Half equations at electrodes

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.

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 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 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

The reaction between hydrogen and oxygen releases energy.

A student drew a reaction profile for the reaction between hydrogen and oxygen. Figure 3 shows the student’s reaction profile.

The student made two errors when drawing the reaction profile. Describe the two errors.

The reaction between hydrogen and oxygen in a hydrogen fuel cell is used to produce electricity. Hydrogen fuel cells and rechargeable cells are used to power some cars. Give two advantages of using hydrogen fuel cells instead of using rechargeable cells to power cars.

Reactions occur at the positive electrode and at the negative electrode in a hydrogen fuel cell. Write a half equation for one of these reactions.

The three states of matter can be represented by a simple particle model. Figure 4 shows a simple particle model for hydrogen gas.

Give two limitations of this simple particle model for hydrogen gas.

The hydrogen gas needed to power a car for 400 km would occupy a large volume. Suggest one way that this volume can be reduced.

The energy needed for a car powered by a hydrogen fuel cell to travel 100 km is 58 megajoules (MJ). The energy released when 1 mole of hydrogen gas reacts with oxygen is 290 kJ The volume of 1 mole of a gas at room temperature and pressure is 24 dm³ Calculate the volume of hydrogen gas at room temperature and pressure needed for the car to travel 100 km

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