So far we’ve examined a number of extremely common techniques for the chemistry laboratory.
Today we are going to do something a little different, we are going to talk about how
batteries work.
A battery works by converting chemical energy into electrical energy, and this is covered
in a branch of chemistry we call electrochemistry, which we discussed in the lecture series.
In the case of a type of electrochemical cell called a galvanic cell, the production of
electricity is achieved by a spontaneous oxidation-reduction reaction, or a redox reaction.
These reactions work by coupling oxidation and reduction, or a transfer of electrons
from one substance to another.
In an oxidation, there is a loss of electrons.
In a reduction, there is a gain of electrons.
Since the redox reaction involves a flow of electrons, it can do work to produce electricity,
just like how flowing water in a river can do work to push a waterwheel.
This is because electricity is a flow of electrons.
To construct a galvanic cell in the laboratory, we need to set up the oxidation and reduction
reactions separately and connect them together.
These two sides are what we call half-cells.
The electrons are produced by oxidation on the left side, which is called the anode,
where neutral metal atoms donate electrons to become ions and join the solution.
On the right side, called the cathode, the reduction takes place as ions in the solution
gain electrons and deposit onto the electrode.
In a common example of a galvanic cell, we have zinc on the anode and copper on the cathode.
The zinc metal loses two electrons to become Zn2+ and goes into solution.
The solution is usually one of zinc ions such as zinc sulfate.
On the cathode, we have copper metal and a solution of copper ions such as copper sulfate.
The copper ions gain two electrons to become neutral copper and deposit on the copper electrode.
We must connect these two cells by what we call a salt bridge.
A salt bridge is a salt solution that is used to transfer ions and reduce the accumulation
of charge on either side.
When the half-cells are prepared and ready to go, they will be producing electricity.
All you have to do is connect the metal electrodes to a light bulb, or simply a voltmeter to
measure the voltage that's going through.
We are going to build a cell like this today.
To build a galvanic cell, we're going to start with the two half-cells.
We’re just going to use two beakers for that.
And in the beakers, we’ll add the two solutions, zinc sulfate and copper sulfate.
We're then going to place the metal electrodes into each solution like this.
The zinc metal goes into the zinc sulfate solution and the copper metal into the copper
sulfate solution.
We’ll then connect the two solutions with a salt bridge.
A salt bridge can be a U tube that's filled with a salt solution.
But it can also be a paper towel that has been dipped in the salt solution.
Today, we're going to use potassium iodide for our salt bridge.
Let's cut up some filter paper and dip it into our solution.
This is what we will use to connect the two beakers.
Now that we've created our two cells and connected them by a salt bridge, let's add a voltmeter
so that we can measure the voltage.
And look at that!
We are already producing electricity.
The reactions in the two half-cells are producing a flow of electrons that are being picked
up by our voltmeter.
As you can see here we have a reading of approximately 1 volt.
In the lecture series, we talked about how to calculate the theoretical voltage of a
galvanic cell.
It is the case that each of these half-cell reactions has a standard potential that's
already known and tabulated.
The potential of the cell as a whole is derived by the standard reduction potential at the
cathode minus the standard reduction potential at the anode.
And that is how you calculate the total voltage.
Once calculated, we can compare our theoretical voltage to the experimental voltage.
For our experiment, as we can see, our experimental voltage is pretty close to the theoretical
voltage.
Another experiment we can do is to figure out how the concentrations of the solutions
in the half cells affect the voltage.
This can be calculated using the Nernst equation.
As you can see here, it is really about the ratio between the two.
So if we measure this when both solutions are at half the original concentration, the
voltage will stay the same.
However, if we change only one concentration and not the other, the voltage will change.
Once we are done with this particular galvanic cell, we can replace one solution and electrode
with another solution and electrode, such as nickel in this case.
We can then measure the voltage between that half-cell and that of copper, or zinc.
So the more half-cells you have, the more pairings you can make.
With 3 half cells, you can make 3 different galvanic cells.
If you have 4 half cells, you can make 6 different cells, and so forth.
But you still measure the voltage the same way every time.
So that’s how chemical reactions can produce electricity, and that’s also how the battery
in your cell phone or your car works.
Of course the batteries in your devices will have more sophisticated reactions happening
at the anode and cathode.
But they still utilize the same concept of a redox reaction in half cells connected by
a salt bridge and some way to harvest
the electricity.