Electrolysis of aqueous solutions

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

This question is about ionic compounds and electrolysis.

Calcium chloride is an ionic compound.

Calcium and chlorine react to produce calcium chloride.

Describe what happens to calcium atoms and chlorine atoms when the ionic compound calcium chloride is formed.

Solid calcium chloride cannot be electrolysed.

Give one reason why.

Name the product formed at the negative electrode when aqueous calcium chloride solution is electrolysed.

What is the half equation for the reaction at the positive electrode when aqueous calcium chloride solution is electrolysed?

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A student investigated the electrolysis of green copper chromate solution.

Figure 4 shows the apparatus.

Figure 5 shows the results.

Copper chromate solution contains the ions Cu²⁺ and CrO₄^2–

Explain the results shown in Figure 5.

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 electrolysis. Molten sodium chloride is electrolysed in an industrial process to produce sodium. Figure 8 shows a simplified version of the electrolysis cell used.

Which is the correct half equation for the production of sodium? Tick (✓) one box.

A mesh is used to keep the products of the electrolysis apart.

Suggest one reason why the products of the electrolysis must be kept apart.

Which type of particle passes through the mesh in the electrolysis of molten sodium chloride? Tick (✓) one box.

Aqueous sodium chloride solution is electrolysed in a different industrial process. Two gases and an alkaline solution are produced.

Which two ions are present in aqueous sodium chloride solution in addition to sodium ions and chloride ions?

Name the alkaline solution produced.

Explain how the alkaline solution is produced. You should refer to the processes at the electrodes.

This question is about acids and alkalis.

Explain why the pH of an acid depends on:

• the strength of the acid

• the concentration of the acid.

A student titrated 25.00 cm³ of hydrochloric acid with 0.100 mol/dm³ barium hydroxide solution. Table 2 shows the results.

The student calculated the volume of barium hydroxide solution to be used in the

Explain why the student used a volume of 23.50 cm³ of barium hydroxide solution in the titration calculation.

25.00 cm³ of the hydrochloric acid reacted with 23.50 cm³ of the 0.100 mol/dm³ barium hydroxide solution. The equation for the reaction is:

Calculate the concentration of the hydrochloric acid in mol/dm³.

Another student titrated sulfuric acid with barium hydroxide solution. The equation for the reaction is:

The student measured the electrical conductivity of the mixture during the titration. The better a conductor, the higher the electrical conductivity value. Figure 10 shows the results.

Explain why the electrical conductivity of the mixture was zero when the sulfuric acid had just been neutralised. Use the equation for the reaction. Refer to ions in your answer.

The student then added a further 10 cm³ of barium hydroxide solution. The electrical conductivity of the mixture increased. Give one reason why.

This question is about electrolysis.

Some students investigated the electrolysis of silver nitrate solution.

This electrolysis produces silver at the negative electrode.

Figure 11 shows the apparatus.

This is the method used.

1. Weigh the negative electrode.

2. Set up the apparatus shown in Figure 11.

3. Switch on the power supply.

4. Switch off the power supply after five minutes.

5. Rinse the negative electrode with water and allow to dry.

6. Reweigh the negative electrode.

7. Repeat steps 1 to 6 for different times.

Some silver did not stick to the negative electrode but fell to the bottom of the beaker.

The students needed to weigh this silver.

How could the students separate the silver from the silver nitrate solution?

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Table 5 shows the students’ results.

Draw a graph on Figure 12.

You should:

• use a suitable scale for the x-axis

• plot the data from Table 5

• draw a line of best fit.

Determine the mass of silver that would be produced after 12 minutes.

Use Figure 12.

A student investigated the electrolysis of two aqueous salt solutions.

Hydrogen is produced at the negative electrode when the metal in the salt solution is more reactive than hydrogen.

Complete Table 6 to show what the student would observe at the negative electrode for each salt solution.

A teacher demonstrates the electrolysis of molten lead bromide.

The products at the electrodes are lead and bromine.

Why should the teacher do the demonstration in a fume cupboard?

Two other molten compounds are electrolysed.

Complete Table 7 to show the molten compounds and the products.

This question is about chemical reactions and electricity.

Electrolysis and chemical cells both involve chemical reactions and electricity. Explain the difference between the processes in electrolysis and in a chemical cell.

A teacher demonstrates the electrolysis of molten lead bromide. Bromine is produced at the positive electrode. Complete the half equation for the production of bromine. You should balance the half equation.

Two aqueous salt solutions are electrolysed using inert electrodes. Complete Table 4 to show the product at each electrode.

Some students investigated the electrolysis of copper nitrate solution using inert electrodes. Figure 4 shows the apparatus.

The students investigated how the mass of copper produced at the negative electrode varied with:

• time

• current.

This is the method used.

1. Weigh the negative electrode.

2. Set up the apparatus shown in Figure 4.

3. Adjust the power supply until the ammeter shows a current of 0.3 A

4. Switch off the power supply after 5 minutes.

5. Rinse the negative electrode with water and allow to dry.

6. Reweigh the negative electrode.

7. Repeat steps 1 to 6 for different times.

8. Repeat steps 1 to 7 at different currents.

Some of the copper produced did not stick to the negative electrode but fell to the bottom of the beaker. Suggest how the students could find the total mass of copper produced.

The students plotted their results on a graph. Figure 5 shows the graph.

A student correctly concluded that the total mass of copper produced is directly proportional both to the time and to the current.

How do the results in Figure 5 support the conclusion that the total mass of copper produced is directly proportional to the time?

How do the results in Figure 5 support the conclusion that the total mass of copper produced is directly proportional to the current? Use data from Figure 5 in your answer.

Copper nitrate solution is blue. Suggest why the blue colour of the copper nitrate solution fades during the electrolysis.

Determine the number of atoms of copper produced when copper nitrate solution is electrolysed for 20 minutes at a current of 0.6 A Give your answer to 3 significant figures. Use Figure 5. Relative atomic mass (A_r): Cu = 63.5 The Avogadro constant = 6.02 10²³ per mole

This question is about electrolysis. A student investigated the hypothesis: ‘The electrolysis of a salt solution produces a metal at the negative electrode and a gas at the positive electrode.’

Figure 4 shows the apparatus used.

What observation would be made at each electrode if the hypothesis is correct?

Table 3 shows the student’s results.

Which salt solution in Table 3 does not match the student’s hypothesis? Give one reason why.

Give two reasons why graphite is used for the electrodes.

A different student investigated what happens during electrolysis. Figure 5 shows the apparatus.

The purple crystal contained:

• colourless positive ions

• purple coloured negative ions.

The purple crystal dissolved in the electrolyte solution.

What happens to the purple coloured ions? Give one reason for your answer. Tick (✓) one box.

This question is about electrolysis. A student investigated the electrolysis of copper chromate solution. Copper chromate solution is green. Copper chromate contains:

• blue coloured Cu²⁺ ions

• yellow coloured CrO₄^2– ions.

Figure 1 shows the apparatus used.

The student switched the power supply on. The student observed the changes at each electrode. Table 4 shows the student’s observations.

Explain why the colour changed at the positive electrode.

The gas produced at the positive electrode was oxygen. The oxygen was produced from hydroxide ions. Name the substance in the solution that provides the hydroxide ions.

Describe how the solid forms at the negative electrode.

The student repeated the investigation using potassium iodide solution instead of copper chromate solution. Name the product at each electrode when potassium iodide solution is electrolysed. Negative electrode Positive electrode

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

A student investigated the electrolysis of sodium chloride solution.

Figure 4 shows the apparatus.

The student measured the volume of gas collected in each measuring cylinder every minute for 20 minutes.

Figure 5 shows the volume of hydrogen gas collected in the measuring cylinder after 8 minutes.

What is the volume of hydrogen gas collected?

Figure 6 shows the results of the investigation.

Which of the lines on Figure 6 show that the volume of gas collected is directly proportional to the time?

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Which of the lines on Figure 6 show a positive correlation between the volume of gas collected and time?

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A teacher demonstrates the electrolysis of different substances using graphite electrodes.

Figure 7 shows the apparatus used.

Figure 7

Why can graphite conduct electricity?

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The teacher demonstrates the electrolysis of:

molten zinc chloride

potassium bromide solution.

Complete Table 3 to predict the products.

Choose answers from the box.