9701 · 24.1
Electrolysis flashcards
Revision flashcards for Cambridge 9701 Electrolysis (syllabus 24.1). Flip, recall, then mark a real past-paper question.
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What is electrolysis?
The decomposition of a chemical compound (the electrolyte) by passing a direct electric current through it. It forces a non-spontaneous redox reaction to occur.
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What is an electrolyte?
A substance that contains mobile ions and can conduct electricity. It can be a molten ionic compound or an aqueous solution of an ionic compound.
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What is the cathode in an electrolytic cell?
The negative electrode. Cations are attracted to it, and reduction occurs here. (Remember 'Red Cat' - REDuction at the CAThode).
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What is the anode in an electrolytic cell?
The positive electrode. Anions are attracted to it, and oxidation occurs here. (Remember 'An Ox' - ANode for OXidation).
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Mnemonic for electrode polarity in electrolysis?
PANIC: Positive Anode, Negative Is Cathode. This is for electrolytic cells only, not voltaic cells.
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Why must the electrolyte be molten or in solution for electrolysis?
The ions must be mobile (free to move) to travel to the electrodes and carry the charge. In a solid ionic lattice, ions are in fixed positions.
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What is the key difference between electrolysing a molten salt and an aqueous solution?
In an aqueous solution, water molecules can also be oxidised or reduced, creating H⁺ and OH⁻ ions which compete with the solute ions for discharge at the electrodes.
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Rule for cation discharge in aqueous solution?
The cation of the less reactive metal (higher reduction potential) is preferentially discharged. For metals more reactive than hydrogen, H⁺ ions (from water) are reduced to H₂ gas instead.
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Rule for anion discharge in aqueous solution?
For simple anions, halide ions (Cl⁻, Br⁻, I⁻) are preferentially discharged over OH⁻ (from water), which is discharged over complex anions like SO₄²⁻ or NO₃⁻. Fluoride (F⁻) is an exception; OH⁻ is discharged instead.
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What is the Faraday constant (F)?
The charge carried by one mole of electrons. Its value is approximately 96500 Coulombs per mole (C mol⁻¹).
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What is the formula relating charge, current, and time?
Q = I × t, where Q is charge in Coulombs (C), I is current in Amperes (A), and t is time in seconds (s).
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What is an 'inert' electrode?
An electrode, typically made of platinum or graphite, that does not take part in the chemical reaction. It serves only as a surface for electron transfer.