Electrolysis: Half Equations at the Electrodes · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Splitting melts with electricity

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. In molten lead bromide, what forms at the cathode?

    • Bromine gas
    • Lead metal
    • Oxygen gas

  2. Why must lead bromide be melted before electrolysis?

    • Melting adds the missing electrons
    • Melting stops the electrodes reacting
    • Melting frees the ions to migrate

  3. Negative ions move to the positive anode.

    Circle one:   True   False

  4. Which half equation shows bromine forming at the anode?

    • Br2 + 2e- -> 2Br-
    • Pb2+ + 2e- -> Pb
    • 2Br- -> Br2 + 2e-

  5. Which half equation shows lead forming at the cathode?

    • 2Br- -> Br2 + 2e-
    • Pb2+ + 2e- -> Pb
    • Pb -> Pb2+ + 2e-

  6. The halves Pb2+ + 2e- -> Pb and 2Br- -> Br2 + 2e- combine. What is the overall reaction?

    • 2Pb + Br2 -> 2PbBr
    • Pb + Br2 -> PbBr2
    • PbBr2 -> Pb + Br2

  7. Molten sodium chloride is electrolysed. What forms at each electrode?

    • Chlorine at the cathode, sodium at the anode
    • Sodium at the cathode, chlorine at the anode
    • Hydrogen at the cathode, oxygen at the anode

  8. A student writes the cathode half as Pb2+ -> Pb + 2e-. What is wrong?

    • Electrons belong on the left, since the ion gains them
    • Lead should form at the anode instead
    • The charge on lead should be 1+
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Splitting melts with electricity W1-mt_6FvxaD8wTY-s1

  1. Lead metal · Lead cations move to the negative cathode and discharge there.
  2. Melting frees the ions to migrate · Only free ions can carry charge to the electrodes.
  3. True · Opposite charges attract, so anions drift to the positive anode.
  4. 2Br- -> Br2 + 2e- · Bromide ions lose electrons at the anode to form bromine gas.
  5. Pb2+ + 2e- -> Pb · Lead ions gain two electrons at the cathode to become lead atoms.
  6. PbBr2 -> Pb + Br2 · The electrons cancel and the ions reunite as the starting compound splitting up.
  7. Sodium at the cathode, chlorine at the anode · Sodium cations discharge at the cathode and chloride anions at the anode.
  8. Electrons belong on the left, since the ion gains them · Gaining electrons means they are reactants, so they sit on the left.
Worksheet · LightMySky