Electrolysis Exam Questions

(a) The decomposition of an ionic compound / electrolyte when an electric current is passed through it while molten or in aqueous solution [1]

(b) An ionic compound that conducts electricity when molten or dissolved in water / a substance that is decomposed during electrolysis [1]

(c) A rod or plate (usually metal or carbon/graphite) that carries current into and out of the electrolyte [1]

Examiner note: For electrolysis, you need both “decomposition” and “electric current.” For electrolyte, stating “a liquid that conducts” is not enough — you must specify it is ionic. Pure water is not an electrolyte.

• In solid NaCl, the ions are held in fixed positions and cannot move [1]
• In molten NaCl, the ions are free to move and carry charge [1]
• At the cathode (negative electrode): sodium ions gain electrons and form sodium metal. At the anode (positive electrode): chloride ions lose electrons and form chlorine gas [1]

Examiner note: Cathode = negative = attracts cations (positive ions). Anode = positive = attracts anions (negative ions). Remember: CAtions go to the CAthode.

(a) Pb2+ + 2e- → Pb [1 for equation, 1 for balanced]

(b) 2Br- → Br2 + 2e- [1 for equation, 1 for balanced]

Examiner note: At the cathode, positive ions gain electrons (reduction). At the anode, negative ions lose electrons (oxidation). Electrons must balance — two electrons are transferred in both half equations here.

(a) At the cathode: lead (metal) [1]. At the anode: bromine (gas/liquid) [1]

(b) A silvery-grey bead of metal forms at the cathode, and brown/orange fumes of bromine are produced at the anode [1]

Examiner note: Lead is a dense silvery metal that forms as a bead at the bottom. Bromine at this temperature appears as brown/red-brown fumes. Observations must describe what you see, not what happens chemically.

(a) The ions must be free to move to carry the current / to migrate to the electrodes [1]

(b) Cryolite lowers the melting point of aluminium oxide, reducing the energy cost / allowing electrolysis at a lower temperature [1]

(c) Al3+ + 3e- → Al [1]

(d) 2O2- → O2 + 4e- [1]

Examiner note: The anode equation is the most commonly written incorrectly. Each O2- loses 2 electrons, and you need 2 oxide ions to make one O2 molecule. The carbon anodes burn away because O2 reacts with them at the high temperature — this is why they need regular replacement.

(a) Cations are positive ions and are attracted to the cathode, which is the negative electrode [1]

(b) Anions are negative ions and are attracted to the anode, which is the positive electrode [1]

Examiner note: Opposite charges attract. The cathode is connected to the negative terminal of the power supply, so it attracts positive ions. The anode is connected to the positive terminal, attracting negative ions.

• At the cathode, cations gain electrons — this is reduction (gain of electrons) [1]
• At the anode, anions lose electrons — this is oxidation (loss of electrons) [1]
• Remember: OILRIG — Oxidation Is Loss, Reduction Is Gain (of electrons) [1]

Examiner note: Reduction always occurs at the cathode and oxidation always occurs at the anode. This applies to all electrolysis reactions. The mnemonic OILRIG is useful, but you must be able to explain it in terms of electron transfer.

• An electrolyte (ionic compound) must be present [1]
• The electrolyte must be molten or dissolved in water so that the ions are free to move [1]
• A direct current (d.c.) power supply must be connected to the electrodes [1]

Examiner note: Alternating current (a.c.) would not work because the polarity of the electrodes would keep switching, preventing consistent migration of ions. Direct current is essential.