The electrolysis of other molten substances
2 marks · HardWrite the ionic half-equation for the reaction that occurs at the cathode during the electrolysis of molten magnesium oxide (MgO).
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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Mg^{2+} + 2e^{-} \rightarrow Mg
Must show: Mg^{2+} ion on left, 2 electrons on left, Mg atom on right, Correct charge and atom balance
Common mistakes: Placing electrons on the right; Using the wrong number of electrons; Writing an unbalanced equation
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Marking points
M1 · 1 mark · Shows electrons are gained by Mg^{2+} to form Mg
A1 · 1 mark · Equation is fully balanced; two electrons and charges/atoms balanced
Worked solution
At the cathode, Mg^{2+} ions gain electrons (are reduced) to form Mg:
Balance: 1 Mg atom on both sides; left charge (+2) + (–2) = 0 matches right (neutral).