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Cambridge IGCSE0620

Electrolysis

Chemistry 0620 Chapter Notes

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ElectrolysisHydrogen–oxygen fuel cells
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Electrolysis notes

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In this chapter

  1. 1Electrolysis and its terms
  2. 2Molten compounds
  3. 3Concentrated sodium chloride and dilute sulfuric acid
  4. 4Electroplating
  5. 5Transfer of charge
  6. 6Aqueous copper(II) sulfate
  7. 7Halide solutions: dilute or concentrated
  8. 8Ionic half-equations
  9. 9Hydrogen and oxygen in, water out
  10. 10Chemical energy to electrical energy
  11. 11A fuel cell in a car
  12. 12Fuel cell or petrol engine?

1. Electrolysis and its terms

Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

In a simple electrolytic cell the anode is the positive electrode and the cathode is the negative electrode. The electrolyte is the molten or aqueous substance that undergoes electrolysis.

An electrode is not the electrolyte. Graphite and platinum are used as inert electrodes.

Examiner insight

“Many of the candidates did not use the terms ‘ionic’ and either ‘electricity’ or ‘electric current’ in their definition.” “Option C gives a substance used an as electrode, not an electrolyte.”

Worked example 12 marks

Define the term electrolysis.

  1. 1

    decomposition of an ionic compound (when molten or in aqueous solution) [1]

  2. 2

    by the passage of an electric current [1]

2. Molten compounds

Metals or hydrogen are formed at the cathode. Non-metals (other than hydrogen) are formed at the anode.

For a molten binary compound, the metal forms at the negative electrode and the non-metal at the positive electrode. Molten lead(II) bromide gives lead and bromine. Bromine is seen as a brown vapour.

Molten cobalt(II) chloride gives cobalt and chlorine. Chlorine is seen as a (yellow-)green vapour. Name the element, not the ion: chlorine, not chloride.

PbBr₂(l) → Pb(l) + Br₂(g)

Examiner insight

“The commonest errors were to suggest ‘chloride’ or ‘chloride ions’ instead of ‘chlorine’ or ‘lithium ions’ instead of lithium.” “A considerable number of candidates did not read the word molten in the stem of the question and suggested that hydrogen is formed at one of the electrodes.”

Worked example 13 marks

Molten cobalt(II) chloride is electrolysed using inert graphite electrodes.

Name the products formed at each electrode during the electrolysis of molten cobalt(II) chloride, and give the observations at the positive electrode.

  1. 1

    negative electrode: cobalt [1]

  2. 2

    positive electrode: chlorine [1]

  3. 3

    observations: (yellow-)green vapour [1]

3. Concentrated sodium chloride and dilute sulfuric acid

In both of these aqueous solutions the cathode product is hydrogen. Sodium is not formed.

Concentrated aqueous sodium chloride gives hydrogen at the cathode and chlorine at the anode. Dilute sulfuric acid gives hydrogen at the cathode and oxygen at the anode.

Bubbles are seen at both electrodes. Hydrogen gives a squeaky pop with a lighted splint. Chlorine is a yellow-green gas that bleaches damp litmus paper.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g)

2H₂O(l) → 2H₂(g) + O₂(g)

Examiner insight

“The commonest error was to ignore the word aqueous in the stem of the question and therefore suggest sodium instead of hydrogen.” “‘A gas is formed’ was not credited because this is not considered an observation.”

Worked example 11 mark

Substance X is electrolysed.

A different gas is produced at each electrode. The gases are collected and tested at room temperature.

One of the gases produces a squeaky pop when tested with a lighted splint. The other gas bleaches damp litmus paper.

What is substance X?

A concentrated aqueous sodium chloride · B dilute sulfuric acid · C molten lead(II) bromide · D molten potassium chloride

  1. 1

    squeaky pop: hydrogen. Bleaches damp litmus: chlorine

  2. 2

    hydrogen and chlorine: concentrated aqueous sodium chloride. A [1]

4. Electroplating

Metal objects are electroplated to improve their appearance and resistance to corrosion.

The object to be plated is the cathode. The anode is the plating metal. The electrolyte is an aqueous solution of a salt of the plating metal.

To plate nickel with silver: cathode nickel, anode silver, electrolyte aqueous silver nitrate. A power supply completes the circuit.

Examiner insight

“For example, using the spoon as the anode rather than the cathode, using an inert electrode rather than silver as one of the electrodes, and using molten silver nitrate rather than a solution of aqueous silver nitrate as the electrolyte.” “Few candidates realised that the electrolyte needed have mobile silver ions and so be able to conduct electricity. These candidates omitted to state that the electrolyte should be aqueous or molten.”

Worked example 12 marks

A student uses electrolysis to electroplate a zinc rod with silver.

Suggest a suitable electrolyte.

  1. 1

    silver nitrate [1]

  2. 2

    aqueous / (aq) [1]

5. Transfer of charge

In the external circuit, electrons carry the charge. They leave the anode and travel to the cathode.

At the anode, ions lose electrons: oxidation. At the cathode, ions gain electrons: reduction.

In the electrolyte, ions carry the charge. Positive ions move to the cathode. Negative ions move to the anode.

Examiner insight

“For both of these options, candidates identified the correct movement of either the electrons or the positive ions but not both.” “Weaker candidates assumed mobile or delocalised electrons were responsible for the passage of current through an electrolyte.”

Worked example 11 mark

Oxidation is occurring at electrode 1 and reduction at electrode 2.

Which row shows the directions of movement of the electrons in the external circuit and of the positive ions in the electrolyte?

direction of movement of electrons in external circuitdirection of movement of positive ions in electrolyte
A1 to 21 to 2
B1 to 22 to 1
C2 to 11 to 2
D2 to 12 to 1
  1. 1

    oxidation at 1: electrode 1 is the anode, electrode 2 the cathode

  2. 2

    electrons leave the anode and travel round the circuit to the cathode: 1 to 2

  3. 3

    positive ions move to the cathode: 1 to 2. A [1]

6. Aqueous copper(II) sulfate

With copper electrodes, copper forms on the cathode, so its mass increases. The anode dissolves, so its mass decreases. No gas is released.

Copper ions removed at the cathode are replaced from the anode. The blue colour of the solution stays the same.

With inert carbon / graphite electrodes, copper still forms on the cathode. Oxygen forms at the anode, seen as bubbles. The copper ions are not replaced, so the blue colour fades.

cathode: Cu²⁺ + 2e⁻ → Cu

anode (copper): Cu → Cu²⁺ + 2e⁻

Examiner insight

“Candidates did not realise that when copper electrodes are used the anode dissolves away and no gas is released.” “A common error was to write an unbalanced equation, the most common of which had one copper ion on the left but two atoms of copper on the right; presumably, candidates confused the magnitude of the charge with a stoichiometric coefficient.”

Worked example 11 mark

Aqueous copper(II) sulfate is a blue solution. The solution is electrolysed using copper electrodes.

Which row describes the changes that take place during the electrolysis?

mass of cathodemass of anodecolour of solution
Adecreasesincreasesbecomes paler
Bdecreasesincreasesstays the same
Cincreasesdecreasesbecomes paler
Dincreasesdecreasesstays the same
  1. 1

    cathode: Cu²⁺ + 2e⁻ → Cu, so its mass increases

  2. 2

    anode: Cu → Cu²⁺ + 2e⁻, so its mass decreases and the copper ions are replaced

  3. 3

    the blue colour stays the same. D [1]

7. Halide solutions: dilute or concentrated

In aqueous sodium chloride, the cathode product is hydrogen. Hydrogen ions are positive and are attracted to the negative cathode. Hydrogen is less reactive than sodium.

At the anode the concentration matters. A concentrated solution of a chloride or bromide gives the halogen: chlorine or bromine. A dilute solution gives oxygen, and not the halogen.

In the molten state the cathode product is the metal, as in sodium from molten sodium chloride.

Examiner insight

“The effect of halide concentration on the major gas produced at the anode was not well recalled by the weaker candidates.” “Some candidates named the ions that migrated to the cathode or named products formed at the anode.”

Worked example 13 marks

Molten and aqueous sodium chloride are separately electrolysed using inert electrodes.

Complete the table to show the name of the product formed at the cathode during the electrolysis of different forms of sodium chloride.

form of sodium chloridename of product formed at the cathode
molten……
dilute aqueous solution……
concentrated aqueous solution……
  1. 1

    molten: sodium [1]

  2. 2

    dilute aqueous solution: hydrogen [1]

  3. 3

    concentrated aqueous solution: hydrogen [1]

8. Ionic half-equations

A half-equation shows one electrode reaction. Electrons are written on the left at the cathode, where ions gain them: reduction. They are written on the right at the anode, where ions lose them: oxidation.

The equation must balance in atoms and in charge. Write the product as the molecule: Br₂, not Br.

cathode: 2H⁺(aq) + 2e⁻ → H₂(g)

anode: 2Br⁻ → Br₂ + 2e⁻

cathode: Al³⁺ + 3e⁻ → Al

anode: 2O²⁻ → O₂ + 4e⁻

Examiner insight

“Many candidates were unable to give the correct balanced ionic half-equation for the reaction taking place at the anode.” “The ionic half equation was known by many candidates; state symbols were frequently omitted or were incorrect due to H+ ions being given as gaseous.”

Worked example 12 marks

Concentrated aqueous lithium bromide undergoes electrolysis. The products are bromine gas at the anode and hydrogen gas at the cathode.

Write an ionic half-equation for the formation of bromine at the anode.

  1. 1

    bromide ions are negative and lose electrons [1]

  2. 2

    2Br⁻ → Br₂ + 2e⁻ [1]

Recap

  • Electrolysis = decomposition of an ionic compound, molten or aqueous, by an electric current.
  • Anode = positive. Cathode = negative. Electrolyte = molten or aqueous substance electrolysed.
  • Metals or hydrogen at the cathode. Non-metals (other than hydrogen) at the anode.
  • Concentrated aqueous sodium chloride: hydrogen and chlorine. Dilute sulfuric acid: hydrogen and oxygen.
  • Electroplating: object is the cathode, anode is the plating metal, aqueous salt of that metal.
  • Extended: electrons travel in the wires, ions travel in the electrolyte. Anode = oxidation, cathode = reduction.
  • Extended: copper electrodes in aqueous copper(II) sulfate: cathode gains mass, anode loses mass, blue colour stays.
  • Extended: dilute halide gives oxygen at the anode. Concentrated gives the halogen.

9. Hydrogen and oxygen in, water out

A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity.

The only chemical product is water. Hydrogen and oxygen are the inputs. Water is the output.

2H₂ + O₂ → 2H₂O

Examiner insight

“Many candidates could not recall an element used as a reactant in fuel cells. The most common incorrect answer was H2O.” “Very few candidates were aware that the behaviour of hydrogen within a fuel cell was equivalent to the combustion of hydrogen.”

Worked example 11 mark

Fuel cells are used as energy sources in cars. Which row gives a fuel used in a fuel cell and the products formed?

fuel in a fuel cellproducts formed
Ahydrogencarbon dioxide and water
Bhydrogenwater only
Cpetrolcarbon dioxide and water
Dpetrolwater only
  1. 1

    B: the fuel is hydrogen and the only product is water. [1]

10. Chemical energy to electrical energy

In the cell, hydrogen combines with oxygen. This reaction converts chemical energy into electrical energy.

Hydrogen is the fuel. It is not extracted from the air.

Examiner insight

“Candidates did not know about fuel cells and answered about a combustion engine.”

Worked example 11 mark

Four statements about hydrogen fuel cells are listed.

1 Carbon dioxide and water are produced in the fuel cell.

2 In the fuel cell, hydrogen combines with oxygen.

3 The fuel cell converts chemical energy into electrical energy.

4 The hydrogen fuel is extracted from the air.

Which statements are correct? A 1 and 3 · B 1 and 4 · C 2 and 3 · D 2 and 4

  1. 1

    C: statements 2 and 3 are correct. [1]

11. A fuel cell in a car

In a car, the fuel cell produces electricity. The electricity powers an electric motor.

The fuel cell does not make heat to power the car. Its only emission is water, not nitrogen.

Examiner insight

“Candidates knew that hydrogen and methane both produce energy but did not appreciate the question related to a fuel cell.”

Worked example 11 mark

Which statement about a fuel cell in a car is correct?

A The fuel cell produces heat, which powers the car. B The fuel cell is supplied with hydrogen directly from the air. C The only emission from a fuel cell is nitrogen gas, which is non-polluting. D The fuel cell produces electricity, which powers an electric motor.

  1. 1

    D: the fuel cell produces electricity, which powers an electric motor. [1]

12. Fuel cell or petrol engine?

A fuel cell car does not produce carbon dioxide, the gas which causes global warming. A petrol engine does. Fuel cells are also more efficient than using petrol.

A petrol car can travel a greater distance after the fuel tank has been filled. Hydrogen is a gas, so it is difficult to store and transport: it must be stored under high pressure. There are also more petrol filling stations than hydrogen filling stations.

Fire risk is not a difference. Petrol is also highly flammable.

Examiner insight

“Many did not read the question properly and gave an advantage instead.” “Answers that described hydrogen as dangerous because it is highly flammable were not accepted as petrol is also highly flammable, neither were answers describing inefficiency as fuel cells are more efficient than using petrol.”

Worked example 11 mark

Which row describes an advantage of using the hydrogen–oxygen fuel cell and an advantage of using the petrol engine?

advantage of using the hydrogen–oxygen fuel celladvantage of using the petrol engine
Acan travel a greater distance after the fuel tank has been filleddoes not produce carbon dioxide gas which causes global warming
Bthere are more petrol filling stations than hydrogen filling stationsno risk of fire or explosion if the fuel tank is damaged
Cdoes not produce carbon dioxide gas which causes global warmingcan travel a greater distance after the fuel tank has been filled
Dno risk of fire or explosion if the fuel tank is damagedthere are more petrol filling stations than hydrogen filling stations
  1. 1

    C: the fuel cell does not produce carbon dioxide and the petrol engine can travel a greater distance on a full tank. [1]

Recap

  • A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity.
  • The only chemical product is water: 2H₂ + O₂ → 2H₂O.
  • Hydrogen and oxygen are inputs. Water is the output.
  • The cell turns chemical energy into electrical energy, which powers an electric motor.
  • Extended: a fuel cell car does not produce carbon dioxide, which causes global warming.
  • Extended: a petrol car can travel a greater distance after the tank has been filled.
  • Extended: hydrogen is a gas and must be stored under high pressure.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the name of the positive electrode and the name of the negative electrode in an electrolytic cell.2 marks
  2. A student tests four substances in an electrolysis cell: graphite, molten lead(II) bromide, aqueous sodium chloride and solid sodium chloride. State which two are electrolytes.2 marks
  3. Molten potassium bromide is electrolysed using inert electrodes. Name the product at each electrode and give the observation at the positive electrode.3 marks
  4. Molten calcium chloride is electrolysed using inert electrodes. Predict the product at the anode and at the cathode.2 marks
  5. Dilute sulfuric acid is electrolysed using inert electrodes. Name the product at each electrode and give the test for the gas formed at the cathode.3 marks
  6. Concentrated aqueous sodium chloride is electrolysed using inert electrodes. A gas forms at the anode. Name the gas, give the test for it and state the result.3 marks
  7. An iron key is electroplated with copper. Name the anode, the cathode and a suitable electrolyte. State one reason for electroplating.4 marks
  8. Molten sodium chloride is electrolysed. Describe the movement of electrons in the external circuit and of the ions in the electrolyte.3 marks
  9. A student electrolyses blue aqueous copper(II) sulfate in two cells. Cell X has graphite electrodes. Cell Y has copper electrodes. After 20 minutes the cathode in each cell has gained 0.32 g of copper. (a) Write the ionic half-equation for the reaction at both cathodes. State whether it is oxidation or reduction. (b) State the change in mass of the anode in cell Y and explain it. (c) Give one observation at the anode in cell X that is not made in cell Y. (d) State which solution becomes paler and explain why.7 marks
  10. Magnesium bromide is electrolysed in three cells using inert electrodes. One cell holds molten magnesium bromide, one a concentrated aqueous solution and one a dilute aqueous solution. The labels are lost. Cell P: bubbles of colourless gas at both electrodes. Cell Q: no bubbles at the cathode, brown vapour at the anode. Cell R: bubbles of colourless gas at the cathode, a brown colour at the anode. (a) Identify the contents of each cell. (b) Name the product at the anode in cell P and explain why it is not bromine. (c) Write the ionic half-equation for the reaction at the cathode in cell R. State why it is reduction. (d) Explain why magnesium is not formed at the cathode in cell R.8 marks
  11. Name the two substances supplied to a hydrogen–oxygen fuel cell.2 marks
  12. State the only chemical product of a hydrogen–oxygen fuel cell.1 mark
  13. State the energy change that happens in a hydrogen–oxygen fuel cell.2 marks
  14. Write the balanced equation for the overall reaction in a hydrogen–oxygen fuel cell.2 marks
  15. A student writes: 'A hydrogen fuel cell does not need oxygen. Its waste is an acidic gas.' Write a correct version of each sentence.2 marks
  16. A bus uses a hydrogen–oxygen fuel cell to power its electric motor. (a) State what is supplied to the fuel cell. (b) State the only substance in the exhaust. (c) State what the fuel cell gives to the motor.3 marks
  17. State one advantage of a hydrogen–oxygen fuel cell car compared with a petrol car.1 mark
  18. State two disadvantages of a hydrogen–oxygen fuel cell car compared with a petrol car.2 marks
  19. A ferry uses a hydrogen–oxygen fuel cell and an electric motor. A leaflet about the ferry says: 'The fuel cell makes hydrogen and oxygen from water. The fuel burns to give heat, which turns the propeller. The only exhaust gas is nitrogen, so the ferry causes no pollution.' Each sentence in the leaflet is wrong. Write a correct version of all three sentences.5 marks
  20. A firm tests a petrol van and a hydrogen–oxygen fuel cell van. It plans a 1400 km journey, and each van starts with a full tank. The table below gives the test results. (a) Calculate how many times each van must stop to refill during the journey. (b) Calculate the mass of carbon dioxide, in kg, that the petrol van produces on the journey. (c) The firm's main aim is to do less to cause global warming. State which van it should choose. Use your answers to (a) and (b) to give one reason for the choice and one cost of it. | | petrol van | fuel cell van | | --- | --- | --- | | distance travelled on one full tank / km | 900 | 500 | | carbon dioxide produced for each km travelled / g | 150 | 0 |6 marks

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