Electrolysis

The One Thing

Electrolysis uses direct electric current to drive the oxidation and reduction of mobile ions in a molten electrolyte or aqueous solution. The products formed depend on the ions present, their concentration, the electrode material, and their relative ease of discharge.

Reactions, Processes and Experiments

Key Terms

• Electrolyte: A compound that conducts electricity in molten form or aqueous solution because it produces mobile ions and undergoes chemical decomposition.
• Strong electrolyte: An electrolyte that ionises almost completely in solution, such as hydrochloric acid, sodium hydroxide, or copper(II) sulfate.
• Weak electrolyte: An electrolyte that ionises only partially in solution, such as ethanoic acid or ammonium hydroxide.
• Non-electrolyte: A substance that does not produce ions in solution and therefore does not conduct electricity, such as sugar solution or alcohol.
• Electrolysis: The chemical decomposition of an electrolyte by the passage of direct electric current.
• Electrolytic cell: An arrangement consisting of an electrolyte, two electrodes, a source of direct current, and connecting wires used for electrolysis.
• Cathode: The electrode connected to the negative terminal of the battery; positive ions, or cations, move towards it and gain electrons.
• Anode: The electrode connected to the positive terminal of the battery; negative ions, or anions, move towards it and lose electrons.
• Cation: A positively charged ion that migrates towards the cathode.
• Anion: A negatively charged ion that migrates towards the anode.
• Electrode: A conducting rod or plate through which electric current enters or leaves the electrolyte.
• Inert electrode: An electrode that does not normally take part in the chemical reaction, for example platinum or graphite.
• Selective discharge: The preferential discharge of one ion at an electrode when several ions are present in the electrolyte.
• Discharge of an ion: The gain or loss of electrons by an ion at an electrode, producing a neutral atom, molecule, or element.
• Oxidation: Loss of electrons, usually occurring at the anode during electrolysis.
• Reduction: Gain of electrons, usually occurring at the cathode during electrolysis.
• Electroplating: The deposition of a thin layer of one metal over another object by electrolysis.
• Electrorefining: The purification of an impure metal by using it as the anode and depositing pure metal at the cathode.

Easily Confused

• Cathode and anode: The cathode is negative and attracts cations; the anode is positive and attracts anions.
• Cation and anion: A cation is positively charged and moves to the cathode; an anion is negatively charged and moves to the anode.
• Reduction and oxidation: Reduction is gain of electrons at the cathode; oxidation is loss of electrons at the anode.
• Molten and solid ionic compounds: Molten ionic compounds conduct because their ions are mobile; solid ionic compounds do not conduct because their ions are fixed in the crystal lattice.
• Copper electrodes and inert electrodes in aqueous copper(II) sulfate: Copper electrodes allow copper to dissolve from the anode, whereas inert electrodes generally produce oxygen at the anode.
• Electroplating and electrorefining: Electroplating coats an object with a metal for protection or appearance; electrorefining purifies an impure metal.
• Electrolytes and non-electrolytes: Electrolytes produce mobile ions and conduct electricity; non-electrolytes, such as sugar solution or alcohol, do not produce ions.
• Ion movement and electron movement: Ions carry current through the electrolyte, whereas electrons carry current through the external metallic wires.

What Gets Asked

• Identify the cathode and anode, their charges, the direction of ion movement, and whether oxidation or reduction occurs at each electrode. Marks are lost by reversing the electrode charges or associating oxidation with the cathode.
• Write electrode and overall equations for molten lead(II) bromide. The specific equations, including Pb2+ + 2e- -> Pb, 2Br- -> Br2 + 2e-, and PbBr2 -> Pb + Br2, must be matched to the correct electrodes.
• Predict products in aqueous electrolysis using selective discharge. Marks are lost by ignoring the electrochemical series, ion concentration, or the nature of the electrode.
• Interpret the electrolysis of acidified water, including the hydrogen-to-oxygen volume ratio of 2:1. The stated ratio must not be reversed.
• Distinguish aqueous copper(II) sulfate with copper electrodes from aqueous copper(II) sulfate with inert electrodes. Copper dissolves at the anode in the first case, whereas oxygen is generally released and the blue colour becomes lighter in the second.
• Apply Q = I x t and Faraday’s first law, stating that deposited mass is directly proportional to the quantity of electricity passed. Marks are lost by omitting the meanings or units of Q, I, and t.
• Describe applications of electrolysis, including electroplating, copper electrorefining, aluminium extraction using molten cryolite, and the manufacture of sodium hydroxide, chlorine, and hydrogen from concentrated aqueous sodium chloride.