Chemistry only
Cells and batteries
A good place to start this topic on cells and batteries is to clear up a common misconception. If you
were to ask most people what the image on the right is, most people would say it's an image of 4
batteries! Unfortunately most people use the words
cell and battery as if they were the same
thing, when they are not. The image actually shows 4 cells, not 4
batteries
Each of the cells in
the image contains chemical which react to produce an electrical current and a voltage of 1.5 volts.
If you were to put all 4 cells into a children's toy, then you would have a
battery producing 6V
(1.5V x 4= 6V). A battery is a group of cells all joined together.
The word battery comes from
the military, where a line of guns is called a battery of guns e.g. the Queen would have a
battery of 21
guns fire to celebrate her birthday. Or a 12 volt car battery contains 6
cells all connected,
with each cell producing 2 volts.
A simple electrochemical cell
Consider a reaction we looked at earlier under the displacement reaction topic. The reaction
between zinc metal and copper sulfate solution.
When a strip of zinc metal is dipped into a blue copper sulfate solution almost immediately a
black coloured layer of copper metal is produced on the zinc metal. The blue copper sulfate
solution slowly fades and forms a clear zinc sulfate solution. This reaction is a displacement reaction
but it is also a redox reaction. Remembers a redox reaction is one where both reduction and
oxidation take place.
Equations for reaction taking place
word equation
zinc(s) + copper sulfate(aq) → zinc sulfate(aq) + copper(s)
symbolic equation
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
ionic equation
Zn(s) + Cu2+SO42- → Zn2+SO42-(aq) + Cu(s)
Spectator ions are ions that take no part in the reaction, that is they are found on both
the reactant and product side of the equation unchanged. In the above equation you can see
that the sulfate ions (SO42-) are spectator ions. If we re-write the ionic equation but omit
the spectator ions it will help us to see more clearly exactly what is happening in the reaction.
Ionic equation with the spectator ions removed
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
We can further split this down and make it even simpler. Consider each reactant in turn
and follow them as the reaction happens:
Zn(s) → Zn2+(aq) + 2e this is oxidation.
The zinc
atoms lose 2 electrons and forms zinc ions (Zn2+)
Cu2+(aq) + 2e → Cu(s) this is reduction
The copper
Ions (Cu2+) gain 2 electrons from the zinc and are reduced.
Each of these equations are called half-equations, for
obvious reasons, they represent half the overall reaction taking place.
From the equations above you can see that a transfer of electrons is taking place, electrons flow
or move from the zinc metal to the copper ions. If we could somehow intercept this flow or
movement of electrons then we would have an electric current- a cell! Unfortunately setting
up the experiment as shown above makes this impossible, any electrical energy produced by the
chemical reaction is lost as heat. However we can modify it to intercept these electrons and
produce an electrical current from the chemical reaction.
Making a cell or battery
Have you ever been in a rush to eat a packet of your favourite sweets and accidentally eaten
some of the metal sweet wrapper by mistake? You might have felt a sharp pain going through
your tooth if the metal sweet wrapper came into contact with any
metal in your fillings
(if you have any!). The reason for this is that to make an electrochemical cell or just
cell all you need is two different metals in a solution that
conducts electricity. So
in your mouth you could have aluminium or tin in the sweet wrapper and there is mercury,
silver, tin and copper in tooth fillings.
All you need to make a cell is two different metals in contact with some moist paper, the paper
needs to be soaked in a solution that conducts electricity (an electrolyte) such as
salt water. Connect the two metals to a voltmeter and you will be able to record the
voltage produced. This basic set-up is shown below. The further apart the two metals
are in the reactivity series the larger the voltage produced. So to increase
the voltage
of the cell shown you could swap the copper for say silver, as silver is lower in the
reactivity series than copper. The set-up shown with Copper and zinc is not efficient
and would not produce a large voltage.
Improving the design of cells
To increase the voltage we can modify the set-up. The two metals we are using are copper and zinc.
These metals are placed in beakers containing a solution of their own ions. This prevents unwanted
side reactions from occurring where the metals can react with the
electrolyte and cause reactions you
did not plan for. The two metals are then connected via electrical leads to a voltmeter. However
if the metals are in beakers containing their own ions then we have a gap in the circuit. This gap
is filled with a salt-bridge. This is simply a piece of filter paper soaked in an inert
electrolyte (that is an electrolyte that will not react with the contents of the beakers
or any of the metal), sodium sulfate solution is often used. In the diagram below a more efficient
salt-bridge is used, here a glass tube is filled with a gel permeated with sodium sulfate.
The salt-bridge completes the circuit and maintains a balance of charge in the two solutions by
allowing the free movement of sodium and sulfate ions. The ions in the salt bridge are inert and take no part in
the cell reactions. These ions migrate into the beakers to ensure electrical neutrality is maintained as the
cell reactions take place.
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