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Chemistry · Practice book · Senior 3 · Unit 7 of 10

Electrolysis and its applications

Senior 3 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 7 Electrolysis and its applications

Key unit competence: Analyse the process of electrolysis and its applications in industry and daily life.

Key ideas

  • Electrolysis is the decomposition of an electrolyte (molten or in solution) by an electric current. The positive electrode is the anode, the negative electrode is the cathode. Cations move to the cathode, anions to the anode.
  • At the cathode, cations gain electrons (reduction): Cu2+ + 2e− → Cu, 2H+ + 2e− → H2. At the anode, anions lose electrons (oxidation): 2Cl− → Cl2 + 2e−, 4OH− → 2H2O + O2 + 4e−.
  • Molten binary compounds give the metal at the cathode and the non-metal at the anode, e.g. molten PbBr2 gives lead and bromine.
  • In aqueous solutions, water also provides H+ and OH−. At the cathode, hydrogen forms unless the metal is less reactive than hydrogen (e.g. copper, silver). At the anode, oxygen forms from OH− unless the solution has concentrated halide ions (chlorine forms) or the anode is active (e.g. copper anode dissolves).
  • Examples: dilute H2SO4 with inert electrodes gives H2 (cathode) and O2 (anode) in a 2 : 1 volume ratio; concentrated NaCl (brine) gives H2 and Cl2, leaving NaOH; CuSO4 with copper electrodes: copper dissolves at the anode and is deposited at the cathode.
  • Applications: electroplating (object as cathode, plating metal as anode), purification of copper, extraction of reactive metals (Na, Al), manufacture of chlorine and sodium hydroxide, anodising aluminium.

Common misconceptions

  • "Electrons flow through the electrolyte." No: In the electrolyte, current is carried by moving ions; electrons flow only in the external wires.
  • "Sodium is deposited at the cathode in the electrolysis of aqueous NaCl." No: Hydrogen is released because sodium is more reactive than hydrogen.
  • "Inert electrodes take part in the reaction." No: Inert electrodes (carbon, platinum) only conduct; they are not changed.

Worked examples

Example 1. Predict the products at each electrode for the electrolysis of molten sodium chloride and of dilute sulfuric acid (inert electrodes).

Solution

Molten NaCl: cathode Na+ + e− → Na (sodium); anode 2Cl− → Cl2 + 2e− (chlorine).

Dilute H2SO4: cathode 2H+ + 2e− → H2 (hydrogen); anode 4OH− → 2H2O + O2 + 4e− (oxygen).

In the acid, water is decomposed and the acid becomes slightly more concentrated.

Example 2. Describe how an iron key can be plated with copper.

Solution

Clean the key and make it the cathode.

Use a piece of pure copper as the anode and copper(II) sulfate solution as the electrolyte.

Pass a small direct current: at the anode Cu → Cu2+ + 2e−; at the cathode Cu2+ + 2e− → Cu.

A thin layer of copper forms on the key; the solution stays blue.

Practice questions

  1. Define electrolysis.

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    Decomposition of an electrolyte (molten or in solution) by an electric current.

    Common mistake: Saying 'passing electricity through a metal'.

  2. What is the cathode?

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    The negative electrode.

    Common mistake: Saying positive electrode.

  3. Name the product at the anode when molten PbBr2 is electrolysed.

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    Bromine.

    Common mistake: Saying lead.

  4. Write the half-equation for the discharge of hydrogen ions.

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    2H+ + 2e− → H2.

    Common mistake: Writing H+ → H + e−.

  5. Why is copper deposited, not hydrogen, at the cathode in copper(II) sulfate solution?

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    Copper is less reactive than hydrogen, so Cu2+ is discharged more easily.

    Common mistake: Saying copper ions are more numerous only.

  6. Give two uses of electrolysis.

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    Electroplating, purifying copper, extracting aluminium/sodium, making chlorine (any two).

    Common mistake: Saying 'making batteries'.

  7. What are the products of electrolysis of concentrated brine?

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    Hydrogen, chlorine and sodium hydroxide.

    Common mistake: Saying sodium metal.

  8. Why are graphite electrodes called inert?

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    They conduct electricity but do not react.

    Common mistake: Saying they produce gas.

  9. Explain why the copper anode gets smaller in copper purification.

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    Copper atoms lose electrons and go into solution as Cu2+.

    Common mistake: Saying copper evaporates.

  10. Write the half-equation at the anode in the electrolysis of dilute sulfuric acid.

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    4OH− → 2H2O + O2 + 4e−.

    Common mistake: Writing SO42− being discharged.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Reduction at the cathode means:a. gain of electrons
    2. The volume ratio of hydrogen to oxygen in the electrolysis of water is:b. cathode in a solution of a gold salt
    3. The half-equation at the cathode when copper is deposited is:c. Cu2+ + 2e− → Cu
    4. Gold-plated jewellery is made by making the jewellery the:d. 2 : 1
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    1 → a (gain of electrons) · 2 → d (2 : 1) · 3 → c (Cu2+ + 2e− → Cu) · 4 → b (cathode in a solution of a gold salt)

    • Cations gain electrons.
    • Water is H2O: twice as much hydrogen.
    • Copper ions gain two electrons.
    • Gold ions deposit on the cathode.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    lose electrons to form chlorine  ·  gain electrons  ·  lead  ·  stays the same
    1. Electrolysis of molten lead(II) bromide gives at the cathode .
    2. In the electrolysis of copper(II) sulfate with copper electrodes, the blue colour .
    3. At the anode, chloride ions .
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    1: lead · 2: stays the same · 3: lose electrons to form chlorine

    • Pb2+ + 2e− → Pb.
    • Copper dissolves at the anode as fast as it deposits.
    • 2Cl− → Cl2 + 2e−.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which gas is produced at the anode in the electrolysis of concentrated brine?a. Chlorine
    2. An inert electrode is made of:b. Aluminium
    3. The positive electrode in electrolysis is the:c. graphite or platinum
    4. Which metal is extracted by electrolysis?d. anode
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    1 → a (Chlorine) · 2 → c (graphite or platinum) · 3 → d (anode) · 4 → b (Aluminium)

    • Concentrated chloride ions are discharged.
    • They do not react during electrolysis.
    • Anions are attracted to the positive anode.
    • Aluminium is too reactive to be reduced by carbon.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    cathode  ·  anode  ·  wire  ·  fades as copper is deposited
    1. Cations move towards the .
    2. During purification of copper, the impure copper is the .
    3. In the electrolysis of copper(II) sulfate with carbon electrodes, the blue colour .
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    1: cathode · 2: anode · 3: fades as copper is deposited

    • Positive ions go to the negative electrode.
    • It dissolves and pure copper deposits on the cathode.
    • Cu2+ ions are removed from solution.

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