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Understand Extracting Substances with Electrolysis

In this worksheet, students will learn how electrolysis is used to extract aluminium from aluminium oxide and chlorine from sodium chloride, using rules of electrolysis to predict the substances produced at each electrode.

Worksheet Overview

QUESTION 1 of 10

Extracting aluminium from aluminium oxide

Aluminium is an incredibly useful metal, because it is light and strong. In nature, we find it as aluminium oxide in rocks, and for a long time it was hard to extract aluminium metal from the oxide. Electrolysis of molten aluminium oxide is the way we can do this. Although the process is scientifically simple, the practicalities of making it work are challenging.

The first problem is making aluminium into a suitable electrolyte. Aluminium oxide does not dissolve in water, so we have to melt aluminium oxide for electrolysis to happen. The melting point of pure aluminium oxide is very high (over 2000 ºC), but adding another mineral, called cryolite, reduces the melting temperature to about 1000 ºC. This is helpful, because it reduces the cost of the process.

Graphite electrodes are used, made of pure carbon. These are placed in the molten aluminium oxide/cryolite mixture. Then electrolysis starts. Since the electrolite is a molten binary compound, it is simple to work out what occurs at each electrode.

At the cathode: Al3+ + 3 e- → Al. The molten metal sinks to the bottom of the vessel, where it falls out and is collected.

At the anode: 2 O2- → O2 + 4 e-.

The oxygen reacts with the carbon electrodes, making carbon dioxide. This gradually destroys the electrodes, so they have to be regularly replaced.

Electrolysis of aluminium oxide is expensive, for several reasons.The main ones are

the cost of heating the aluminium oxide

the cost of electricity for the electrolysis

Because of this, aluminium is expensive, which is why it is so important to recycle aluminium.

Extracting chlorine from sodium chloride

Chlorine is also a very useful element. It is used for cleaning, as a disinfectant and in bleach. Chlorine is added to water (especially in swimming pools) to kill bacteria. Some plastics (especially PVC) also need chlorine as an ingredient. Our main source of chlorine is sodium chloride. We use electrolysis to get chlorine out of sodium chloride.

Our first job is to find sodium chloride. Although there is some sodium chloride in sea water, it is better to find layers of sodium chloride buried underground in mines like this.

The easiest way to bring sodium chloride to the surface is to pump fresh water into the mine. Sodium chloride will dissolve into the water, so salty water will be pumped back to the surface. This gives us a solution which we can use for electrolysis. To be sure of what is produced, we need to compare the elements attracted to each electrode.

The ions in the salt water electrolyte are Na+, Cl-, H+ and OH-. Sodium and hydrogen ions will be attracted to the cathode, and chloride and hydroxide will be attracted to the anode.

At the cathode, sodium stays in solution because it is more reactive, so hydrogen ions are turned into hydrogen atoms; 2 H+ + 2e- → H2

At the anode, hydroxide stays in solution because the chloride is a halogen ion, so chloride ions are turned into chlorine atoms; 2 Cl- → Cl2 + 2e-

Electrolysis of sodium chloride is much cheaper than electrolysis of aluminium oxide. We can make the liquid electrolyte by dissolving salt in water, which doesn't need energy- unlike melting aluminium oxide.

Match these properties of aluminium with their consequences

Column A

Column B

Aluminium is light
so aluminium can be used for electric cables.
Aluminium is a reasonable conductor of electricity
so aluminium is found as oxides, not pure metal.
Aluminium is reactive
so aluminium airplanes can take off more easily.
Aluminium oxide is insoluble in water
so electrolysis is expensive.

Why do we add cryolite to aluminium oxide? Select one option from each part to make a complete explanation.

Column A

Column B

Aluminium is light
so aluminium can be used for electric cables.
Aluminium is a reasonable conductor of electricity
so aluminium is found as oxides, not pure metal.
Aluminium is reactive
so aluminium airplanes can take off more easily.
Aluminium oxide is insoluble in water
so electrolysis is expensive.

Construct a complete half-equation for the cathode in the electrolysis of molten Al2O3.

Column A

Column B

Aluminium is light
so aluminium can be used for electric cables.
Aluminium is a reasonable conductor of electricity
so aluminium is found as oxides, not pure metal.
Aluminium is reactive
so aluminium airplanes can take off more easily.
Aluminium oxide is insoluble in water
so electrolysis is expensive.

Construct a complete half-equation for the anode in the electrolysis of molten Al2O3.

Column A

Column B

Aluminium is light
so aluminium can be used for electric cables.
Aluminium is a reasonable conductor of electricity
so aluminium is found as oxides, not pure metal.
Aluminium is reactive
so aluminium airplanes can take off more easily.
Aluminium oxide is insoluble in water
so electrolysis is expensive.

What is the consequence of production of oxygen at the anode?

Oxygen can be recycled.

Oxygen damages the carbon electrodes.

Oxygen causes the aluminium to oxidise again.

Oxygen makes the process sustainable.

Why is it much cheaper to produce chlorine than aluminium? Pick one part from each section to make a complete explanation.

Oxygen can be recycled.

Oxygen damages the carbon electrodes.

Oxygen causes the aluminium to oxidise again.

Oxygen makes the process sustainable.

Which of these reactions happen during the electrolysis of aqueous sodium chloride?

Na+ + e- → Na

2 H+ + 2 e- → H2

2 Cl- → Cl2 + 2 e-

4 OH- - 4 e- → 2 H2O + O2

Could we, and should we, extract chlorine from sodium chloride by using molten sodium chloride instead of aqueous sodium chloride?

We could, but it's not a good idea.

We could, and we should.

We can't do this at all

It makes no difference which method we use.

Could we extract sodium from sodium chloride by using molten sodium chloride instead of aqueous sodium chloride?

No, it wouldn't work.

Yes, it would work.

It would work, but it would be better to use aqueous sodium chloride.

It's not possible to extract sodium from sodium chloride.

Copper chloride is soluble in water.  What would be the products of electrolysis of aqueous copper chloride?

chlorine

copper

hydrogen

oxygen

  • Question 1

Match these properties of aluminium with their consequences

CORRECT ANSWER

Column A

Column B

Aluminium is light
so aluminium airplanes can take o...
Aluminium is a reasonable conduct...
so aluminium can be used for elec...
Aluminium is reactive
so aluminium is found as oxides, ...
Aluminium oxide is insoluble in w...
so electrolysis is expensive.
EDDIE SAYS
You can\'t revise for questions like this; they are more about thinking than memory. The link between \"insoluble in water\" and \"electrolysis is expensive\" is that we have to melt aluminium oxide, which needs large amounts of energy.
  • Question 2

Why do we add cryolite to aluminium oxide? Select one option from each part to make a complete explanation.

CORRECT ANSWER
EDDIE SAYS
In general, adding impurities to metals does make them stronger- think of adding carbon to iron to make steel. That's not the point here, though- the product of electrolysis should be pure aluminium. The purpose of cryolite is to reduce the melting temperature; a bit like when we add salt to water to stop it freezing.
  • Question 3

Construct a complete half-equation for the cathode in the electrolysis of molten Al2O3.

CORRECT ANSWER
EDDIE SAYS
The metal ion is the cation, so that goes to the cathode. There, it gains electrons, so is reduced to metal atoms.
  • Question 4

Construct a complete half-equation for the anode in the electrolysis of molten Al2O3.

CORRECT ANSWER
EDDIE SAYS
The oxide ion is the anion, so that goes to the anode. There, it gains electrons, so is oxidised to make oxygen atoms. We need two of them to make an O2 molecule.
  • Question 5

What is the consequence of production of oxygen at the anode?

CORRECT ANSWER
Oxygen damages the carbon electrodes.
EDDIE SAYS
The oxygen produced reacts with the electrodes to make carbon dioxide. This gradually destroys the electrodes, so they have to be replaced.
  • Question 6

Why is it much cheaper to produce chlorine than aluminium? Pick one part from each section to make a complete explanation.

CORRECT ANSWER
EDDIE SAYS
Heating anything up needs lots of energy, so is expensive. This is especially true if we have to heat huge amounts of rock to about 1000 ºC.
  • Question 7

Which of these reactions happen during the electrolysis of aqueous sodium chloride?

CORRECT ANSWER
2 H+ + 2 e- → H2
2 Cl- → Cl2 + 2 e-
EDDIE SAYS
Although all these are correct half-equations, both the sodium and the hydroxide stay as ions in the solution. Sodium is more reactive than hydrogen, and chloride discharges in preference to hydroxide.
  • Question 8

Could we, and should we, extract chlorine from sodium chloride by using molten sodium chloride instead of aqueous sodium chloride?

CORRECT ANSWER
We could, but it's not a good idea.
EDDIE SAYS
The reaction would work, and we would produce chlorine. The disadvantage is that melting sodium chloride needs large amounts of energy. Not as much as for aluminium oxide, but enough to make the process expensive and inefficient.
  • Question 9

Could we extract sodium from sodium chloride by using molten sodium chloride instead of aqueous sodium chloride?

CORRECT ANSWER
Yes, it would work.
EDDIE SAYS
If we don't dissolve the sodium chloride in water, there are no hydrogen ions to go to the cathode along with the sodium ions. That means that the sodium has to be reduced to make sodium metal. The process is called Down's process; it's hard to do safely, but it's important. Sodium is one of the ingredients in indigo dye, used in blue denim.
  • Question 10

Copper chloride is soluble in water.  What would be the products of electrolysis of aqueous copper chloride?

CORRECT ANSWER
chlorine
copper
EDDIE SAYS
Chlorine would be produced at the anode, for the same reasons as with sodium chloride. Copper would be produced at the anode, because it is less reactive than hydrogen. The hydrogen ions would stay in the solution.
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