Electrolysis

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?

Tick () one box.

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?

Tick () one box.

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?

Tick () one box.

Hydrogen is produced instead of magnesium at the negative electrode.

Why is hydrogen produced at the negative electrode?

Tick () one box.

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

Which hypothesis explains the results in Figure 7 ?

Tick () one box.

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 electrolysis and the extraction of metals.

Why can some molten substances be electrolysed?

Tick () one box.

Table 3 shows the products of the electrolysis of some molten compounds.

Complete Table 3.

Aluminium is extracted by electrolysing molten aluminium oxide.

Balance the equation for the reaction.

Choose numbers from the box.

Calculate the relative formula mass (M_r) of aluminium oxide (Al₂O₃).

Relative atomic masses (A_r): O = 16 Al = 27

Figure 13 shows part of the reactivity series of metals.

The non-metal carbon has been included.

Metals can be extracted from their compounds by:

electrolysis

reduction with carbon.

Electrolysis is more expensive than reduction with carbon.

Predict one metal that would be extracted by each method.

Use Figure 13.

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?

Tick () one box.

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.

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?

Tick (✓) one box.

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?

Tick (✓) one box.

What is the state symbol for molten sodium chloride?

Tick (✓) one box.

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.

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?

Tick (✓) one box.

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.

A student investigated the electrolysis of different substances. Figure 6 shows the apparatus.

Explain why electrolysis would not take place in the apparatus shown in Figure 6.

Explain why graphite conducts electricity. Answer in terms of the structure and bonding in graphite.

The student investigated how the volume of gases produced changes with time in the electrolysis of sodium chloride solution. Figure 7 shows the apparatus.

The student made an error in selecting the apparatus for this investigation. How should the apparatus be changed? Give one reason for your answer.

Another student used the correct apparatus. This student measured the volumes of gases collected every minute for 20 minutes. Figure 8 shows the student’s results.

Describe the trends shown in the results. Use values from Figure 8.

The number of moles of each gas produced at the electrodes is the same. No gas escapes from the apparatus. Suggest one reason for the difference in volume of each gas collected.

Calculate the amount in moles of chlorine collected after 20 minutes. Use Figure 8. The volume of one mole of any gas at room temperature and pressure is 24.0 dm³ Give your answer in standard form.