Revision notes for AQA GCSE Chemistry Cells and batteries. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Cells and batteries
What you'll learn
- How a chemical cell turns chemical reactions into electricity.
- How to make a simple cell using two metals and an electrolyte.
- Why the voltage of a cell depends on the electrodes and electrolyte.
- How batteries, non-rechargeable cells, and rechargeable cells compare.
The starting idea: reactions can produce electricity
A chemical cell contains chemicals that react and produce a potential difference, also called a voltage. If the cell is connected in a complete circuit, this voltage can make charge flow, producing an electric current.
You do not need to know the detailed chemistry of different commercial batteries for this GCSE section. The key idea is that chemical reactions can transfer electrons, and moving electrons in a wire are what allow electrical energy to be transferred.
Definition
Cell
A cell is a single chemical source of electrical energy. It contains chemicals that react to produce a voltage.
Definition
Voltage
Voltage, or potential difference, is a measure of how much energy is transferred by each unit of charge. It is measured in volts, V.
Making a simple cell
A simple cell can be made using:
- two different metals
- an electrolyte
- wires and a voltmeter or circuit
The two metals act as electrodes. They are placed in the electrolyte and connected by wires. A voltage is produced because the two metals have different tendencies to react.
Definition
Electrode and electrolyte
An electrode is a conductor, usually a metal or graphite, that is in contact with the electrolyte. An electrolyte is a liquid or solution containing ions that can move and carry charge.
A typical school simple cell might use zinc and copper electrodes in a salt solution or copper sulfate solution. The exact voltage depends on the chemicals used.
Key Idea
Simple cell recipe
A simple cell needs two different electrodes in contact with an electrolyte. The chemical reactions in the cell produce a voltage.
Why the metals must be different
Metals differ in how easily they form positive ions. This is linked to their reactivity.
A more reactive metal loses electrons more easily than a less reactive metal. When two different metals are connected in an electrolyte, this difference can produce a voltage.
If the two electrodes are the same metal, there is usually no useful voltage because both electrodes behave in the same way.
Common Mistake
Same metals do not make a useful simple cell
Using two identical metal electrodes in the same electrolyte will not usually produce a useful voltage. A simple cell needs a difference between the electrodes.
What affects the voltage?
The voltage produced by a cell depends on several factors, especially:
- the type of electrode
- the type of electrolyte
- the difference in reactivity between the metals
In GCSE questions, you may be given data and asked to spot patterns. A bigger difference in reactivity between two metal electrodes often gives a bigger voltage, if the electrolyte and conditions are kept the same.
Tip
Comparing cell voltages
When comparing metals in simple cells, make sure the electrolyte is the same. If the electrolyte changes as well, you cannot say the voltage difference is only due to the metals.
Example
Using cell voltage data to compare reactivity
A student uses copper as one electrode each time and tests three other metals in the same electrolyte.
| Cell | Voltage produced |
|---|---|
| magnesium and copper | 2.7 V |
| zinc and copper | 1.1 V |
| iron and copper | 0.8 V |
Which metal appears to have the greatest difference in reactivity from copper?
-
Compare only the results where the conditions are the same. Copper is always one electrode, and the electrolyte is the same, so the voltages can be compared fairly.
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Look for the largest voltage. Magnesium and copper produce 2.7 V, which is larger than 1.1 V and 0.8 V.
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Conclude that magnesium has the greatest difference in reactivity from copper in this data set. The likely order, compared with copper, is magnesium showing the biggest difference, then zinc, then iron.
Cells and batteries
In everyday speech, people often call a single AA cell a “battery”. In chemistry, it is more precise to say that a battery is made from two or more cells connected together.
Definition
Battery
A battery is two or more cells connected together in series to provide a greater voltage.
Cells in series
Cells are connected in series when they are joined one after another in a single path. The positive terminal of one cell is connected to the negative terminal of the next.
When cells are connected in series in the correct direction, their voltages add together:
Vtotal=V1+V2+V3+⋯V_\text{total} = V_1 + V_2 + V_3 + \cdotsVtotal=V1+V2+V3+⋯
Example
Calculating the voltage of cells in series
Three 1.5 V cells are connected in series. Calculate the total voltage.
-
Identify that the cells are in series, so their voltages add.
-
Substitute the cell voltages into the equation:
Vtotal=1.5 V+1.5 V+1.5 VV_\text{total} = 1.5\text{ V} + 1.5\text{ V} + 1.5\text{ V}Vtotal=1.5 V+1.5 V+1.5 V -
Add the voltages:
Vtotal=4.5 VV_\text{total} = 4.5\text{ V}Vtotal=4.5 V
Common Mistake
Forgetting the direction of cells
Voltages add only when the cells are connected the same way round in series. If one cell is reversed, it works against the others and reduces the total voltage.
Non-rechargeable cells and batteries
In a non-rechargeable cell, the chemical reactions continue until one of the reactants has been used up. When that happens, the cell can no longer produce a useful voltage.
Alkaline batteries are common examples of non-rechargeable batteries.
Definition
Reactant
A reactant is a substance that is used up during a chemical reaction.
Key Idea
Why non-rechargeable cells run out
A non-rechargeable cell stops working because the chemicals needed for the reaction are used up.
Non-rechargeable batteries are useful because they are often cheap, portable, and easy to store. However, once used up, they must be replaced and disposed of safely.
Rechargeable cells and batteries
A rechargeable cell can be used again because the chemical reactions can be reversed.
When the cell is supplying electricity, the reaction goes in the direction that releases electrical energy. When the cell is recharged, an external electrical current is supplied, forcing the reaction to go in the opposite direction. This helps remake the reactants.
Definition
Rechargeable cell
A rechargeable cell is a cell in which the chemical reactions can be reversed by supplying an external electrical current.
Rechargeable cells are useful in phones, laptops, electric vehicles, and rechargeable torches. They usually cost more at first, but they can be reused many times.
Common Mistake
Do not recharge non-rechargeable cells
Non-rechargeable alkaline batteries are not designed to be recharged. Trying to recharge them can cause leakage, overheating, or bursting.
Evaluating the use of cells
In exams, you may be asked to choose the best cell or battery for a job. There is often no single “perfect” answer. You need to use the data and justify your choice.
Useful points to compare include:
- voltage needed by the device
- whether the cell is rechargeable
- cost at the start and cost over time
- mass and size
- how long it lasts
- environmental impact and disposal
- safety, such as leakage or toxic materials
Example
Choosing a suitable battery from data
A sensor works safely between 2.8 V and 3.6 V. It is used every day. Three battery options are available.
| Option | Voltage | Other information |
|---|---|---|
| A: two alkaline cells in series | 3.0 V | cheap, non-rechargeable, replaced often |
| B: three rechargeable cells in series | 3.6 V | expensive at first, can be recharged many times |
| C: one coin cell | 3.0 V | small, non-rechargeable, needs careful disposal |
Evaluate which option is best for daily use.
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Check the voltage requirement first. All three options are between 2.8 V and 3.6 V, so all could power the sensor safely.
-
Compare long-term use. The sensor is used every day, so a rechargeable option is likely to reduce replacement cost and waste over time.
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Weigh up the practical drawbacks. Option B costs more at first and may be larger, but it can be recharged many times.
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Make a justified conclusion. Option B is likely to be best for daily use, provided its size is suitable, because it meets the voltage requirement and can be reused.
Exam technique
In the exam
-
For simple cells, mention two different electrodes and an electrolyte when explaining how the cell is made.
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When comparing voltages, check whether the electrode materials and electrolyte have been kept the same so the comparison is fair.
-
For evaluation questions, do not just pick the highest voltage. Use the context: voltage needed, cost, rechargeability, lifetime, safety, and disposal.
Self review
Check yourself
- Why does a simple cell need two different metals?
- What happens to the total voltage when cells are connected in series?
- Why can rechargeable cells be used again, but alkaline batteries cannot?
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