4.1 Electrolysis
- Syllabus
- 0620–2026–2027
- Topic
- 4.1
- Level
- —
Learning objectives
4.1.1Electrolysis as the decomposition of• Define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current4.1.2In simple electrolytic cells: (a) the• Identify in simple electrolytic cells: (a) the anode as the positive electrode (b) the cathode as the negative electrode (c) the electrolyte as the molten or aqueous substance that undergoes electrolysis4.1.8Transfer of charge during electrolysis• Describe transfer of charge during electrolysis to include: (a) the movement of electrons in the external circuit (b) the loss or gain of electrons at the electrodes (c) the movement of ions in the electrolyte4.1.3Products and observations during• Identify products and observations during electrolysis of: (a) molten lead(II) bromide (b) concentrated aqueous sodium chloride (c) dilute sulfuric acid with inert platinum/carbon electrodes4.1.4Metals or hydrogen are formed at the• State: metals or hydrogen are formed at the cathode and that non-metals (other than hydrogen) are formed at the anode4.1.5Predict the identity of the products• Predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state4.1.6Metal objects are electroplated to• State: metal objects are electroplated to improve their appearance and resistance to corrosion4.1.7Metals are electroplated• Describe how metals are electroplated4.1.9Products formed at the electrodes and• Identify the products formed at the electrodes and describe the observations made during the electrolysis of aqueous copper(II) sulfate using inert carbon/graphite electrodes and when using copper electrodes4.1.10Predict the identity of the products• Predict the identity of the products at each electrode for the electrolysis of a halide compound in dilute or concentrated aqueous solution4.1.11Construct ionic half-equations for• Construct ionic half-equations for reactions at the anode (to show oxidation) and at the cathode (to show reduction)
Define electrolysis
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
A solid ionic compound cannot be electrolysed because its ions cannot move.
Identify anode, cathode and electrolyte
In an electrolytic cell the anode is positive, the cathode is negative, and the electrolyte is the molten or aqueous ionic substance that is decomposed.
These electrode signs are for electrolysis; do not infer them only from the names anode and cathode.
Trace charge through an electrolytic cell
Electrons carry charge in the external wires; mobile ions carry charge through the electrolyte. Cations move to the cathode and gain electrons, while anions move to the anode and lose electrons.
Electrons do not travel through the electrolyte and ions do not travel through the metal wire.
Recall products and observations in three electrolyses
Molten PbBr2 gives grey lead at the cathode and orange-brown bromine at the anode. Concentrated NaCl(aq) gives hydrogen, chlorine and remaining NaOH. Dilute H2SO4 gives hydrogen and oxygen with inert electrodes.
Aqueous sodium chloride does not produce sodium metal at the cathode.
Classify products by electrode
Reduction at the cathode forms a metal or hydrogen. Oxidation at the anode forms a non-metal other than hydrogen.
Positive ions go to the negative cathode; electrode sign and ion charge are opposite.
Predict products from a molten binary compound
Only the compound's ions are present: the metal cation gains electrons at the cathode and the non-metal anion loses electrons at the anode.
Do not introduce hydrogen or oxygen from water when the compound is molten.
State why objects are electroplated
Electroplating coats an object with a thin metal layer to improve appearance and resistance to corrosion.
A decorative coating can also protect the underlying metal; the two purposes are distinct.
Configure an electroplating cell
Make the object the cathode, use a solution containing ions of the coating metal as electrolyte, and normally use the coating metal as anode so its ions are replenished.
Making the object the anode would dissolve it rather than plate it.
Compare copper sulfate electrolysis
With inert electrodes, copper deposits at the cathode, oxygen forms at the anode and the blue solution fades. With copper electrodes, copper dissolves from the anode and deposits at the cathode, so Cu2+ concentration and colour stay approximately constant.
The anode product changes when a reactive copper electrode replaces an inert electrode.
Predict products of aqueous halides
At the cathode, reactive-metal salt solutions usually give hydrogen. At the anode, concentrated chloride or bromide gives the halogen, whereas dilute halide solution gives oxygen as the main product.
Concentration affects anode competition; molten-halide rules cannot be copied directly to aqueous solutions.
Construct electrode half-equations
Place electrons on the left for cathode reduction and on the right for anode oxidation, then balance atoms and total charge. Examples: Cu2+ + 2e- → Cu; 2Cl- → Cl2 + 2e-.
A half-equation must balance charge as well as atoms; electrons cannot appear on both sides after simplification.