Using electrolysis to extract metals

This question is about electrolysis.

Table 4 shows some of the products of electrolysis of two molten compounds.

Complete Table 4.

Aluminium is extracted by electrolysis of a molten mixture of aluminium oxide and cryolite.

The products are molten aluminium and oxygen gas.

Figure 4 shows the electrolysis cell.

What is the formula of oxygen gas?

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Aluminium oxide contains the ions Al³⁺ and O^2–

Explain why aluminium is produced at the negative electrode.

The melting point of aluminium oxide is 2045 °C.

The melting point of the mixture of aluminium oxide and cryolite is 950 °C.

Explain why the electrolysis uses the mixture instead of pure aluminium oxide.

The positive electrodes in the electrolysis cell are made of carbon.

Why do the positive electrodes need to be continually replaced?

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A student investigated the electrolysis of magnesium sulfate solution.

Figure 5 shows the apparatus.

This is the method used.

1. Set the power supply to give a current of 0.05 A.

2. Measure the volume of each gas every minute for 5 minutes.

3. Repeat steps 1 and 2 using a current of 0.10 A in step 1.

Figure 6 shows a measuring cylinder during the electrolysis.

What is the volume of gas in the measuring cylinder?

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Hydrogen is produced instead of magnesium at the negative electrode.

Why is hydrogen produced at the negative electrode?

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Figure 7 shows the results for the volume of hydrogen collected.

Which hypothesis explains the results in Figure 7 ?

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Figure 8 is a repeat of part of the graph for current = 0.10 A in Figure 7.

Determine the gradient of the graph in Figure 8.

Use the equation

Aluminium is extracted by the electrolysis of a molten mixture.

The mixture contains aluminium oxide and one other substance.

Aluminium oxide is an ionic compound.

Explain why aluminium oxide has a very high melting point.

Name the substance added to aluminium oxide to form the mixture.

Give one reason why a mixture is used as the electrolyte.

Oxygen is produced at the positive electrode.

Write the half equation for the reaction that produces oxygen at the positive electrode.

The positive electrode must be continually replaced.

Explain why.

This question is about ionic compounds and electrolysis. Sodium chloride is an ionic compound.

Figure 8 represents part of the structure of solid sodium chloride.

Complete Figure 8.

Give one reason why molten sodium chloride conducts electricity. Refer to ions in your answer.

Table 3 shows products of the electrolysis of two molten ionic compounds. Complete Table 3.

Aluminium is extracted by electrolysis. The electrolyte is a molten mixture of aluminium oxide and cryolite. Why is a mixture used instead of pure aluminium oxide as the electrolyte? Tick (✓) one box.

Electrolysis of an aqueous solution of sodium sulfate produces hydrogen and oxygen. What is the source of the hydrogen and the oxygen produced during the electrolysis of aqueous sodium sulfate solution? Tick (✓) one box.

Electrolysis of an aqueous solution of sodium sulfate produces hydrogen and oxygen.

Why is hydrogen produced instead of sodium in the electrolysis of aqueous sodium sulfate solution? Tick (✓) one box.

Figure 9 shows the relationship between the volume of hydrogen and the volume of oxygen produced during the electrolysis.

Give one conclusion that can be made about the volume of hydrogen produced compared to the volume of oxygen produced.

This question is about electrolysis.

Aluminium is manufactured by electrolysing a molten mixture of aluminium oxide

Complete the half equation for the reaction occurring at the negative electrode.

Cryolite contains Na⁺ ions as well as Al³⁺ ions. Suggest one reason why sodium is not a product of the electrolysis.

A student investigated the electrolysis of an aqueous solution of a different compound. Figure 7 shows the apparatus.

Hydrogen was produced at the negative electrode and oxygen was produced at the positive electrode.

Explain how oxygen was produced from water during the electrolysis of this aqueous solution.

The student compared the volumes of the two gases collected. How can the student change the apparatus in Figure 7 to compare the volumes of the two gases produced more accurately? Give one reason for your answer.

The overall equation for the reaction is:

What is the volume of oxygen produced when 20 cm³ of hydrogen has been produced?

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This question is about electrolysis.

Ionic compounds decompose when they are electrolysed.

A student electrolyses sodium sulfate solution.

Figure 13 shows the apparatus used.

Sodium sulfate solution contains:

• hydrogen ions

• hydroxide ions

• sodium ions

• sulfate ions.

Oxygen is produced at the positive electrode.

Which ions are discharged at the positive electrode to produce oxygen?

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Figure 14 shows one of the measuring cylinders during the electrolysis.

What is the volume of gas in the measuring cylinder?

Ionic compounds can be electrolysed when molten or dissolved in water.

Why can ionic compounds not be electrolysed when solid?

You should answer in terms of ions.

Table 7 shows the products of electrolysis of two molten compounds.

Complete Table 7.

The electrolysis of molten sodium chloride is used to extract sodium metal.

Why is sodium metal extracted by electrolysis instead of by reduction with carbon?

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What is the state symbol for molten sodium chloride?

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Titanium can be produced from titanium oxide by electrolysis.

The equation for the reaction is:

Calculate the percentage atom economy for the production of titanium from titanium oxide by electrolysis.

Use the equation:

Relative atomic mass (A_r): Ti = 48

Relative formula mass (M_r): TiO₂ = 80

This question is about aluminium.

Aluminium is a metal. Draw one line from each property of aluminium to the correct reason for that property.

Property Reason

Aluminium can be used to make alloys. What is meant by an ‘alloy’?

Aluminium is extracted from bauxite. Bauxite is a mixture which contains aluminium oxide.

Bauxite contains between 15% and 25% aluminium. Aluminium oxide always contains 53% aluminium. How does this show that bauxite is a mixture and not a compound?

The waste material from the bauxite is stored in lakes of mud. The lakes of mud are held in place by dams. Figure 6 shows one of these lakes.

Suggest two possible problems with storing the waste material in lakes of mud.

Aluminium is extracted by electrolysis. The aluminium oxide is mixed with cryolite and melted. The mixture is then electrolysed.

The formula of cryolite is Na₃AlF₆ Give the total number of atoms in the formula.

What is the reason for adding cryolite to the aluminium oxide? Tick (✓) one box.

Complete the sentences. Choose answers from the box.

When the molten aluminium oxide and cryolite mixture is electrolysed the product

This product reacts with the positive electrode because the positive electrode is

A sample of bauxite contains 25% aluminium. Calculate the maximum mass of aluminium that can be extracted from 300 000 kg of the sample of bauxite. Give your answer in standard form.

This question is about the extraction of metals.

Tungsten is a metal. The symbol of tungsten is W Tungsten is produced from tungsten oxide by reaction with hydrogen. The equation for the reaction is:

Calculate the percentage atom economy when tungsten is produced in this reaction. Use the equation:

Relative formula masses (M_r): WO₃ = 232 H₂ = 2

Aluminium is extracted from aluminium oxide.

38% of a rock sample is aluminium oxide. Calculate the mass of aluminium oxide in 40 kg of the rock sample.

The formula of aluminium oxide is Al₂O₃ Calculate the relative formula mass (M_r) of aluminium oxide. Relative atomic masses (A_r): O = 16 Al = 27

60.0 kg of aluminium oxide produces a maximum of 31.8 kg of aluminium. In an extraction process only 28.4 kg of aluminium is produced from 60.0 kg of aluminium oxide. Calculate the percentage yield. Give your answer to 3 significant figures.

Use the equation:

Extracting metals by electrolysis is a very expensive process. Explain why aluminium is extracted using electrolysis and not by reduction with carbon.

This question is about electrolysis. Aluminium is produced by electrolysing a molten mixture of aluminium oxide and cryolite.

Explain why a mixture is used as the electrolyte instead of using only aluminium oxide.

What happens at the negative electrode during the production of aluminium? Tick (✓) one box.

Oxygen is produced at the positive electrode. Complete the balanced half-equation for the process at the positive electrode.

Explain why the positive electrode must be continually replaced.

The overall equation for the electrolysis of aluminium oxide is:

Calculate the mass of oxygen produced when 2000 kg of aluminium oxide is completely electrolysed. Relative atomic masses (A_r): O = 16 Al = 27

Sodium metal and chlorine gas are produced by the electrolysis of molten sodium chloride.

Explain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal.

Calculate the volume of 150 kg of chlorine gas at room temperature and pressure. The volume of one mole of any gas at room temperature and pressure is 24.0 dm³ Relative formula mass (M_r): Cl₂ = 71