives us power, which is the rate at which a battery does work. A Volt times an Amp is a Watt, the unit of power. If you think about it, a battery will produce the same power whether it is connected in series or in parallel.
And if power is the rate at which work is done, then power times time must be work. And it is! A Watt*sec is a Joule, the unit of work. In the electrical business, electricity gets sold in units of kWatt*hr = 1000 Watt * 3600 sec = 3.6 million Joules = 3.6 MJ.
Just as it's harder to push traffic across a 1 lane bridge than a 6 lane bridge,
it's harder to push electrons through a long skinny wire than through a
short fat one.
This resistance to the flow of electrons is called, oddly enough resistance
and is measured in Ohms. It takes an EMF of 1 Volt to push 1 Amp
of current through a resistance of 1 Ohm:
1 Volt = 1 Amp * 1 Ohm
This relationship is called "Ohm's Law" and is the foundation for understanding electrical circuits. The higher the resistance, the more voltage it takes to push a given current through it. Conversely, for a given voltage, less current will flow through a high resistance than a low one. The resistance of a wire increases with length and decreases with cross-sectional area. That is, long skinny wires have higher resistance than short fat ones. The resistance also depends on the material from which the wire is made. For a given wire length and diameter, a copper wire will have less resistance than an iron or lead one.
Let's look at some common cells and the batteries built from them. The first
is the dry cell, Leclanche Cell, familiar to us as the
"ordinary" (nonalkaline) flashlight battery. This cell consists of a zinc
case filled with a moist paste containing ammonium sulfate. In the center
of this electrolyte paste is a carbon rod coated with manganese dioxide, which
is a strong oxidizing agent. The reaction which takes place is a classic
Zn(s) + 2 MnO2(s) + 2 NH4Cl(aq) -----> ZnCl2(aq) + Mn2O3(s) + 2 NH3(aq) + H2O(l)
The zinc is oxidized so it is the anode, and it forms the negative end of the cell. The manganese dioxide is reduced so it is the cathode, and the positive end of the cell. The voltage of a fresh Leclanche cell is 1.5 V, and the current it can produce depends on the size. Unfortunately, as the reaction proceeds, the ammonium chloride concentration drops and the voltage along with it. This drop of the voltage as the cell ages is one of the major disadvantages of this cell, and it is currently heading for extinction.
The alkaline cell is really based on the same redox reaction as the Leclanche cell
except that potassium hydroxide is used as the electrolyte:
Zn(s) + 2 MnO2(s) + H2O(l) -----> Zn(OH)2(s) + Mn2O3(s)
Notice that unlike the Leclanche Cell, the electrolyte is not consumed in this reaction. That means that the voltage of the alkaline cell stays very close to 1.5 volts right up to the end, when either the zinc or the manganese dioxide are used up. This advantage of constant cell potential is the principle reason that the alkaline cell has all but replaced the Leclanche cell in common usage.
A battery must put out a lot of current to turn over an automobile engine.
Pound for pound, no storage battery in common usage can compete with the
lead storage battery. The lead storage cell consists of a lead plate and a
lead(IV) oxide plate immersed in a sulfuric acid solution. The reaction is:
Pb(s) + PbO2(s) + 2 H2SO4(aq) -----> 2 PbSO4(s) + 2 H2O(l)
Lead is oxidized, is the anode, and the negative end, lead oxide is the cathode and the positive end. It has a EMF of 2.0 V when fully charged and a car battery consists of 6 cells in series to develop 12 V. As the sulfuric acid electrolyte is depleted, the voltage drops, just as in the Leclache cell. Since sulfuric acid is more dense than water, the density of the electrolyte drops as the battery discharges. This makes possible the little bulb tester, which by measuring the density of the acid can determine whether the battery is charged.
The lead acid battery has two points in its favor. First, it can deliver enormous currents for its weight. Second, it can be recharged, that is, electrical power applied with reversed polarity (positive to lead, negative to lead oxide) drives the redox reaction backwards, consuming lead sulfate and regenerating lead and lead oxide.
At the same time, water is converted back to sulfuric acid. One hazard is that
if the reverse voltage is not carefully controlled, water can be reduced in a
competing redox reaction:
2 H2O(l) -----> 2 H2(g) + O2(g)
This combination of hydrogen, oxygen, and electrical sparks makes battery recharging a potential explosion hazard.
The electric car has long been a pipe dream in the automotive industry. The chief obstacle to such a car is that the batteries used must deliver large currents and be rechargeable. For 100 years, the lead acid battery has been the best contender in this area. But even it is too heavy and requires too long to recharge to be practical in a car. For the moment, the electric car must wait for a new kind of cell to be developed.
The battery quiz will consist of three questions on the following topics:
You will build a 6 volt battery from scratch. You may use any commonly available metals
for your electrodes and any commonly available electrolytes for your solution.
The only requirement is that you cannot use materials taken from
other batteries, and your battery must be able to power a small
electric motor. The battery shown here was built by Michael York, Caveman '94.
To pass, your battery must be able to run a small motor.