GCSE · Chemistry · AQA · Spec 8462

Electrolysis to extract aluminium

Aluminium holds on to oxygen so tightly that carbon can't prise it free. Electricity can, but it takes a lot of energy.

Run the aluminium cell

Electrolyte: Molten aluminium oxide + cryolite (molten). Cathode (−), half-equation Al³⁺ + 3e⁻ → Al (Higher), product Aluminium: each Al³⁺ ion gains 3 electrons and becomes an aluminium atom. Anode (+), made of Carbon, half-equation 2O²⁻ → O₂ + 4e⁻ (Higher), product Oxygen gas: oxide ions lose electrons and pair up as O₂ molecules. Al³⁺ (positive) migrates to the cathode. O²⁻ (negative) migrates to the anode. electrons in the external circuit: anode to cathode. static

watch where each ion ends up

Cathode (−)

Al³⁺ + 3e⁻ → Al (Higher)

Product: Aluminium: each Al³⁺ ion gains 3 electrons and becomes an aluminium atom

Anode (+)

2O²⁻ → O₂ + 4e⁻ (Higher)

Product: Oxygen gas: oxide ions lose electrons and pair up as O₂ molecules

Aluminium oxide is made of Al³⁺ and O²⁻ ions. In the solid they're locked in place, so it has to be melted before anything can move. Once it's molten and the current is on, each kind of ion heads for the electrode with the opposite charge.

Exam line: Aluminium forms at the negative electrode. Oxygen forms at the positive electrode, which is made of carbon.

Watch out: Positive ions go to the NEGATIVE electrode, not the positive one. Opposite charges attract.

Choosing the route

Carbon or electricity?

Pick an answer at each question and follow the metal to its extraction route.

Is the metal more reactive than carbon? → Does the metal react with carbon?

3 extraction routes.

On A metal oxide: how do we get the metal out?. 2 branches to choose from.

Every metal extraction starts with the same question: can carbon do the job? Walk each path to see where it ends.

Why a mixture?

What is the cryolite for?

The electrolyte isn't pure aluminium oxide. The aluminium oxide is mixed with another substance, called cryolite, and it's the molten mixture that's electrolysed.

Why do you think the cryolite is there? Pick the idea closest to yours.

Cryolite is a catalyst. It speeds up the electrolysis.Aluminium oxide can't conduct on its own, so cryolite is added to carry the current.The aluminium comes from the cryolite. The aluminium oxide just helps it melt.The mixture melts at a lower temperature than aluminium oxide on its own, so less energy is needed to melt it.

How sure are you?

I'm sureI think soI'm guessing

Cryolite is a catalyst. It speeds up the electrolysis.

This is a common misconception

It's a tempting word, because the cryolite isn't the thing being split up. But its job isn't about how fast anything happens at the electrodes. Its job comes before that: it changes the temperature the electrolyte melts at.

Aluminium oxide can't conduct on its own, so cryolite is added to carry the current.

This is a common misconception

Conducting isn't the problem. Once aluminium oxide is molten, its Al³⁺ and O²⁻ ions are free to move, and moving ions carry the current. The problem is getting it molten in the first place.

The aluminium comes from the cryolite. The aluminium oxide just helps it melt.

This is a common misconception

It's the other way round. The aluminium comes from the aluminium oxide: its Al³⁺ ions are what travel to the negative electrode in the cell. Cryolite is the helper.

The mixture melts at a lower temperature than aluminium oxide on its own, so less energy is needed to melt it.

This matches the evidence

Exactly. Nothing moves until the electrolyte is molten, and melting pure aluminium oxide would take a huge amount of energy. The mixture melts at a lower temperature, so less energy goes into melting it.

Cryolite is there to cut the melting bill: the mixture melts at a lower temperature than aluminium oxide alone, so less energy is needed to melt the electrolyte.

Exam line: The mixture melts at a lower temperature than aluminium oxide alone, so less energy is needed to melt the electrolyte.

The disappearing electrode

Reason it through

Why does the positive electrode have to be continually replaced?

Link 1 of 4

First link · your turn

What forms at the positive carbon electrode?

Locked — reveal the link above first

Higher

Half equations (Higher)

Balance the two half equations

Write balanced half equations for the reactions at the two electrodes in the aluminium cell.

Start from the ions: aluminium oxide contains Al³⁺ ions and O²⁻ ions.A half equation shows what happens to one kind of ion at one electrode, with the electrons written in.
missing step

Which line is step 2?

Negative electrode: Al³⁺ → Al + 3e⁻Negative electrode: Al³⁺ + 3e⁻ → AlNegative electrode: Al³⁺ + e⁻ → Al

Exam line: Electrons go on the left when an ion gains them, and on the right when an ion loses them. Then check the atoms AND the total charge.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Carbon can't pull aluminium out of its oxide. Electricity can, but it costs a lot of energy.

What you need to know

Metals can be extracted from molten compounds using electrolysis.
Electrolysis is used when a metal is too reactive to be extracted by reduction with carbon, or when the metal reacts with carbon.
Large amounts of energy are used to melt the compound and to produce the electric current.
Aluminium is made by electrolysing a molten mixture of aluminium oxide and cryolite. The mixture melts at a lower temperature than aluminium oxide alone, so less energy is needed to melt it.
The positive electrode is made of carbon. Oxygen forms on it and reacts with the carbon to make carbon dioxide, so the electrode wears away and must be continually replaced.
Higher: write and balance the half equations at each electrode, Al³⁺ + 3e⁻ → Al and 2O²⁻ → O₂ + 4e⁻.

The big picture

When a metal is too reactive to be extracted by reduction with carbon, or reacts with carbon, it is extracted by electrolysis of a molten compound. Aluminium is made by electrolysing a molten mixture of aluminium oxide and cryolite. The mixture melts at a lower temperature than aluminium oxide alone, so less energy is needed to melt it. Aluminium forms at the negative electrode and oxygen at the positive carbon electrode, which reacts with the oxygen to form carbon dioxide and so must be continually replaced. Melting the compound and producing the current both use large amounts of energy.

Key points

1Carbon can't reduce a metal that is more reactive than it, and can't be used for a metal that reacts with it. That's when electrolysis is used.

2The compound must be molten: in a solid the ions are held in place, and only moving ions can reach the electrodes.

3In the aluminium cell, Al³⁺ ions move to the negative electrode and form aluminium; O²⁻ ions move to the positive electrode and form oxygen.

4Cryolite lowers the temperature the electrolyte melts at. It isn't a catalyst and it isn't where the aluminium comes from.

5The carbon positive electrode reacts with the oxygen made on it, forming carbon dioxide, so it has to be replaced.

6Higher: electrons go on the left when an ion gains them (Al³⁺ + 3e⁻ → Al) and on the right when an ion loses them (2O²⁻ → O₂ + 4e⁻). Atoms and charge must both balance.

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Worked example

Problem

Metal M is more reactive than carbon. Its oxide is made of M²⁺ and O²⁻ ions. Explain how M could be extracted, at which electrode it would form, and why the process would use a lot of energy. (Higher: also write the half equation for the formation of M.)

⚠ Watch out

Saying electrolysis is used because it is cheaper or easier than heating with carbon. It's used because carbon can't do the job, and it uses large amounts of energy.

🧠

Memory hook

Carbon can't, so current can. Cryolite cuts the melting bill, and the positive electrode burns in its own oxygen.

✓

Check yourself

Cover the page. Explain, in two linked steps each, why the electrolyte is a mixture and why the positive electrode has to be replaced.

Flashcards

(11)

When is electrolysis used to extract a metal?

When the metal is too reactive to be extracted by reduction with carbon, or when the metal reacts with carbon.

Why must the compound be molten for electrolysis?

In a solid the ions are held in place. Molten, they are free to move to the electrodes.

What is the electrolyte in the extraction of aluminium?

A molten mixture of aluminium oxide and cryolite.

Why is a mixture of aluminium oxide and cryolite used, not aluminium oxide alone?

The mixture melts at a lower temperature, so less energy is needed to melt the electrolyte.

What are the two big energy costs of extracting a metal by electrolysis?

Melting the compound, and producing the electric current.

At which electrode does aluminium form, and why there?

The negative electrode (cathode). Al³⁺ ions are positive, so they are attracted to it.

What forms at the positive electrode in the aluminium cell?

Oxygen, from the oxide ions losing electrons.

What is the positive electrode made of in aluminium extraction?

Carbon.

What happens to the oxygen made at the carbon positive electrode?

It reacts with the hot carbon to form carbon dioxide, which escapes, so the electrode wears away.

(Higher) Half equation at the negative electrode in aluminium extraction?

Al³⁺ + 3e⁻ → Al

(Higher) Half equation at the positive electrode in aluminium extraction?

2O²⁻ → O₂ + 4e⁻

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Run the aluminium cell

Carbon or electricity?

What is the cryolite for?

Balance the two half equations

More AQA GCSE Chemistry topics