The electrolysis of other molten substances

3 marks · Normal

During the electrolysis of molten aluminium oxide, explain how you can check that each ionic half-equation is correctly balanced. Include checks for both the numbers of atoms and the charges.

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Must show: Describes checking atoms, Describes checking charges, Correctly refers to example equations

Also accept: Checks using other valid equations

Common mistakes: Only checks atoms, not charges; Only mentions one equation

Show mark scheme and worked solution

Marking points

E1 · 1 mark · Describes checking the number of each type of atom on both sides of the equation

E2 · 1 mark · Describes checking total charges on both sides

B1 · 1 mark · Correctly illustrates method using cathode and/or anode equations from aluminium oxide, e.g., Al^{3+} + 3e^- \to Al or 2O^{2-} \to O_2 + 4e^-

Worked solution

For Al3+ + 3e- → Al, check atoms: 1 Al on both sides. Check charges: 3+ + 3(-1) = 0 on left; 0 on right.

For 2O2- → O2 + 4e-, atoms: 2 O on both sides. Charges: 2(-2) = -4 on left, 0 + 4(-1) = -4 on right.