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IGCSE Chemistry

IGCSE Chemistry electrolysis: molten vs aqueous, and how to tell which product forms at the anode and cathode

Electrolysis is where students most often mix things up: when you get the metal, when H₂, when O₂ or the halogen. This guide boils it down to one set of rules that works for every question, lets you switch electrolytes in an animated cell and watch the ions move, and gives the half-equations and the words that lose marks.

The short answer

Remember 3 lines: PANIC (Positive is Anode, Negative Is Cathode) · positive ions go to the cathode, negative ions to the anode · OIL RIG (Oxidation Is Loss, Reduction Is Gain): the cathode gains electrons, so reduction; the anode loses them, so oxidation.

A molten compound always gives the metal at the cathode and the non-metal at the anode. An aqueous solution also has H⁺ and OH⁻ from water competing, so you need the selection rules in the next table.

In Edexcel 4CH1 this is points 1.55C–1.60C (C = Chemistry only, not in Double Award). The content matches 0620 almost exactly.

Molten vs aqueous: which ion is discharged

This table is every rule you need. Extended papers like to ask about a halide you have not met, such as concentrated KBr(aq), and the table answers it at once: K is more reactive than H, so H₂ at the cathode; Br⁻ is concentrated, so Br₂ at the anode.

Tap a row to highlight it

  • Ions in the cell

    MoltenJust 2 kinds, from the compound, e.g. Pb²⁺ and Br⁻

    Aqueous (in water)The compound's ions plus H⁺ and OH⁻ from water, competing to be discharged

  • Cathode (−) gives

    MoltenAlways the metal

    Aqueous (in water)The metal if it is less reactive than hydrogen (Cu, Ag); H₂ if it is more reactive (K, Na, Ca, Mg, Al, Zn, Fe)

  • Anode (+) gives

    MoltenAlways the non-metal (a halogen, or O₂ from an oxide)

    Aqueous (in water)The halogen from a concentrated halide; O₂ if it is dilute, or the anion is SO₄²⁻ / NO₃⁻ (never discharged)

  • Examples

    MoltenPbBr₂ → Pb + Br₂ · Al₂O₃ (in cryolite) → Al + O₂

    Aqueous (in water)Concentrated NaCl → H₂ + Cl₂ · CuSO₄ → Cu + O₂

The cathode rule uses the reactivity series from the metals topic: K Na Ca Mg Al (C) Zn Fe H Cu Ag Au.

Try it: switch the electrolyte and watch the ions

Close the switch and watch every positive ion drift to the cathode and every negative ion to the anode, while only one kind of ion is discharged at each electrode (thick outline); the rest bounce back as spectators. Compare concentrated with dilute NaCl, and CuSO₄ with graphite against copper electrodes.

An aqueous solution also contains H⁺ and OH⁻ from water. Thick outline = the ion that is discharged.
Cathode (−) · Product
—
Anode (+) · Product
—

Rule: every + ion drifts to the cathode and every − ion to the anode, but only one ion is discharged at each electrode; the rest bounce back (spectators).

At the cathode: if the metal is more reactive than hydrogen (K Na Ca Mg Al Zn Fe) you get H₂ from water instead of the metal; if less reactive (Cu Ag) the metal plates the electrode. At the anode: a concentrated halide gives the halogen, otherwise OH⁻ from water is discharged and you get O₂. Everything else is a spectator. With copper electrodes the anode itself dissolves instead. Switch between the six cells and watch who gets discharged.

Products and half-equations for every cell that comes up

The 7 cells that come up most, with the products, ionic half-equations and observations in one place. In a half-equation the charges on both sides must match: electrons on the left at the cathode, on the right at the anode.

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  • Molten PbBr₂ (graphite) · Core

    Cathode (−)Pb · Pb²⁺ + 2e⁻ → Pb

    Anode (+)Br₂ · 2Br⁻ → Br₂ + 2e⁻

    What you seeSilvery-grey lead below the cathode · red-brown vapour at the anode

  • Concentrated NaCl(aq) (graphite) · Core

    Cathode (−)H₂ · 2H⁺ + 2e⁻ → H₂

    Anode (+)Cl₂ · 2Cl⁻ → Cl₂ + 2e⁻

    What you seeColourless bubbles at the cathode · pale green gas at the anode that bleaches damp litmus · NaOH left in solution

  • Dilute NaCl(aq) (graphite) · Extended

    Cathode (−)H₂ · 2H⁺ + 2e⁻ → H₂

    Anode (+)O₂ · 4OH⁻ → O₂ + 2H₂O + 4e⁻

    What you seeColourless bubbles at both electrodes · the anode gas relights a glowing splint

  • Dilute H₂SO₄(aq) (graphite/Pt) · Core

    Cathode (−)H₂ · 2H⁺ + 2e⁻ → H₂

    Anode (+)O₂ · 4OH⁻ → O₂ + 2H₂O + 4e⁻

    What you seeBubbles at both electrodes · twice the volume of H₂ as O₂

  • CuSO₄(aq) (graphite) · Extended

    Cathode (−)Cu · Cu²⁺ + 2e⁻ → Cu

    Anode (+)O₂ · 4OH⁻ → O₂ + 2H₂O + 4e⁻

    What you seeRed-brown copper on the cathode · bubbles at the anode · the blue fades

  • CuSO₄(aq) (copper electrodes) · Extended

    Cathode (−)Cu · Cu²⁺ + 2e⁻ → Cu

    Anode (+)Cu²⁺ · Cu → Cu²⁺ + 2e⁻

    What you seeCathode gains mass · anode loses mass, no bubbles · the blue stays the same

  • Al₂O₃ dissolved in molten cryolite (carbon) · Extended

    Cathode (−)Al · Al³⁺ + 3e⁻ → Al

    Anode (+)O₂ · 2O²⁻ → O₂ + 4e⁻

    What you seeMolten aluminium collects at the bottom · O₂ burns the carbon anodes to CO₂, so they are replaced regularly

Core: name the products and the observations. Extended: also write the ionic half-equations (0620 4.1.11). Electrons gained at the cathode = reduction; electrons lost at the anode = oxidation.

Electroplating, purifying copper and extracting aluminium

The real-world uses the exam asks about follow the same rules. What you must remember is which is the cathode, which is the anode, and what the electrolyte is.

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  • Electroplating

    Cathode (−)The object to be plated

    Anode (+)The plating metal, e.g. Ag, Ni, Cu

    ElectrolyteA solution of a salt of the plating metal

    What the question wantsDone to improve appearance and resistance to corrosion

  • Purifying copper

    Cathode (−)Pure copper (gets thicker)

    Anode (+)Impure copper (dissolves)

    ElectrolyteCuSO₄(aq)

    What the question wantsImpurities fall below the anode; the blue stays the same

  • Extracting aluminium

    Cathode (−)Carbon lining, gives Al

    Anode (+)Carbon, gives O₂

    ElectrolyteAl₂O₃ dissolved in molten cryolite

    What the question wantsCryolite lowers the melting point, saving energy · the anodes burn to CO₂ and are replaced

  • Brine (concentrated NaCl)

    Cathode (−)Gives H₂

    Anode (+)Gives Cl₂

    ElectrolyteConcentrated NaCl(aq)

    What the question wants3 products: H₂, Cl₂, and NaOH left in the solution

Words that lose marks

Most marks here go on wording, not knowledge. The most common slip is writing that electrons move through the electrolyte. Ions move in the electrolyte; electrons move in the wires (the external circuit).

For questions that start with Explain, see the phrases examiners want in Explain answers.

Tap a row to highlight it

  • How charge moves (Extended)

    Loses the markelectrons move through the electrolyte

    Scoresions move through the electrolyte; electrons move through the external circuit

  • What moves to the electrodes

    Loses the markatoms / sodium moves to the cathode

    Scorespositive ions (cations) move to the cathode; negative ions (anions) move to the anode

  • Why the ionic solid does not conduct

    Loses the markthere are no free electrons

    Scoresthe ions cannot move in the solid; they are free to move when molten or in solution

  • Cathode of NaCl(aq)

    Loses the marksodium is formed

    Scoreshydrogen is formed, because sodium is more reactive than hydrogen

  • Naming the anode gas

    Loses the markCl (or 'chloride gas')

    Scoreschlorine, Cl₂

  • The anode half-equation

    Loses the mark2Cl⁻ + 2e⁻ → Cl₂

    Scores2Cl⁻ → Cl₂ + 2e⁻ (electrons lost = oxidation)

  • A question asks what you see

    Loses the markchlorine is produced

    Scoresbubbles of pale green gas at the anode

  • Electroplating

    Loses the markthe object is the anode

    Scoresthe object is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal

Green = a sentence you can write in the exam. The wording is our summary in the style of a 0620 mark scheme, not a published one.

Where Boom-Tutor helps

Topic 4 of the course (Electrochemistry) has 6 on-screen stations: cells and ions · products at the electrodes · electroplating · CuSO₄ with 2 kinds of electrode · halides and half-equations · fuel cells. The student builds the cell, sees the result, then answers exam-style questions straight away.

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