Studentactivity

Name ...... Class ...... Date ......

The extraction of aluminium

Specification requirements

  • C4.3.3 Using electrolysis to extract metals

Aims

In this activity you will learn how electrolysis is used to extract aluminium from its ore.

Learning outcomes

After completing this activity, you should be able to:

  • state that aluminium can be extracted from aluminium oxide using electrolysis
  • write a word equation to describe the electrolysis of aluminium oxide
  • describe the electrolysis of aluminium oxide
  • explain why electrolysis is an expensive metal extraction method and illustrate this with the extraction of aluminium
  • explain why cryolite is added to aluminium oxide in the industrial extraction of aluminium
  • explain why electrolysis is used to extract aluminium from compounds
  • describe electrolysis with half equations at the electrodes
  • explain the classification of the reactions at each electrode as oxidation or reduction.

Setting the scene

Aluminium is the most abundant metal in the Earth’s crust. It occurs naturally in the form of aluminium oxide, which is contained in anore called bauxite.

You will look at how aluminium is extracted from aluminium oxide using electrolysis.

Task

1Explain why aluminium cannot be extracted from its oxide by reduction with carbon.(1 mark)

2Here is an unlabelled diagram of the electrolysis of molten aluminium oxide.

Add the appropriate labels.(5 marks)

3Describe the electrolysis in terms of the ions moving.(2 marks)

4Write an equation to summarise the overall process.(2 marks)

5Explain why cryolite is added.(1 mark)

6Explain why the carbon electrodes must be replaced frequently.(2 marks)

7Give two reasons why the extraction of aluminium is expensive.(2 marks)

8Give three uses of aluminium and explain why it is used for each particular purpose. (3 marks)

9Write half equations for the reactions at the anode and cathode, and determine whether each reaction is an oxidation or reduction. (4 marks)

Questions

1Potassium was the first metal isolated by electrolysis. It was isolated in 1807 by Sir Humphry Davy.

aDavy first tried to electrolyse a solid potassium salt to produce potassium. Explain why this electrolysis did not work.

(2 marks)

bHe then electrolysed dry molten caustic potash (KOH, potassium hydroxide) and discovered that potassium collected at the cathode.

iDescribe how potassium atoms are formed from potassium ions.

(1 mark)

iiWrite a half equation for the reaction at the cathode and state whether it is oxidation or reduction.

(3 marks)

cA student dissolved some potassium chloride in water. The student tried to electrolyse the potassium chloride solution to produce potassium. The student expected to see potassium metal at the negative electrode, but instead saw bubbles of a gas.

iName the gas produced at the negative electrode.

(1 mark)

iiExplain why this gas was produced at the negative electrode and why potassium was not produced. Use the reactivity series of metals to help you answer this question.

(2 marks)

2a Calculate the % by mass of aluminium in aluminium oxide (Al2O3) (Ar values: Al  27, O  16).

(2 marks)

bChromium can be extracted from chromium oxide (Cr2O3) by displacement with aluminium.

iWrite an equation to show this reaction.

(2 marks)

iiWhat does this tell you about the reactivity of chromium compared to aluminium?

(1 mark)

iiiCalculate how many tonnes of aluminium are needed to make 208 tonnes of chromium (Ar values: Al  27, Cr  52).

(2 marks)

ivList two important uses of chromium.

(2 marks)

© Oxford University Press 2016:

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