Cells and Batteries

Chemical cells and batteries convert chemical energy into electrical energy through chemical reactions. This lesson covers how cells work, the difference between cells and batteries, and the factors that affect voltage. This is part of the Energy Changes topic in the AQA GCSE Combined Science Trilogy specification (8464).

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What Is a Chemical Cell?

A chemical cell (also called an electrochemical cell or voltaic cell) is a device that produces a voltage (potential difference) by converting chemical energy into electrical energy.

A basic cell contains:

• Two electrodes made from different metals (or one metal and a non-metal conductor such as carbon).
• An electrolyte — a solution that conducts electricity by allowing ions to flow.

graph TD
    subgraph "Simple Chemical Cell"
        A["Electrode 1 \n(more reactive metal \n— NEGATIVE terminal)"] -->|"Electrons flow \nthrough wire"| B["Electrode 2 \n(less reactive metal \n— POSITIVE terminal)"]
        C["Electrolyte \n(ionic solution)"] -->|"Ions flow \nbetween electrodes"| A
        C -->|"Ions flow \nbetween electrodes"| B
    end

Component	Role
Electrode 1 (more reactive metal)	Negative terminal — atoms lose electrons (oxidation)
Electrode 2 (less reactive metal)	Positive terminal — ions gain electrons (reduction)
Electrolyte	Allows ions to flow between electrodes, completing the internal circuit
External wire	Carries electrons from negative to positive electrode (the current)

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How a Chemical Cell Works

When two different metals are placed in an electrolyte and connected by a wire:

1. The more reactive metal loses electrons more readily — these electrons flow through the external wire to the less reactive metal.
2. The flow of electrons is an electric current.
3. Inside the cell, ions in the electrolyte move between the electrodes to complete the circuit.

Exam Tip: The more reactive metal always becomes the negative electrode because it loses electrons more easily. Think of the reactivity series — the further apart two metals are on the series, the greater the voltage produced.

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What Determines the Voltage of a Cell?

The voltage produced by a chemical cell depends on:

Factor	Effect
The metals used as electrodes	Greater difference in reactivity → greater voltage
The electrolyte	Different electrolytes can affect the voltage slightly

Example Voltages

Electrode Pair	Approximate Voltage
Magnesium and copper	~2.7 V (large difference in reactivity)
Zinc and copper	~1.1 V
Iron and copper	~0.8 V
Zinc and iron	~0.3 V (small difference in reactivity)
Same metal and same metal	0 V (no difference in reactivity)

Exam Tip: If both electrodes are made of the same metal, no voltage is produced because there is no difference in reactivity — no electrons flow.

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Cells vs Batteries

Term	Definition
Cell	A single electrochemical device that produces a voltage
Battery	Two or more cells connected together in series

A battery produces a higher voltage than a single cell. For example, a typical AA cell is 1.5 V, but a 9 V battery contains six 1.5 V cells in series ($6 \times 1.5 = 9$6×1.5=9 V).

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Non-Rechargeable vs Rechargeable Cells

Feature	Non-Rechargeable	Rechargeable
Reaction type	Irreversible	Reversible
Lifespan	Used once, then discarded	Can be recharged and reused many times
How they stop	Reactants are used up	Can be restored by passing a current through in reverse
Examples	Alkaline batteries (AA, AAA)	Lithium-ion (phones, laptops), lead-acid (cars)
Environmental impact	More waste — must be disposed of	Less waste, but contain toxic materials

Advantages of Rechargeable Cells

• Can be reused hundreds of times, reducing waste.
• More cost-effective over time.
• The chemical reaction can be reversed by applying an external electrical current.

Disadvantages of Rechargeable Cells

• Higher initial cost.
• Eventually lose capacity and must be replaced.
• Contain toxic materials (e.g., lithium, cadmium) that need careful disposal.

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Environmental Considerations

Issue	Detail
Toxic materials	Cells and batteries contain heavy metals and toxic chemicals that can pollute soil and water
Recycling	Many components can be recycled, reducing environmental impact
Energy to manufacture	Producing cells requires significant energy and raw materials
Disposal	Must be disposed of at proper recycling points, NOT in general waste

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Common Mistakes

Mistake	How to Avoid It
"A battery is a single cell"	A battery is two or more cells in series
"The less reactive metal is negative"	The MORE reactive metal is the negative electrode
"Using the same metal gives a small voltage"	Same metal = NO voltage (zero)
Confusing current direction with electron flow	Electrons flow negative → positive; conventional current flows positive → negative

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Summary

• A chemical cell converts chemical energy to electrical energy using two different metal electrodes and an electrolyte.
• A battery is two or more cells connected in series.
• The voltage depends on the difference in reactivity of the metals — greater difference = greater voltage.
• Non-rechargeable cells use irreversible reactions; rechargeable cells use reversible reactions.
• The more reactive metal is the negative electrode.
• Cells and batteries contain toxic materials and should be recycled properly.

Exam Tip (AQA 8464): A common question asks: "What determines the voltage of a simple cell?" Answer: the type of metals used as electrodes — the greater the difference in reactivity between the two metals, the greater the voltage.

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Important Note on Trilogy Coverage