Electrolysis of aqueous solutions

Magnesium sulfate is an ionic compound.

A student adds magnesium sulfate to water to form a solution.

Which of these will also form a solution when added to water?

Calculate the percentage by mass of oxygen in magnesium sulfate, MgSO₄.

(relative atomic mass: O = 16.0 relative formula mass: MgSO₄ = 120)

Give your answer to 3 significant figures.

percentage by mass =_........

Calculate the number of oxygen atoms in 7.20 g of MgSO₄.

(relative formula mass: MgSO₄ = 120 Avogadro constant = 6.02 × 10²³)

number of oxygen atoms =_........

*(c) Magnesium sulfate solution can be electrolysed using the apparatus shown in Figure 7.

Explain how the products are formed at the electrodes in terms of

• the ions present in the solution

• the oxidation and reduction reactions that take place at the electrodes.

You should include ionic equations as part of your answer.

A 250 cm³ solution of copper sulfate contains 6.52 g of dissolved solid.

Calculate the concentration of this copper sulfate solution in g dm^–3.

concentration =_........ g dm

Sodium hydroxide solution and copper sulfate solution were reacted together completely.

The result was a mixture of a precipitate of copper hydroxide in a solution of sodium sulfate.

Describe how to obtain

• a pure sample of solid copper hydroxide from the mixture

• a pure sample of solid sodium sulfate from the mixture.

Figure 9 shows the equipment used to electrolyse a sample of sodium sulfate solution.

Graphite electrodes are used in the electrolysis.

Give two reasons why graphite is a suitable material for the electrodes.

Sodium sulfate solution contains ions.

Which ions are attracted to the positive electrode during the electrolysis?

Draw one straight line from each electrode to the product formed at that electrode during the electrolysis of sodium sulfate solution.

Water, acidified with dilute sulfuric acid, was electrolysed for 10 minutes using inert electrodes.

Figure 5 shows the apparatus used.

In this electrolysis, the acidified water is an electrolyte.

Explain why acidified water is an electrolyte.

Hydrogen collects at the negative electrode and oxygen collects at the positive electrode.

Which of these shows the results after 10 minutes of this electrolysis?

Complete and balance the half equation for the formation of oxygen at the positive electrode in this electrolysis.

Copper sulfate solution was electrolysed for 10 minutes using copper electrodes.

Figure 6 shows the mass of the cathode and the appearance of the copper sulfate solution before electrolysis and after electrolysis.

Describe what should be done to the copper cathode, after it has been removed from the copper sulfate solution, before its final mass is determined.

Explain, in terms of ions, the change in mass of the cathode shown in Figure 6.

Explain why the appearance of the copper sulfate solution did not change during the electrolysis.

This question is about electrolysis.

Which statement describes what happens during electrolysis?

Figure 5 shows the electrolysis of copper chloride solution.

Use the words from the box to complete the labelling of the diagram in Figure 5.

The products of the electrolysis shown in Figure 5 are solid X and gas Y.

Draw one straight line from each product to its name.

The experiment is repeated using powdered solid copper chloride instead of copper chloride solution.

Nothing happens and no products are formed.

Explain why nothing happens and no products are formed.

A student investigates the mass of copper produced when copper chloride solution in a beaker is electrolysed using inert electrodes.

Where is copper formed during the electrolysis?

The student investigated the change in the mass of copper formed when the current was altered.

The results are shown in Figure 7.

State and explain the trend shown in these results.

Describe how, after the power supply has been switched off, the mass of copper formed can be measured.

In another experiment, 74 mg of copper is formed.

Calculate the number of copper atoms in 74 mg of copper.

(relative atomic mass Cu = 63.5; Avogadro constant = 6.0210²³)

number of atoms =_........

3.14 g of solid copper sulfate was dissolved in water and made up to 250 cm³ of solution.

Calculate the concentration of this copper sulfate solution in g dm⁻³.

concentration =_........ g dm

Sodium hydroxide solution was added to a solution of copper sulfate. A precipitate of copper hydroxide and a solution of sodium sulfate were formed.

State what would be seen in the reaction.

Complete the balanced equation for the reaction by adding a number in front of NaOH.

Describe how to obtain a pure, dry sample of the precipitate of copper hydroxide from the reaction mixture.

Figure 11 shows the ions in the sodium sulfate solution.

Draw a circle around each of the ions in Figure 11 that are attracted to the negative graphite electrode during the electrolysis.

State why it is important that the electrodes are inert.

Explain, in terms of its structure, how graphite conducts electricity.

When copper sulfate solution is electrolysed using copper electrodes, the mass of each electrode changes.

Draw a labelled diagram to show the apparatus that can be used to electrolyse copper sulfate solution using copper electrodes.

Before the electrolysis is carried out, the mass of each electrode is determined.

Explain what should be done to the copper electrodes before their masses are determined.

Figure 4 shows the results obtained from an electrolysis experiment when copper sulfate solution was electrolysed for 10 minutes.

Explain, in terms of ions, the changes in mass of the two electrodes shown in the results in Figure 4.

The electrolysis was repeated using another pair of copper electrodes of the same masses.

Explain a change that could be made to the electrolysis experiment to cause the mass of the cathode to increase by 2.34 g in 10 minutes.

Hydrochloric acid reacts with solid B. Solid B is an alkali.

A student carries out an experiment to see how the pH changes when different masses of solid B are added to dilute hydrochloric acid.

The student uses the following method.

step 1 use a measuring cylinder to measure out 100cm³ of dilute hydrochloric acid

step 2 pour the acid into a beaker

step 3 measure the pH with a pH probe

step 4 add half a spatula of solid B and stir

step 5 repeat steps 3 and 4 until the pH stops changing.

Give a safety precaution that should be taken during the experiment.

Give an improvement to step 4 that would produce more accurate results.

What is the most likely change in pH during the experiment?

If some methyl orange indicator is added to the acid in step 2, the mixture changes colour during the experiment.

State the colour change.

colour at start in acid_........ colour at end_........

Concentrated hydrochloric acid can be broken down using electricity. The apparatus that can be used is shown in Figure 6.

Give the name of the piece of apparatus labelled X.

The rod labelled Y in Figure 6 is made of graphite.

What is the name of this piece of apparatus?

Give one reason why graphite is a suitable material to make Y.

Complete the balanced equation for the reaction that occurs.

This question is about electrolysis.

A sample of molten potassium bromide is electrolysed.

What are the two products formed?

Zinc chloride and zinc carbonate contain ions.

Zinc chloride mixed with water can be electrolysed. Zinc carbonate mixed with water cannot be electrolysed.

Explain this difference.

In the electrolysis of sodium chloride solution, bubbles of a colourless gas form at the cathode. This gas, when mixed with air, burns with a squeaky pop.

Identify this gas.

Explain how this gas is formed at the cathode.

A solution of copper sulfate in a beaker is electrolysed using copper electrodes.

Draw a labelled diagram to show how this experiment would be set up.

The beaker has been drawn for you.

During the electrolysis, the anode gets smaller, the cathode gets larger and the solution remains the same shade of blue.

Give the reason for each of these observations.

the anode gets smaller_........

the cathode gets larger_........

the solution remains the same shade of blue_........

Figure 11 shows the apparatus that can be used to electrolyse sodium sulfate solution using inert electrodes.

Hydrogen is produced at the negative electrode during electrolysis.

Describe the test to show the gas is hydrogen.

What is the name of gas X that forms at the positive electrode?

State what is meant by the term electrolysis.

The sodium sulfate solution was made by dissolving 28.4 g of sodium sulfate in water to make 250 cm³ of solution.

Calculate the concentration of this solution in g dm⁻³.

Give your answer to three significant figures.

concentration =_........ g dm

The ions present in sodium sulfate are

Write the formula of sodium sulfate using this information.

In Figure 11, the gases given off at the electrodes are collected in test-tubes. However, the actual volume of gases cannot be measured using these test-tubes.

Suggest what apparatus could be used in place of the test-tubes in Figure 11 to measure the volume of gases given off.

State what could be added into the circuit to show a current is flowing during electrolysis.

(a) Water, acidified with sulfuric acid, is decomposed by electrolysis. The water is decomposed to produce hydrogen and oxygen.

A sample of hydrogen is mixed with air and ignited.

State what would happen.

Throughout the experiment the volume of hydrogen and the volume of oxygen are measured at two-minute intervals.

The results are shown in Figure 2.

Describe, using the data in Figure 2, what the results show about the volumes of hydrogen and of oxygen produced in this experiment.

Molten lead bromide is electrolysed.

The products of this electrolysis are

Calcium nitrate and calcium carbonate are both ionic compounds.

Calcium nitrate mixed with water behaves as an electrolyte. Calcium carbonate mixed with water does not behave as an electrolyte.

Explain, in terms of solubility and movement of ions, this difference in behaviour.

When molten zinc chloride is electrolysed, zinc ions, Zn²⁺, form zinc atoms.

Write the half equation for this reaction.

Two compounds of barium are barium sulfide and barium chloride.

The hazard symbol shown in Figure 5 is on bottles containing barium metal.

State the meaning of this hazard symbol.

Give the names of the elements combined in barium sulfide.

Barium chloride is toxic.

Explain one safety precaution that should be taken when using barium chloride.

A beaker of barium chloride solution and a beaker of dilute sulfuric acid were placed on a balance, as shown in Figure 6.

The total mass reading on the balance was 25.7 g.

The dilute sulfuric acid was poured into the barium chloride solution and the beaker replaced on the balance, as shown in Figure 7.

Figure 7

The mixture formed contained a white precipitate.

State the total mass reading on the balance after the reaction.

Give the name of the white precipitate formed by the reaction of barium chloride solution with dilute sulfuric acid.

Solid sodium chloride is dissolved in water.

The sodium chloride solution is electrolysed in the apparatus shown in Figure 8.

State why sodium chloride solution, rather than solid sodium chloride, must be used in this experiment.

The formulae of the ions present in the sodium chloride solution are

Circle the ions that would be attracted to the anode.

Molten lead bromide can be electrolysed to form molten lead and bromine gas.

Explain how a student could modify the apparatus shown in Figure 8 to carry out this electrolysis.

(a) Molten zinc chloride is an electrolyte.

Which row shows the products formed at the anode and at the cathode when molten zinc chloride is electrolysed?

Which of the following is the reason why molten zinc chloride is an electrolyte?

Copper sulfate solution was electrolysed using copper electrodes.

Draw a labelled diagram to show the apparatus that is used to carry out this electrolysis in the laboratory.

Before the electrolysis, the masses of the electrodes were determined. After the electrolysis, the electrodes were washed and dried and their masses re-determined.

Figure 6 shows these masses and the resulting changes in masses of the electrodes.

Explain these results.

When sodium sulfate solution is electrolysed, using inert electrodes, hydrogen is formed at the cathode.

Write the half equation for the formation of hydrogen gas, H₂, from hydrogen ions, H⁺.