Chapter 7 of 8

All 8 chapters
  1. 1Chemical formulae, readunderstood
  2. 2Balancing equations, readunderstood
  3. 3The mole, readunderstood
  4. 4Reacting masses and yield, readunderstood
  5. 5Acids, bases and salts, readunderstood
  6. 6The reactivity series, readunderstood
  7. 7Electrolysis, readunderstood
  8. 8Energy changes and rates, readunderstood

S4.7.7 Chapter 7

Electrolysis

Carbon cannot take the oxygen from aluminium oxide. So how is aluminium made?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Predict the products of electrolysis of a molten ionic compound
  • Explain why aluminium is extracted by electrolysis, not with carbon

In the last chapter, carbon took the oxygen from iron oxide. Aluminium is above carbon in the series. Carbon cannot take its oxygen. The way out is a flow of electricity.

Electrolysis is the splitting of a compound by passing an electric current through it. The liquid that is split is the electrolyte. The current enters and leaves it by two electrodes, made of a material that conducts, such as graphite.

An ionic compound must be molten for this. In the solid, the ions are held in fixed places, so no charge can flow. When the compound is melted, the ions can move. Moving ions carry the current.

A model of an ionic solid in one flat layer. Small hatched circles with a plus sign are metal ions and larger white circles with a minus sign are non-metal ions. They alternate in rows and columns. A key says metal ion and non-metal ion.metal ionnon-metal ion
A solid ionic compound. Each ion is held in its place, so the solid does not conduct.

Think of the cell for lead bromide, PbBr₂. It melts at 371 °C. The melted compound has two kinds of ion, Pb²⁺ and Br⁻. Two electrodes dip into it and are joined to a cell, or battery.

One electrode is joined to the negative end of the battery. It is the cathode. The other is joined to the positive end. It is the anode. Opposite charges attract.

See it three ways

As a picture

A crucible of molten lead bromide, with heat arrows and the word heat. Two electrodes dip into it, joined to a cell symbol. The short thick plate (−) of the cell symbol is wired to the left electrode, labelled cathode (−). The long thin plate (+) of the cell symbol is wired to the right electrode, labelled anode (+). Pb²⁺ ions, hatched, move to the cathode and Br⁻ ions, plain, move to the anode. Lead forms at the cathode and bromine forms at the anode. A key names both ions.−+molten leadbromidePb²⁺Br⁻cathode (−)leadanode (+)bromineheat

In words

The positive Pb²⁺ ions go to the negative cathode. There each gains two electrons and becomes a lead atom. The negative Br⁻ ions go to the positive anode. There each loses an electron, and pairs of bromine atoms join to make Br₂. Each disc in the picture shows the sign of the charge, and the key gives the size.

In symbols

PbBr₂ → Pb + Br₂

So the rule is short. Positive ions go to the cathode. A metal forms there. Negative ions go to the anode. A non-metal forms there. The charges of the ions tell you which way each one moves.

A table with the columns Compound, Cathode and Anode. PbBr₂: Pb at the cathode, Br₂ at the anode. NaCl: Na and Cl₂. ZnCl₂: Zn and Cl₂. Al₂O₃: Al and O₂. MgO: Mg and O₂.CompoundCathodeAnodePbBr₂PbBr₂NaClNaCl₂ZnCl₂ZnCl₂Al₂O₃AlO₂MgOMgO₂
Products of electrolysis of five molten compounds. Br₂, Cl₂ and O₂ are bromine, chlorine and oxygen gas, made of two-atom molecules.
Five discs in a row. Two hatched discs are Al³⁺. Three plain discs are O²⁻. Under them the sum reads +3 +3 −2 −2 −2 = 0, and under the sum is the formula Al₂O₃.Al³⁺Al³⁺O²⁻O²⁻O²⁻+3 +3 −2 −2 −2 = 0Al₂O₃
Aluminium oxide has 2 Al³⁺ ions for each 3 O²⁻ ions.

Aluminium is the most common metal in the Earth's crust, but it is always combined. The ore is bauxite. It is purified to make aluminium oxide, Al₂O₃. Aluminium oxide melts above 2 000 °C, which is too hot to be practical.

The trick is to dissolve the oxide in molten cryolite, another aluminium compound. The oxide dissolves in the molten cryolite, so the mixture is liquid at about 950 °C. It is still very hot, but far lower. Cryolite does not split.

A crucible of molten aluminium oxide, with heat arrows and the word heat. Two electrodes dip into it, joined to a cell symbol. The short thick plate (−) of the cell symbol is wired to the left electrode, labelled cathode (−). The long thin plate (+) of the cell symbol is wired to the right electrode, labelled anode (+). Al³⁺ ions, hatched, move to the cathode and O²⁻ ions, plain, move to the anode. Aluminium forms at the cathode and oxygen forms at the anode. A key names both ions.−+moltenaluminiumoxideAl³⁺O²⁻cathode (−)aluminiumanode (+)oxygenheat
The same idea for aluminium. Two Al³⁺ ions to three O²⁻ ions balance the charges. In a real cell the oxide is dissolved in molten cryolite. Each disc shows the sign of the charge; the key gives the size.

At the cathode, Al³⁺ ions gain electrons and make molten aluminium, which sinks to the bottom and is tapped off. At the anode, O²⁻ ions lose electrons and make oxygen gas. The anodes are made of carbon. The hot oxygen burns the carbon and makes carbon dioxide, so the anodes wear away and must be replaced.

Worked example

Balance the equation for the electrolysis of aluminium oxide: Al₂O₃ → Al + O₂

  1. Count the atoms. Left: Al 2, O 3. Right: Al 1, O 2.
  2. O is 3 on the left and 2 on the right. The smallest number that both go into is 6. Write 2 in front of Al₂O₃ and 3 in front of O₂.
  3. Now Al is 2 × 2 = 4 on the left. Write 4 in front of Al.
  4. Check: Al is 4 and 4. O is 6 and 6. The equation is balanced.

Answer: 2Al₂O₃ → 4Al + 3O₂

Worked example

What mass of aluminium is made from 204 g of aluminium oxide? Use 2Al₂O₃ → 4Al + 3O₂.

  1. Mr of Al₂O₃ = 2 × 27 + 3 × 16 = 102.
  2. Moles of Al₂O₃ = 204 ÷ 102 = 2 mol.
  3. The ratio of Al₂O₃ to Al is 2 to 4, which is 1 to 2. So 2 mol of Al₂O₃ make 4 mol of Al.
  4. Mass of Al = 4 × 27 = 108 g.

Answer: 108 g

Four boxes joined by arrows, from top to bottom. Bauxite is purified to aluminium oxide. The oxide is dissolved in molten cryolite at about 950 °C. A current splits it: aluminium at the cathode and oxygen at the anode. The aluminium is tapped off and the carbon anodes are replaced.Bauxite is purified toaluminium oxideThe oxide is dissolved inmolten cryolite at about 950°CA current splits it:aluminium at the cathode andoxygen at the anodeThe aluminium is tapped offand the carbon anodes arereplaced
Making aluminium. The step with the current needs a great deal of electricity. A smelter is built where there is plenty of electricity, often from water power. Recycled aluminium needs far less energy.

Try it

Move the ions on paper

You need: a sheet of paper and a pencil, three small torn pieces of paper.

Be safe: Use only paper and a pencil. Do not try any of this with real electricity or real salts.

  1. Draw a box in the middle of the sheet. Write a minus sign on the left side for the cathode. Write a plus sign on the right side for the anode.
  2. Write Pb²⁺ on one torn piece and Br⁻ on the other two. Lay them in a row in the box. In the solid they cannot move.
  3. Now melt the compound. Slide the pieces about freely.
  4. Send the Pb²⁺ piece to the minus side. Send both Br⁻ pieces to the plus side.
  5. At the minus side, write "lead" on the back of the Pb²⁺ piece. At the plus side, join the two Br⁻ pieces and write "bromine, Br₂".

What you will see: The ions of opposite charge go to opposite sides. One Pb²⁺ is paired with two Br⁻, because +2 and −1 −1 add to zero. The metal forms at the negative side and the non-metal at the positive side.

If you cannot try it, this is what happens.
Three discs in a row. A hatched disc is Pb²⁺. Two plain discs are Br⁻. Under them the sum reads +2 −1 −1 = 0, and under the sum is the formula PbBr₂.Pb²⁺Br⁻Br⁻+2 −1 −1 = 0PbBr₂

A wrong idea many people have

In electrolysis the current in the liquid is a flow of electrons.

Why it feels right: In a copper wire the current is a flow of electrons, so it seems the same in a liquid.

The test that breaks it: Look at the cell. Ions move through the liquid, and lead and bromine are made where they arrive. Free electrons would not make new substances.

What is true: Electrons flow in the wires. In the liquid, the ions carry the charge. Electrons are gained and lost only at the electrodes.

Key words

electrolysis
splitting a compound by passing an electric current through it.
electrolyte
the molten or dissolved compound that is split.
electrodes
the two conductors that carry the current into and out of the liquid.
cathode
the negative electrode, where positive ions go.
anode
the positive electrode, where negative ions go.
cryolite
a molten compound that aluminium oxide dissolves in, so it is liquid at a lower temperature.

Back to your guess

The answer is B: Electricity splits aluminium oxide into aluminium and oxygen.

Aluminium is above carbon in the reactivity series, so carbon cannot remove its oxygen at any temperature we use. A current of electricity does it instead. That is electrolysis.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Molten lead bromide is electrolysed. At which electrode does the lead form?

    Tap your answer.

    1. Not this one. Pb²⁺ is positive, so it is attracted to the negative electrode. The positive electrode attracts the Br⁻ ions. Try another.

    2. Yes, that is right. Pb²⁺ ions are positive and go to the negative electrode, the cathode. There they gain electrons and become lead.

    3. Not this one. Each kind of ion goes to one electrode only. Lead forms at one and bromine at the other. Try another.

    See the answer

    The cathode, the negative electrode

    Pb²⁺ ions are positive and go to the negative electrode, the cathode. There they gain electrons and become lead.

    • If you chose A: Pb²⁺ is positive, so it is attracted to the negative electrode. The positive electrode attracts the Br⁻ ions.
    • If you chose C: Each kind of ion goes to one electrode only. Lead forms at one and bromine at the other.

  2. Why does solid lead bromide not conduct electricity, but molten lead bromide does?

    Tap your answer.

    1. Not this one. The solid is made of ions. They are fixed in a lattice and cannot move. Go to S4.6.6. Try another.

    2. Not this one. It is ions that move in the liquid, not electrons. The heat only frees the ions from the lattice. Try another.

    3. Yes, that is right. A current needs charged particles that can move. The ions are fixed in the solid and free in the liquid.

    See the answer

    The ions are free to move when it is molten.

    A current needs charged particles that can move. The ions are fixed in the solid and free in the liquid.

    • If you chose A: The solid is made of ions. They are fixed in a lattice and cannot move. Go to S4.6.6.
    • If you chose B: It is ions that move in the liquid, not electrons. The heat only frees the ions from the lattice.

  3. Molten zinc chloride, ZnCl₂, is electrolysed. What forms at each electrode?

    Tap your answer.

    1. Yes, that is right. Zn²⁺ goes to the cathode and becomes zinc. Cl⁻ goes to the anode and becomes chlorine, Cl₂.

    2. Not this one. Zn²⁺ is positive and goes to the negative cathode. Cl⁻ is negative and goes to the positive anode. Try another.

    3. Not this one. Ions of opposite charge go to opposite electrodes. Chlorine forms at the anode. Try another.

    See the answer

    Zinc at the cathode and chlorine at the anode

    Zn²⁺ goes to the cathode and becomes zinc. Cl⁻ goes to the anode and becomes chlorine, Cl₂.

    • If you chose B: Zn²⁺ is positive and goes to the negative cathode. Cl⁻ is negative and goes to the positive anode.
    • If you chose C: Ions of opposite charge go to opposite electrodes. Chlorine forms at the anode.

  4. Why is aluminium made by electrolysis and not by heating its oxide with carbon?

    Tap your answer.

    1. Not this one. Heat is not the problem. Carbon is below aluminium in the series, so in a furnace it cannot take the oxygen. Go to S4.7.6. Try another.

    2. Yes, that is right. A less reactive element cannot take oxygen from a more reactive one. Carbon is below aluminium in the series, so electricity is used instead.

    3. Not this one. It is not about hardness. Carbon cannot take oxygen from aluminium oxide in a furnace. Go to S4.7.6. Try another.

    See the answer

    Carbon is less reactive than aluminium, so it cannot take the oxygen.

    A less reactive element cannot take oxygen from a more reactive one. Carbon is below aluminium in the series, so electricity is used instead.

    • If you chose A: Heat is not the problem. Carbon is below aluminium in the series, so in a furnace it cannot take the oxygen. Go to S4.7.6.
    • If you chose C: It is not about hardness. Carbon cannot take oxygen from aluminium oxide in a furnace. Go to S4.7.6.

  5. What is the job of cryolite in the aluminium cell?

    Tap your answer.

    1. Not this one. Cryolite does not react. The current splits the oxide. Try another.

    2. Not this one. The aluminium comes from aluminium oxide. Cryolite only dissolves it. Try another.

    3. Yes, that is right. Aluminium oxide melts above 2 000 °C. Dissolved in molten cryolite, it can be electrolysed at about 950 °C, which saves energy.

    See the answer

    It dissolves the aluminium oxide, so it is liquid at a much lower temperature.

    Aluminium oxide melts above 2 000 °C. Dissolved in molten cryolite, it can be electrolysed at about 950 °C, which saves energy.

    • If you chose A: Cryolite does not react. The current splits the oxide.
    • If you chose B: The aluminium comes from aluminium oxide. Cryolite only dissolves it.

  6. Why must the carbon anodes in the aluminium cell be replaced from time to time?

    Tap your answer.

    1. Yes, that is right. O²⁻ ions become oxygen at the anode. Oxygen and hot carbon make carbon dioxide, so the carbon is used up.

    2. Not this one. The aluminium forms at the cathode. The anodes are where the oxygen is made. Try another.

    3. Not this one. A current does not wear a conductor away like that. The carbon burns in the hot oxygen. Try another.

    See the answer

    The hot oxygen made at the anode burns the carbon away.

    O²⁻ ions become oxygen at the anode. Oxygen and hot carbon make carbon dioxide, so the carbon is used up.

    • If you chose B: The aluminium forms at the cathode. The anodes are where the oxygen is made.
    • If you chose C: A current does not wear a conductor away like that. The carbon burns in the hot oxygen.

  7. From an earlier chapter

    Which of these metals can be made by heating its oxide with carbon?

    Tap your answer.

    1. Not this one. Aluminium is above carbon in the series. Carbon cannot take its oxygen. Go to S4.7.6. Try another.

    2. Not this one. Sodium is far above carbon in the series, so it is also made by electrolysis. Go to S4.7.6. Try another.

    3. Yes, that is right. Iron is below carbon in the series, so carbon removes the oxygen from iron oxide in the blast furnace.

    See the answer

    Iron

    Iron is below carbon in the series, so carbon removes the oxygen from iron oxide in the blast furnace.

    • If you chose A: Aluminium is above carbon in the series. Carbon cannot take its oxygen. Go to S4.7.6.
    • If you chose B: Sodium is far above carbon in the series, so it is also made by electrolysis. Go to S4.7.6.

  8. What mass of aluminium is made from 102 g of aluminium oxide? Use 2Al₂O₃ → 4Al + 3O₂. Mr of Al₂O₃ is 102 and the relative atomic mass of Al is 27.

    See the answer

    54 g

    102 ÷ 102 = 1 mol of Al₂O₃. The ratio of Al₂O₃ to Al is 1 to 2, so 2 mol of Al form. Mass = 2 × 27 = 54 g.

  9. Molten lead bromide has Pb²⁺ ions and Br⁻ ions. How many Br⁻ ions are there for each Pb²⁺ ion?

    See the answer

    2 bromide ions

    The charges must add to zero. Pb²⁺ is +2 and each Br⁻ is −1, so 2 ÷ 1 = 2 bromide ions.

Keep it

Copy this into your exercise book.

A molten ionic compound conducts because its ions move. Positive ions go to the cathode and make a metal. Negative ions go to the anode. Aluminium needs electrolysis.

Draw: A cell with a cathode and anode, Pb²⁺ moving to the cathode and Br⁻ to the anode, and PbBr₂ → Pb + Br₂.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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