cells and batteries

Chemistry only

Cells and batteries

4 cells being charged in a charger 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 and not 4 batteries. However if you were to walk into your local hardware store and ask for 4 cells I doubt the shop assistant would know what you were asking for, however if you asked for 4 batteries then you would probably get the 4 cells you need!

Each of the cells in the image contains chemicals which react to produce an electrical current and a voltage of 1.5 volts. If you were to put all 4 cells in a child'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.

Making a electrical cell or battery

Young girl getting an electric shock 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? If yes then 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 an electrical cell all you need is two different metals in a solution that conducts electricity (an electrolyte).

In your mouth you could have the metals aluminium or tin in the sweet wrapper and there are the metals mercury, silver, tin and copper in any tooth fillings while the alkaline saliva acts as the electrolyte. This means that in your mouth you have all the requirements to make an electrical cell and the electrical current produced shoots up your metal filling to the nerve in your tooth and you feel a sharp pain.

Fruity cells!

All you need to make a cell is two different metals in contact with a solution that conducts electricity (an electrolyte). Perhaps the first simple electrical cell you ever made was using a lemon or an orange or as shown below a lime and a potato with two strips of different metals stuck into them.

Making a simple cell using a piece of fruit such as an orange, lemon or a piece of lime.  You also need two different metals

We can replace the fruit and vegetables in the "fruit cells" above with a solution of say sodium chloride or sodium nitrate. Both of these solutions contain an ionic compound dissolved in water. You may recall that solutions of ionic compounds are electrical conductors. Any ionic compound which dissolves in water will conduct electricity, we call these solutions electrolytes. However caution should be used in the choice of the ionic compound to be dissolved in water to form an electrolyte; it is best to use one that will not react with any metal that is placed in it when making cells. For this reason a sodium nitrate solution is often used as it is a fairly inert electrolyte.

The image below shows three cells; in each cell there are two different metals dipped in a sodium nitrate electrolyte. A voltmeter or an ammeter can be placed in the circuit to detect if an electrical voltage or current is flowing. In each cell there is a flow of electrons; that is an electrical current; from the metal highest in the reactivity series to the metal lowest in the reactivity series.

Diagram to show how to set up a simple electrical cell using two different metals and an electrolyte

There are several factors that will affect the size of the voltage produced in each of the cells but perhaps the most significant factors are:

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 colour of the blue copper sulfate solution slowly fades and to form a clear zinc sulfate solution. This reaction is a metal displacement reaction but it is also a redox reaction. Recall that a redox reaction is one where both reduction and oxidation take place. In this reaction the zinc metal loses two electrons and is oxidised to form zinc ions (Zn2+) while the copper ions (Cu2+) in the copper sulfate solution gain two electrons are reduced to form copper atoms.

displacement reaction of zinc 
and copper sulfate

Equations for oxidation and reduction reactions 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 metal and are reduced. Each of these equations are called half-equations since 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 more reactive 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 as any electrical energy produced by the chemical reaction is lost as heat. However we can modify the experiment to force these electrons to flow through a wire or external circuit and produce an electrical current from this chemical reaction. This is outlined below.

Improving the design of cells

The two metals we are using in the cell above 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 unwanted reactions you did not plan for. The two metals are then connected by electrical leads to a voltmeter. However if the two metals copper and zinc are in beakers containing solutions of their own ions then we will 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 metals); sodium nitrate or sodium sulfate solutions are 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 or sodium nitrate solution. This salt-bridge completes the circuit and maintains a balance of charge in the two beakers 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 with salt bridge.

Key points

A 12V car battery consists of six 2V cells.