7.5 Chemical Cells and Batteries

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how chemical cells produce electricity, what affects their voltage, batteries, and rechargeable versus non-rechargeable cells. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe a simple chemical cell made from two different metals and an electrolyte
• Explain how the voltage depends on the metals (electrodes) and the electrolyte
• Use voltage data to compare the reactivity of metals
• Describe a battery as two or more cells connected in series
• (Higher tier) write half equations for the reactions at the electrodes of a simple cell
• Compare rechargeable and non-rechargeable cells

Key words

• Cell: Chemicals that react to produce electricity: two different electrodes in contact with an electrolyte.
• Battery: Two or more cells connected together in series to give a bigger voltage.
• Voltage: The potential difference of a cell: the “push” it gives to electrons, in volts (V).
• Rechargeable: A cell whose chemical reactions can be reversed by passing an electric current through it from an external supply.

A simple chemical cell

Dip two different metals into an electrolyte and join them with wires, and a current flows. The more reactive metal loses electrons more easily: it becomes the negative terminal. Electrons flow through the wire from the more reactive metal to the less reactive metal (the positive terminal).

Key fact: If both electrodes are the same metal, the voltage is zero. A cell needs two different electrodes.

What affects the voltage?

The voltage of a cell depends on several factors, including:

• the type of electrodes: the bigger the difference in reactivity between the two metals, the bigger the voltage
• the electrolyte: its type and its concentration

Worked example: using voltages to compare reactivity

Order of voltage with copper: magnesium (2.0 V) > zinc (1.0 V) > iron (0.7 V) > lead (0.4 V).
So the reactivity order is magnesium > zinc > iron > lead > copper.
Estimate the voltage of a zinc–iron cell: 1.0 − 0.7 = 0.3 V. Zinc is more reactive, so zinc is the negative terminal.

Answer: about 0.3 V, zinc negative

What happens at the electrodes (Higher tier)

A cell uses the same chemistry as a displacement reaction. When zinc is put into copper(II) sulfate solution, zinc displaces copper: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). The sulfate ions do not change, so they are spectator ions. Leaving them out gives the ionic equation:

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

In a cell, the two halves of this redox reaction happen at different electrodes, so the electrons have to travel through the wire – and that flow of electrons is the electric current.

Key fact: Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain. The more reactive metal is oxidised and is the negative terminal. The cell keeps working until one reactant (here zinc metal or copper(II) ions) is used up.

If a voltmeter is connected the “wrong” way round, with the more reactive metal on its positive terminal, it shows a negative voltage. A negative reading tells you the metal you expected to be more reactive is actually the less reactive one.

Batteries

A battery is two or more cells connected in series. Their voltages add together. For example, a 9 V battery contains six 1.5 V cells: 6 × 1.5 = 9 V.

Maths skill: Total voltage of cells in series = voltage of one cell × number of cells (if the cells are identical).

Worked example: cells needed for a battery

A device needs 6.0 V. Each cell gives 1.5 V.
Number of cells = 6.0 ÷ 1.5 = 4

Answer: 4 cells in series

Non-rechargeable and rechargeable cells

• Why it stops; Non-rechargeable (e.g. alkaline) The chemical reactions stop when one of the reactants has been used up.; Rechargeable (e.g. lithium-ion) Reactants are used up, but the reactions can be reversed by applying an external electrical current.
• Advantages; Non-rechargeable (e.g. alkaline) cheaper to buy; hold charge for a long time when not used; Rechargeable (e.g. lithium-ion) can be used hundreds of times; less waste; cheaper over the product’s life
• Disadvantages; Non-rechargeable (e.g. alkaline) thrown away after one use; waste and resources; Rechargeable (e.g. lithium-ion) more expensive to buy; need a charger; lose capacity over time
• Uses; Non-rechargeable (e.g. alkaline) remote controls, clocks, smoke alarms; Rechargeable (e.g. lithium-ion) phones, laptops, electric cars

Common non-rechargeable cells include zinc–carbon “dry” cells (about 1.5 V, cheap, but can leak if left in a device for a long time) and alkaline cells, which last longer and supply a larger current. In a rechargeable cell, connecting a charger (an external power supply) drives the electrode reactions backwards, re-forming the original reactants.

All batteries contain metals and other chemicals that can be harmful. Used batteries should be recycled, not put in household waste.

Common mistakes

• Saying the voltage depends only on the size of the electrodes. It depends on the type of metals and the electrolyte.
• Making the less reactive metal the negative terminal. The more reactive metal is negative.
• Saying a rechargeable battery “makes new chemicals” when charged. The reactions are reversed, re-forming the reactants.
• Confusing cell and battery. A battery is several cells in series.

Revision questions

Try these, then check your answers below each one.

Question 1. What is a simple chemical cell made from? (Choose from: Two different metals in contact with an electrolyte, Two identical metals in pure water, One metal in a vacuum, A metal and a plastic in oil.)

Answer: Two different metals in contact with an electrolyte

Working: Two different electrodes in an electrolyte produce a voltage.

Question 2. True or false? A cell made from two strips of the same metal in salt solution gives a voltage of zero.

Answer: True

Working: True. There is no difference in reactivity, so there is no voltage.

Question 3. A battery contains four 1.5 V cells connected in series. What is its total voltage?

Answer: 6 V

Working: 4 × 1.5 = 6.0 V.

Related lessons

• Previous lesson: 7.4 Bond Energy Calculations
• Next lesson: 7.6 Fuel Cells
• In this chapter: 7.1 Exothermic and Endothermic Reactions
• In this chapter: 7.2 Using Energy Transfers from Reactions
• In this chapter: 7.3 Reaction Profiles

Continue in the full course

This lesson is section 7.5 (AGCCH0705) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.