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(i) Electrolysis

Syllabus
2024
Topic
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Level
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Learning objectives

155C Covalent compounds and conductivityUnderstand why covalent compounds do not conduct electricity156C Ionic compounds and conductivityUnderstand why ionic compounds conduct electricity only when molten or in aqueous solution157C Anions and cationsKnow that anion and cation are terms used to refer to negative and positive ions respectively158C Electrolysis experimentsDescribe experiments to investigate electrolysis, using inert electrodes, of molten compounds (including lead(II) bromide) and aqueous solutions (including sodium chloride, dilute sulfuric acid and copper(II) sulfate) and to predict the products159C Electrolysis half-equationsWrite ionic half-equations representing the reactions at the electrodes during electrolysis and understand why these reactions are classified as oxidation or reduction160C Electrolysis practicalPractical: investigate the electrolysis of aqueous solutions

Explain why covalent compounds do not conduct

Covalent compounds do not conduct electricity because they do not contain charged particles that are free to move through the substance.

Their molecules are neutral, and their electrons are held in covalent bonds or localized around atoms. Applying a potential difference therefore provides no mobile ions or delocalised electrons to carry charge between electrodes.

Possible carrier Covalent compound
mobile ions absent in the pure compound
mobile delocalised electrons absent in a typical covalent compound

Do not say that covalent compounds contain no electrons: they contain electrons, but those electrons are not free to move through the whole substance. A covalent substance that reacts with water to form ions is a separate aqueous case.

Explain when ionic compounds conduct

Ionic compounds conduct electricity only when their charged ions are free to move.

State Ion mobility Conducts?
solid ions fixed in a giant lattice no
molten ions free to move after the lattice breaks down yes
aqueous separated ions free to move through water yes

Electrolysis therefore requires an ionic substance to be molten or dissolved. Cations move toward the negative electrode and anions toward the positive electrode, carrying charge through the electrolyte.

The moving charge carriers are ions, not electrons. Heating a solid until warm is not enough: it must melt before its ions become mobile.

Distinguish cations from anions

A cation is a positive ion; an anion is a negative ion.

Ion Charge Electrode attracted to during electrolysis
cation, e.g. NaX+\ce{Na+} or CuX2+\ce{Cu^{2+}} positive cathode, the negative electrode
anion, e.g. ClX−\ce{Cl^-} or SOX4X2−\ce{SO4^{2-}} negative anode, the positive electrode

Opposite charges attract: cations move toward the negative cathode and anions move toward the positive anode. The electrode name stays the same throughout this electrolytic-cell context.

Do not infer charge from the first letter of anode or cathode. 'Anion' means negative ion, even though it moves to the positive anode.

Predict products of electrolysis with inert electrodes

Connect two inert conducting electrodes, such as graphite or platinum, to a d.c. supply and place them in a molten ionic compound or aqueous ionic solution. Inert electrodes conduct but do not supply the products.

Electrolyte Negative electrode (cathode) Positive electrode (anode)
molten PbBrX2\ce{PbBr2} lead, Pb\ce{Pb} bromine, BrX2\ce{Br2}
aqueous NaCl\ce{NaCl} hydrogen, HX2\ce{H2} chlorine, ClX2\ce{Cl2}
dilute HX2SOX4\ce{H2SO4} hydrogen, HX2\ce{H2} oxygen, OX2\ce{O2}
aqueous CuSOX4\ce{CuSO4} copper, Cu\ce{Cu} oxygen, OX2\ce{O2}

For a molten compound, only its ions compete: the cation forms the element at the cathode and the anion forms the element at the anode. In an aqueous solution, water also supplies hydrogen and hydroxide ions; sodium is not deposited because hydrogen is reduced more readily, while copper is deposited from CuX2+\ce{Cu^{2+}}.

Record bubbles, colour, deposits and solution changes. Copper forms a pink-brown cathode coating and the blue CuX2+\ce{Cu^{2+}} solution becomes paler; bromine gives brown fumes. Chlorine is toxic, bromine is harmful and molten lead(II) bromide is hot and hazardous, so these demonstrations require appropriate school controls and ventilation.

Write electrode half-equations and classify redox

A half-equation shows the species discharged at one electrode and includes electrons so that both atoms and total charge balance.

Electrode process Half-equation Classification
copper ions form copper CuX2++2 eX−→Cu\ce{Cu^{2+} + 2e^- -> Cu} reduction: electron gain
hydrogen ions form hydrogen 2 HX++2 eX−→HX2\ce{2H+ + 2e^- -> H2} reduction: electron gain
bromide ions form bromine 2 BrX−→BrX2+2 eX−\ce{2Br^- -> Br2 + 2e^-} oxidation: electron loss
hydroxide ions form oxygen 4 OHX−→OX2+2 HX2O+4 eX−\ce{4OH^- -> O2 + 2H2O + 4e^-} oxidation: electron loss

At the cathode, put electrons on the left because cations gain them. At the anode, put electrons on the right because anions or hydroxide ions lose them. Recount atoms, then confirm that total charge is equal on both sides.

Oxidation is loss of electrons and reduction is gain of electrons. Classify the ion or species undergoing the electron change—not the electrode itself—and never balance charge by changing a chemical formula.

Investigate electrolysis of aqueous solutions

Investigate an aqueous electrolyte using two inert electrodes connected to a low-voltage d.c. supply, keeping the electrodes separated and immersed to a consistent depth.

Stage Action and evidence
1 Add a known aqueous electrolyte to the cell and identify its ions.
2 Insert inert electrodes and connect their positive and negative terminals correctly.
3 Switch on for a controlled time; record current and observations at each electrode.
4 Collect gases separately when required and use appropriate gas tests; record any metal deposit or solution-colour change.
5 Repeat under the same conditions if comparing current, time or gas volume.

When investigating current against gas volume, keep electrolyte, concentration, electrode area, separation and time constant. Plot volume against current, identify anomalies from the pattern and use a best-fit line; at fixed time, greater current produces more gas.

Use small quantities, eye protection and the specified school risk controls. Keep collected gases separate, avoid ignition sources unless performing a controlled hydrogen test, and use a fume cupboard for chlorine or bromine. A lower-than-expected gas volume may result from leakage, gas dissolving or an early reading.