Topic 4
3 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Electrolysis is the decomposition of an ionic compound (electrolyte) when molten or in aqueous solution, using an electric current.
Explanation
Key terms: - Electrolyte: ionic compound that conducts electricity when molten or dissolved. - Anode (+): positive electrode. Anions move here and are oxidised (lose electrons). - Cathode (-): negative electrode. Cations move here and are reduced (gain electrons). Memory aid: AN OX — RED CAT - ANOde = OXidation (loss of electrons) - REDuction at CAThode (gain of electrons)
💡 Analogy
Think of electrolysis like a magnet attracting iron filings: the positive anode attracts negative ions (anions), and the negative cathode attracts positive ions (cations). Opposite charges attract!
Worked Examples
Practice Questions
During electrolysis, which statement correctly describes what happens at the cathode?
Key Idea
In aqueous solution, water also provides H+ and OH- ions. The product at each electrode depends on the position of the ion in the electrochemical series and the concentration.
Explanation
Rules for aqueous electrolysis: AT CATHODE (-): Metal ions vs H+ ions - If metal is below hydrogen in reactivity series: metal is deposited. - If metal is above hydrogen in reactivity series: H2 gas is produced. AT ANODE (+): Halide ions vs OH- ions - If halide ions (Cl-, Br-, I-) are present in HIGH concentration: halogen gas is produced. - If halide ions are absent or in LOW concentration: O2 gas is produced. Electrolysis of dilute H2SO4: - Cathode: H2 gas - Anode: O2 gas - Volume of H2 = twice the volume of O2 (2:1 ratio) Electrolysis of concentrated NaCl (brine): - Cathode: H2 gas - Anode: Cl2 gas - Solution becomes NaOH (chlor-alkali industry)
Worked Examples
Practice Questions
Dilute sulfuric acid is electrolysed using platinum electrodes. a) Name the gas produced at the cathode. [1] b) Name the gas produced at the anode. [1] c) Compare the volumes of gas produced at each electrode. [1]
Key Idea
Electroplating uses electrolysis to coat a metal object with a thin layer of another metal. The object to be plated is the cathode; the plating metal is the anode.
Explanation
Electroplating setup: - Cathode = object to be plated (gains metal coating). - Anode = metal used for plating (dissolves to replenish ions). - Electrolyte = solution of the plating metal's salt. Industrial applications: - Aluminium extraction: electrolysis of molten Al2O3 (alumina) dissolved in molten cryolite. Cathode: Al deposited. Anode: O2 produced (carbon anodes burn away and must be replaced). - Chlor-alkali industry: electrolysis of brine (concentrated NaCl). Products: Cl2 (anode), H2 (cathode), NaOH (solution).
Practice Questions
A steel spoon is to be electroplated with silver. Describe the setup needed, identifying the cathode, anode, and electrolyte. [3 marks]