Electrolysis (OCR GCSE Combined Science A (Gateway): Chemistry): Exam Questions

Electrolysis can be used to separate the elements in some compounds using electricity.

i) Look at the diagram of an electrolysis experiment.

Complete the diagram using the words in the list.

You may use each word once, more than once or not at all.

[2]

ii) A teacher demonstrates the electrolysis of molten lead bromide.

Predict the products made at each electrode.

Positive electrode ......................................................

Negative electrode ....................................................

[2]

iii) Molten lead bromide contains lead ions, Pb²⁺, and bromide ions, Br⁻.

What is the formula for lead bromide? Tick () one box.

[1]

The student investigates the mass of copper made during the electrolysis of aqueous copper chloride.

The student varies the electric current and passes the current for the same time in each experiment. Here is a table of their results.

i) Plot a graph of the student’s results and draw a line of best fit.

[3]

ii) Use your graph to estimate the current needed to make 2.25 g of copper.

Current = ....................................................... A

[1]

iii) Use your graph, and a calculation, to find the mass of copper that would be produced using 15 A.

Give your answer to 2 significant figures.

Mass of copper produced = ....................................................... g

[2]

A student electrolyses dilute sulfuric acid.

Hydrogen gas is made at the cathode.

The student measures the volume of hydrogen made at the cathode every 2 minutes for 10 minutes.

Look at his results.

Plot the results on the grid. The first 3 points have been done for you.

Draw a line of best fit.

One of the results is anomalous.

Circle the anomalous result on the graph.

Sulfuric acid contains these particles.

H⁺                   OH^-                   H₂O               SO₄^2-

Which particles are attracted to the anode?

The student also investigates the electrolysis of some molten (liquid) salts.

Complete the table.

A student electrolyses copper sulfate using copper electrodes.

Look at the diagram. It shows the apparatus she uses.

She investigates the change in mass at each electrode before and after the electrolysis.

Look at her method.

1. Using a balance, measure the mass of the copper cathode and copper anode.

2. Set up the apparatus and run the electrolysis for 30 seconds.

3. Remove the copper cathode and the copper anode and immediately place them on the balance and measure their masses again.

What improvements could you make to the student’s experiment?

Explain your answers.

The student finds that:

• the cathode gains mass

• the anode loses mass.

Explain these observations in terms of the reactions at each electrode.

A student investigates the electrolysis of aqueous solutions of ionic compounds

Write the formulae of the ions that are present in aqueous copper sulfate solution.

Why is it important that the investigation is done with inert electrodes?

Electroplating is used to cover a metal with another metal.

i) Which aqueous solution would you use to electroplate a metal spoon with copper using a safe method?

Tick () one box.

[1]

ii) Give two reasons for your answer to (c)(i).

[2]

i) Predict the product made at the anode when sodium sulfate solution is electrolysed.  

[1]

ii) Hydrogen gas is made at the cathode instead of sodium metal.

Explain why.

[1]

iii) Write the balanced half equation for the formation of hydrogen gas.

Use e⁻ to represent an electron.

[2]

The electrolysis products of ionic compounds can be different in the molten or aqueous states.

Suggest why.

A student is investigating the electrolysis of copper sulfate solution.

He does two experiments.

Experiment 1 uses platinum electrodes. Experiment 2 uses copper electrodes.

Complete the table to show the products at each electrode.

Copper electrodes are non-inert electrodes.

What is meant by non-inert electrodes?

Higher Only

Look at the results for experiment 2 using copper electrodes.

At the cathode copper ions, Cu²⁺ gain electrons to make copper atoms.

Write the half equation for this reaction.

Use e^–  to represent an electron.

The student also electrolyses sodium chloride solution using platinum (inert) electrodes.

At the cathode, hydrogen gas is made rather than sodium metal.

Explain why.