Electrolysis
6 marks · NormalDuring the electrolysis of molten calcium chloride (CaCl2), calcium forms at one electrode and chlorine gas is released at the other.
Write the balanced ionic half-equation for the reaction at the electrode where chlorine gas is produced.
Explain what is meant by the term 'discharging an ion' using Ca^2+ or Cl^- as an example from this electrolysis process.
Recall the meaning of the mnemonic OILRIG and use it to decide whether the reaction at the chlorine-producing electrode is an oxidation or a reduction.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 2Cl^- → Cl2 + 2e^-
(b) Discharging an ion means it loses its charge to become a neutral atom. For example, Cl^- ions lose electrons at the anode to become Cl atoms, which then form Cl2 gas.
(c) OILRIG stands for 'Oxidation Is Loss, Reduction Is Gain (of electrons)'. At the anode, Cl^- ions lose electrons, so this is oxidation.
Common mistakes: Failing to balance the half-equation for chlorine evolution; Confusing which process is oxidation or reduction
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Marking points
B1 (a) · 2 marks · Correct balanced ionic half-equation for chlorine at the anode (2Cl^- → Cl2 + 2e^-)
B1 (b) · 2 marks · Explains 'discharging an ion' clearly using an example: ion loses or gains electrons to form a neutral atom, with reference to Cl^- or Ca^2+
B1 (c) · 1 mark · Correctly recalls what OILRIG stands for
B1 (c) · 1 mark · Applies OILRIG to correctly classify the electrode reaction as oxidation and gives reasoning
Worked solution
a) The half-equation for chlorine at the anode is:
b) Discharging an ion means converting it to a neutral atom or molecule by the loss or gain of electrons. In this case, Cl^- ions lose electrons (are oxidised) at the anode to become chlorine atoms, which then form Cl2 gas.
c) OILRIG stands for Oxidation Is Loss, Reduction Is Gain (of electrons). At the anode, chloride ions lose electrons, so it is an oxidation reaction.