cells and batteries

Chemistry only

Cells and batteries

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, this is shown in the diagram below. 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.

displacement reaction of zinc 
and copper sulfate

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

man with toothache 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.

simple cell set-up

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.

zinc copper cell