Batteries
In a chemical reaction, atoms swap partners. The electrons are now shared in a different way. This
almost always means that the energy of the system has changed, and the reactions that happen
spontaneously are the ones where the energy decreases. The excess energy is usually released as heat,
or perhaps light. But still another possibility is that the energy is given to an electrical system; then the
chemical reaction becomes a source of electrical power: a battery.
To do this we need two reactions which are happening in different places. In one reaction the
rearrangement of atoms produces excess electrons; in the other, electrons need to be supplied. The
electrons are themselves “chemical reactants”! Here is an example: an old-fashioned kind of battery
called the Daniell cell. This has two compartments with a porous separation (so that ions can move
through it). On one side there is a copper plate, and on the other, a zinc plate. The side with the
copper plate in it starts out bathed in a high concentration of copper sulfate. This provides copper ions
(copper atoms lacking two electrons Cu++).
On the zinc side there is a sulfuric acid solution; it contains sulfate ions (SO4++).
We can represent the Daniell cell by a pair of chemical reactions:
Cu++ + 2 e- ==> Cu
Zn ==> Zn++ + 2 e-
The copper ions acquire a pair of electrons and become copper metal, while the zinc metal gives up a
pair of electrons to become zinc ions. The electrons coming from one reaction are used in the other –
they both have to take place, which means that there has to be a wire connecting the two sides to move
the electrons from one side to the other. The result of having the two reactions take place is that some
energy is released. This is the energy that the electrical current delivers to a circuit element (like a light
bulb) along the way. The amount of energy per electron is directly proportional to the voltage of the
battery.
This picture is trying to show that the copper changes energy
more in acquiring the electrons than is needed to pull the
electrons off of the zinc atom.
We can think of the two reactions taking place separately, and assign a voltage to each one. The
division is a bit arbitrary, because in reality you always have to get rid of the electron, so there are
always two reactions going on. The convention is to call the reaction that turns hydrogen gas into
hydrogen ions "zero volts." Then we can make a table:
Zn++ + 2 e-
==> Zn
- 0.76 V
2H+ + 2 e-
==> H2
0V
Cu++ + 2 e-
==> Cu
O2 + 4 H+ + 4 e- ==> 2 H2O
+0.34 V
+1.23 V
In the case of the Daniell cell, we are using the first and third, and running the first one backwards, so
the total voltage is
0.34 V – (-0.76 V)=1.10 V
The Daniell cell can be recharged by applying a backwards voltage slightly larger than 1.10 V and forcing
the current to go the other way. Then we are running the first reaction forwards (zinc ions turning into
zinc metal) and the third one backwards (copper metal turning into copper ions).
However, according to the table there is another reaction
2H+ + 2 e-
==> H2
O2 + 4 H+ + 4 e- ==> 2 H2O
0V
1.23 V
This will split H2O into H2 +O2 -- we get hydrogen and oxygen gases, instead of a charged battery. Since
1.23 V is only slightly more than 1.10 V, recharging is a little tricky: you can't be in a hurry. Hydrogen
gas will explode, and it was a property of this kind of battery that they exploded occasionally.
Rusting, and other kinds of oxidation
There are chemical reactions that decrease the energy (and thus are allowed to happen) but which
happen very slowly. Diamond in air should turn into carbon dioxide. Iron in air should turn into iron
oxide. These reactions don’t take place at room temperature, because the intermediate steps involve
pulling apart molecules, and this is temporarily a high energy configuration. So the situation is similar
to having two lakes with different levels and a hill between them: the water would flow downhill if it
could, but the need for a small increase in energy prevents the large release in energy from taking place.
There are two ways to overcome the obstacle:
*A catalyst. This is a side reaction that avoids the barrier. It is the chemical equivalent of a river that
goes around (or through) the hill:
We could regard the current-carrying circuit as a catalyst that lets the Daniell cell reactions go.
Iron rusts a lot faster when it is wet. It isn’t really the water that causes the rust; instead, the water
helps the oxygen from the air do its thing: water catalyzes the rusting reaction.
*Raise the temperature. This is the chemical equivalent of putting big waves on the lake, that splash
over the barrier. Iron and copper normally don’t react with the oxygen in the air, but at high
temperatures they will acquire a tarnish.
If the temperature gets high enough, reactions will proceed fast enough that the energy released will
replace the energy that is escaping, maintaining the temperature: this is combustion. Oil won’t react
with air at room temperature, but oil will burn if you get it hot enough.
An example of this process occurred in the Japanese nuclear power plants after the tsunami. The
reactors got very hot, because the water pumps weren’t working. The nuclear fuel is contained in thinwalled pipes made of zirconium (to keep the radioactive stuff from dissolving in the water). Zirconium
normally doesn’t react with water, but at high temperature it will:
Zirconium + H2O  Zirconium oxide + hydrogen gas
Eventually the pressure of the hydrogen gas got so high they had to release it. But hydrogen gas will
react with oxygen (again, at high enough temperature); perhaps the operators were hoping the gases
would stay cool and not react, but instead there was an explosion that blew the roof off of the power
plant.
As before the zinc plate gets eroded away, but the copper plate gets thicker. When the battery is
discharged, all of the copper ions are gone, and there are lots of zinc ions instead.