IGCSE Chemistry: electrolysis, products and half-equations

The electrolytes you must know (inert electrodes unless stated)

The first three are Cambridge Core. Copper(II) sulfate with carbon and copper electrodes is Cambridge Supplement. Edexcel lists lead(II) bromide, sodium chloride, dilute sulfuric acid and copper(II) sulfate with inert electrodes.

Key ideas board by board

Cambridge 0620 Core: define electrolysis, identify anode, cathode and electrolyte, give the products and observations for molten lead(II) bromide, concentrated aqueous sodium chloride and dilute sulfuric acid with platinum or carbon electrodes, predict products for any molten binary compound, and describe electroplating: the object to be plated is the cathode, the anode is the plating metal, and the electrolyte contains ions of that metal. Metal objects are electroplated for appearance and resistance to corrosion. Core also states that a hydrogen-oxygen fuel cell produces electricity with water as the only chemical product.

Cambridge Supplement adds the transfer of charge (electrons move in the external circuit, ions move in the electrolyte, electrons are lost and gained at the electrodes), copper(II) sulfate with carbon and with copper electrodes, predicting products for dilute or concentrated aqueous halides, ionic half-equations showing oxidation at the anode and reduction at the cathode, and the advantages and disadvantages of fuel cells compared with petrol engines.

Edexcel 4CH1 starts from why covalent compounds do not conduct and why ionic compounds conduct only when molten or in solution, defines anion and cation, then covers the same experiments, half-equations and the practical investigating electrolysis of aqueous solutions (1.60C). All of these statements carry a C reference and are assessed in Paper 2.

Worked example 1: predicting products in solution

Question: predict the products of electrolysing concentrated aqueous potassium bromide with inert electrodes, and write the half-equations.

Ions present: K+ and Br- from the salt, H+ and OH- from water. Cathode: potassium is more reactive than hydrogen, so hydrogen is discharged: 2H+ + 2e- -> H2. Anode: bromide is a halide in concentrated solution, so bromine is discharged: 2Br- -> Br2 + 2e-. Observations: bubbles at the cathode; orange-brown colour forms at the anode. Potassium and hydroxide ions remain, so the solution becomes alkaline.

If the solution were very dilute potassium bromide, oxygen would form at the anode instead: 4OH- -> O2 + 2H2O + 4e-.

Worked example 2: electroplating and redox

Question: describe how to silver-plate a steel spoon. Name the electrodes and electrolyte and write the cathode half-equation.

The spoon is the cathode (negative). The anode is a bar of pure silver. The electrolyte is an aqueous silver salt, such as silver nitrate. At the cathode: Ag+ + e- -> Ag, so silver coats the spoon. At the anode: Ag -> Ag+ + e-, so the bar dissolves and replaces the silver ions used. The cathode reaction is reduction because silver ions gain electrons; the anode reaction is oxidation because silver atoms lose electrons.

Common mistakes

• Swapping the electrodes. The anode is positive and attracts negative ions (anions); the cathode is negative.
• Saying solid ionic compounds conduct. The ions must be free to move, so the compound must be molten or dissolved.
• Saying electrons move through the electrolyte. Electrons flow in the wires; ions carry the charge in the electrolyte.
• Writing sodium as the cathode product of aqueous sodium chloride. Hydrogen forms because sodium is more reactive than hydrogen.
• Writing Br as the anode product instead of Br2, or unbalanced half-equations (2Br- -> Br2 + 2e-).
• Giving "bubbles" as the only observation for chlorine. Add the colour and the bleaching test.

Exam technique

Start every product question by listing the ions present, including H+ and OH- from water for aqueous solutions. Then apply the two rules in order: cathode (hydrogen unless the metal is below hydrogen in reactivity), anode (halogen if a concentrated halide, otherwise oxygen). Writing the ion list first makes the answer almost automatic and shows the examiner your reasoning.

Check half-equations by charge as well as atoms: in 4OH- -> O2 + 2H2O + 4e- the left has a charge of 4- and the right also totals 4-. Use "discharged" for ions turning into atoms or molecules at an electrode; it is the word mark schemes use.

How one-to-one lessons help

Students usually find electrolysis confusing because there are several rules and several electrolytes. A tutor builds one ion-list method on the shared whiteboard and applies it to each electrolyte in turn, then asks the student to predict products for unfamiliar salts and to write half-equations from scratch. For Edexcel students, the tutor makes sure this Paper 2 topic is not left until the end of the course.

Self-check

1. Define electrolysis, anode, cathode and electrolyte.
2. Give products and observations for molten lead(II) bromide, concentrated sodium chloride and dilute sulfuric acid.
3. Explain the difference between copper(II) sulfate electrolysis with carbon and with copper electrodes.
4. Predict products for any aqueous halide, dilute or concentrated.
5. Write half-equations at both electrodes and label oxidation and reduction.
6. Describe how an object is electroplated, and why.
7. State the product of a hydrogen-oxygen fuel cell.