GCSE · Chemistry · Edexcel · Spec 1CH0

Chemical cells

There's no electricity stored in your phone's battery, just chemicals waiting to react. 'Flat' simply means one of them has run out.

Watch a simple cell run

Electrolyte: Electrolyte (a solution containing ions). Metal B, made of less reactive, half-equation Particles from the electrolyte take in the electrons. Metal A, made of more reactive, half-equation Atoms of Metal A give up electrons, product Ions of Metal A, which go into the electrolyte, observation Metal A slowly wears away as the cell runs. + (positive) migrates to the cathode. − (negative) migrates to the anode. electrons: A → B in the external circuit: anode to cathode. static

Follow the electrons from A to B

Metal B

Particles from the electrolyte take in the electrons

Metal A

Atoms of Metal A give up electrons

Product: Ions of Metal A, which go into the electrolyte

Observation: Metal A slowly wears away as the cell runs

Two different metals stand in an electrolyte, joined by a wire. At Metal A, the more reactive one, atoms give up electrons. Those electrons are pushed along the wire into Metal B, where particles from the electrolyte take them in. Meanwhile ions drift through the electrolyte, which completes the circuit. That push on the electrons is the cell's voltage.

Watch out: The cell only works while its reaction can keep going. Once one of the reacting substances runs out, whether that's Metal A or the particles taking in electrons at Metal B, the reaction stops. No reaction means no electron flow and no voltage. That's what a flat cell is.

What do you really think?

What's actually inside a cell?

A torch gets dimmer and dimmer, then stops. Everyone says its cell has gone 'flat'.

Which of these is closest to what you think right now?

A cell is a little tank of electricity. The torch drains it until it's empty.Any two electrodes in an electrolyte give the same voltage, even two pieces of the same metal.The electrolyte just holds the metals. The metals do all the work.A cell stores chemical energy. A reaction releases it as moving electrons.

How sure are you?

I'm sureI think soI'm guessing

A cell is a little tank of electricity. The torch drains it until it's empty.

This is a common misconception

It's the most common picture, and it's worth letting go of. There's no electricity sitting in a cell waiting to be poured out. A cell stores energy in its chemicals. Connect a circuit and those chemicals react, and the reaction pushes electrons round. A flat cell isn't empty. It has run out of one of its reactants, so the reaction can't carry on.

Any two electrodes in an electrolyte give the same voltage, even two pieces of the same metal.

This is a common misconception

Try it with two identical pieces of the same metal and you get no voltage at all. A cell needs two different electrodes. And the size of the voltage depends on which substances you pick: change one of the electrodes, or the electrolyte, and the voltage can change.

The electrolyte just holds the metals. The metals do all the work.

This is a common misconception

The electrolyte is part of the chemistry, not a stand. In the cell you watched, particles from the electrolyte take in the electrons arriving at Metal B, and that's part of the reaction. That's why the voltage depends on the electrolyte as well as on the electrodes.

A cell stores chemical energy. A reaction releases it as moving electrons.

This matches the evidence

Yes. The energy is stored in the chemicals, and the reaction turns it into a flow of electrons round the circuit. The voltage lasts only as long as the reaction does, and its size depends on the substances used for the electrodes and the electrolyte.

A cell is a store of chemical energy, not a tank of electricity. Its voltage lasts as long as its reaction does, and the substances you choose decide how big it is.

Rechargeable cells

Rebuild the recharge loop, then break it

2 stages are missing. Use the rest of the loop to work out which goes where.

Stage 1: The cell runs
Its reaction gives a voltage and uses up the reactants as it goes.
Stage 3: The cell is flat
The reaction has stopped, so there's no voltage.
Stage 4: A charger passes a current through it
An external supply pushes an electric current through the cell.

…and stage 5 leads back to stage 1.

Predict, then check

Plenty of devices take more than one cell, lined up end to end. Why bother?

You join a second, identical cell to the first, in series: one after the other, the + end of one touching the − end of the next. What happens to the total voltage?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A cell doesn't hold electricity. It holds chemicals, and while they react they push electrons round a circuit.

What you need to know

A chemical cell transfers energy stored in chemicals into electrical energy. A reaction inside the cell makes electrons flow round a connected circuit, so the cell provides a voltage (potential difference).
A simple cell is two different electrodes, for example two different metals, in contact with an electrolyte and joined by an external circuit. In a cell made of two metals, the more reactive metal gives up electrons.
The size of the voltage depends on which substances are used for the electrodes and the electrolyte.
The reaction uses up its reactants. Once one reacting substance has run out, the reaction stops and the cell no longer gives a voltage.
In a non-rechargeable cell the reaction can't be reversed, so the cell is thrown away when a reactant runs out. In a rechargeable cell, passing a current through it from an external supply reverses the reaction and makes the reactants again.
A battery is two or more cells connected in series, which gives a larger total voltage than one cell.

The big picture

A chemical cell stores energy in chemicals, not electricity. Two different electrodes sit in an electrolyte, joined by a circuit, and a reaction pushes electrons round that circuit: that push is the voltage, and its size depends on the substances used. When one reactant runs out, the reaction and the voltage stop. A rechargeable cell's reaction can be reversed by a current from an external supply; a non-rechargeable cell's can't. A battery is two or more cells in series, giving a bigger voltage.

Key points

1A cell stores energy in chemicals, not electricity.

2Two different electrodes + an electrolyte + an external circuit = a simple cell.

3The reaction pushes electrons round the circuit: that's the voltage.

4Voltage size depends on the electrode and electrolyte substances.

5One reactant runs out → reaction stops → no voltage (flat).

6Rechargeable: a current from an external supply reverses the reaction. Non-rechargeable: it can't, so the cell is thrown away.

7Battery = two or more cells in series, for a bigger voltage.

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Worked example

Problem

A TV remote runs on two non-rechargeable cells joined in series. After several months it stops working. (a) Why does the remote use two cells rather than one? (b) Explain why the cells stopped producing a voltage. (c) Explain why these cells cannot simply be recharged.

⚠ Watch out

Saying a flat cell has 'run out of electricity' or 'run out of charge'. A cell never contained a store of electricity. What has run out is one of its reactants, so the reaction, and with it the voltage, has stopped.

🧠

Memory hook

A cell is a reaction waiting to happen. Reacting? You get a voltage. Reactant gone? Flat. Rechargeable? Run the reaction backwards. Battery? Cells in series.

✓

Check yourself

A cell made from two different metals in an electrolyte goes flat, yet there is still plenty of the more reactive metal left. How can the cell be flat?

Flashcards

(12)

What does a chemical cell store?

Energy in its chemicals. It does not store electricity.

What makes electrons flow round the circuit connected to a cell?

A chemical reaction inside the cell.

What are the three parts of a simple chemical cell?

Two different electrodes, an electrolyte they are in contact with, and an external circuit joining them.

Why can't you make a working cell from two pieces of the same metal?

A cell needs two different electrodes. Two identical ones give no voltage.

What decides the size of a cell's voltage?

Which substances are used for the electrodes and the electrolyte.

In a simple cell made of two metals, which metal gives up electrons?

The more reactive metal. It is slowly used up as the cell runs.

Why does a cell eventually stop producing a voltage?

One of its reactants runs out, so the reaction stops.

Does a cell have to use up ALL of its reactants before it goes flat?

No. Just one reacting substance running out is enough to stop the reaction.

Once a non-rechargeable cell goes flat, what happens to it, and why?

It gets discarded, because its reaction only goes one way.

How is a rechargeable cell recharged?

A current from an external supply is passed through it. This reverses the reaction and makes the reactants again.

What is a battery?

Two or more chemical cells connected together in series.

Why do many devices take several cells lined up end to end?

Joined in series they make a battery, with a bigger voltage than one cell gives.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Watch a simple cell run

What's actually inside a cell?

Rebuild the recharge loop, then break it

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