Electrolysis (AQA GCSE Chemistry)

Magnesium chloride is an ionic compound that can be electrolysed. 

Figure 1 shows two experiments in which MgCl₂ undergoes this process.

Figure 1 

Explain why solid magnesium chloride cannot be electrolysed and give the necessary conditions for electrolysis to occur.

Justify the production of magnesium metal at the cathode in Experiment A.

Identify the gas produced at the cathode in Experiment B.

Explain why magnesium is not produced at the cathode in Experiment B.

Balance the half equation which occurs at the anode.

________ Cl^–       →      Cl₂ + _______

When copper(II) sulfate undergoes electrolysis, it produces oxygen gas at the positive electrode.

The unbalanced half equation below represents the reaction that occurs at the positive electrode.

Which numbers will correctly balance the half equation?

___ OH^-    →    O₂   +   ___ H₂O  +  ___ e^-

This question is about the extraction of aluminium using electrolysis. The process is displayed in Figure 3.

Figure 3

Why isn’t aluminum extracted by heating with carbon?

Explain why aluminium is formed at the cathode.

Explain why carbon dioxide is formed at the anode and why the anode must be continually replaced.

The electrolyte in the process consists of a mixture of aluminium oxide and cryolite. 

Explain why a mixture is used instead of just the aluminium oxide.

This question is about the electrolysis of sulfuric acid. The apparatus used in this process is shown in Figure 4. 

Oxygen is formed at the anode.

Figure 4

Solutions which decompose when electricity is passed through them have a specific name. 

What is the name given to these solutions?

The half equation for the reaction which occurs at the anode is:

4OH^– → 2H₂O + O₂ + 4e^–

Describe what type of reaction this is and give a reason for your answer.

Write a balanced half equation for the reaction which occurs at the cathode.

As the electricity is passed through the electrolyte what happens to the concentration of the sulfuric acid? 

Explain your answer.

A student electrolysed a solution of potassium chloride with the intention of isolating potassium. 

The apparatus was set up as shown in Figure 5. 

Figure 5 

The student expected to see potassium metal being formed at the cathode but instead observed bubbles of gas being formed.

Name the gas produced at the cathode and explain why this gas is produced, giving a reason why potassium is not produced.

The student then electrolysed molten potassium chloride instead of a solution and found that potassium metal was produced at the negative electrode.

Describe how potassium atoms are produced from ions of potassium.

During the the electrolysis of copper sulfate solution, copper ions move to the negative electrode.

What is the reason for this?

A student was investigating the electrolysis of zinc chloride. 

What is the correct reason for electrolysis not working? 

Hydrogen, chlorine and sodium hydroxide are commercially important chemicals which can all be made in one process. 

The process involves passing electricity through an aqueous solution of sodium chloride (brine). 

Figure 2 shows an electrolytic cell in which this process can be carried out.

Figure 2 

Identify the substances produced at points A, B and C.

A _________________________

B _________________________

C _________________________

Complete and balance the equations for the reactions at the electrodes.

___Cl^– – ___e^– → Cl₂
___H + ___e^– → H₂

Identify the compound in the cell responsible for producing the hydrogen ions.

What kind of particles must be able to pass through the barrier so that electrolysis can take place?

This question is about electrolysis.

A student investigated the electrolysis of copper chromate solution.

Chromate ions, , are yellow.

Explain the colour of copper chromate solution.

The student's apparatus to electrolyse copper chromate solution using zinc electrodes is shown in Figure 1.

Explain why the negative electrode increases in mass.

The student replaces the zinc electrodes with carbon electrodes. 

Explain why the mass of one of the electrodes decreases.

This question is about the electrolysis of aluminium oxide.

Explain why aluminium is extracted from its ore by electrolysis.

Explain why cryolite is used in the extraction of aluminium.

Aluminium oxide undergoes a redox reaction during electrolysis. 

Explain whether aluminium is produced as the result of a reduction or an oxidation reaction. 

Your answer should include an appropriate equation including state symbols.

Explain what happens to the anode during the electrolysis of aluminium oxide.

This question is about the electrolysis of zinc(II) chloride, ZnCl₂.

Zinc(II) chloride can conduct electricity when it is molten or dissolved in solution.

Explain why zinc(II) chloride cannot conduct electricity when it is solid.

In terms of structure and bonding, explain why graphite is commonly used as the electrode in electrolysis reactions.

Explain how the product at each electrode is formed during the electrolysis of molten zinc chloride.

The electrolysis of molten zinc chloride gives different products compared to the electrolysis of aqueous zinc chloride. 

Explain any differences in the products from each electrolysis experiment.

Sodium chloride solution can undergo electrolysis. 

What product will be formed at the negative electrode?

This question is about the electrolysis of sulfuric acid.

A student investigated how the volume of gases produced changes with time in the electrolysis of dilute sulfuric acid.

The apparatus is shown in Figure 1.

Figure 1

The student has made an error in the apparatus. 

Explain how the apparatus should be changed.

With the help of suitable equations, explain why it is more likely that the hydrogen formed in this electrolysis comes from the sulfuric acid than water.

Sulfate ions and hydroxide ions are both attracted to the anode. 

Hydroxide ions are less stable than sulfate ions so they are discharged at the anode. 

OH^– (aq) → OH (aq) + e^–  

The OH groups produced exist for a fraction of a second before they form water and oxygen. 

Write a balanced symbol equation, including state symbols, for this reaction.

Electrolysis can be used to separate molten lead bromide. 

Which statement is true for what happens at the negative electrode?

This question is about sodium chloride.

Describe, in terms of electrons and with the help of suitable equations, what happens during the electrolysis of molten sodium chloride.

The products of electrolysis of molten sodium chloride are different to those of aqueous sodium chloride. 

Explain the different products from the electrolysis of aqueous sodium chloride.

Nuclear submarines can stay underwater for months without needing to come to the surface. 

Oxygen for the crew to breathe is produced using a piece of equipment called an automated electrolytic oxygen generator. 

Many people assume that seawater is electrolysed to produce the oxygen, this assumption is incorrect. 

Explain why the assumption is not only incorrect but potentially hazardous and how the oxygen can be produced by electrolysis.