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6.2.7Additional lessons

Electrolytic Cells — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 90 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6Electrolysis Basics

Remember RED CATreduction at the cathode, in both voltaic and electrolytic cells.

Electrolytic cell
  • Electrolytic cell An electrolytic cell uses electrical energy to drive a non-spontaneous redox reaction.
Electrodes
  • Electrodes Reduction occurs at the cathode (negative) and oxidation at the anode (positive).
Molten ionic compounds
  • Molten ionic compounds Ions are free to move only when the compound is molten or dissolved, so solids do not conduct.
Ion movement
  • Ion movement Cations move to the cathode and anions to the anode, carrying the current through the electrolyte.

Part 2 of 6Electrolysis of Molten Salts

Electrolysis of Molten Salts
Molten NaCl
  • Molten NaCl Cathode: Na⁺ + e⁻ → Na(l). Anode: 2Cl⁻ → Cl₂(g) + 2e⁻.
Molten PbBr₂
  • Molten PbBr₂ Cathode: Pb²⁺ + 2e⁻ → Pb(l). Anode: 2Br⁻ → Br₂(g) + 2e⁻.
Molten MgCl₂
  • Molten MgCl₂ The overall change is the decomposition MgCl₂(l) → Mg(l) + Cl₂(g).
Inert electrodes
  • Inert electrodes Graphite or platinum electrodes do not react and only supply or accept electrons.

Part 3 of 6Aqueous Solutions and Comparison

Concentrated NaCl(aq)
  • Concentrated NaCl(aq) Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Anode: 2Cl⁻ → Cl₂ + 2e⁻, leaving NaOH(aq).
Water competes
  • Water competes In aqueous electrolysis water can also be oxidised or reduced, so the products depend on reactivity and concentration.
Uses
  • Uses Electrolysis extracts reactive metals such as aluminium and produces chlorine and sodium hydroxide.
FeatureVoltaic cellElectrolytic cell
Energy changeChemical → electricalElectrical → chemical
ReactionSpontaneousNon-spontaneous
Anode polarityNegativePositive
Cathode polarityPositiveNegative

Part 4 of 6Choosing Products in Aqueous Electrolysis

Choosing Products in Aqueous Electrolysis
Less reactive metals
  • Less reactive metals Ions of unreactive metals such as copper are reduced to the metal at the cathode in preference to water.
Reactive metals
  • Reactive metals Ions such as Na⁺ stay in solution and hydrogen forms at the cathode from water.
Halide ions
  • Halide ions Concentrated chloride ions are oxidised to chlorine at the anode, while dilute solutions tend to give oxygen.
Electrode material
  • Electrode material Reactive electrodes such as copper can dissolve at the anode, which is how copper is purified.

Part 5 of 6Worked Example

Question Write the electrode equations and overall equation for the electrolysis of molten magnesium chloride using inert electrodes.

  1. Mg²⁺ ions move to the cathode and gain electrons: Mg²⁺ + 2e⁻ → Mg(l).
  2. Cl⁻ ions move to the anode and lose electrons: 2Cl⁻ → Cl₂(g) + 2e⁻.
  3. Two electrons are transferred in each half equation, so add them directly.

Answer MgCl₂(l) → Mg(l) + Cl₂(g).

Part 6 of 6Exam Checklist & Tips

6.2.7 – Key Points for the Exam

  • Electrolysis The decomposition of an ionic compound by an electric current.
  • Electrolyte A molten or dissolved ionic substance that conducts electricity.
  • Cathode The electrode where reduction takes place.
  • Anode The electrode where oxidation takes place.

Common Mistakes

  • Saying electrons flow through the electrolyte; ions carry the current there.
  • Giving the anode a negative sign in an electrolytic cell.
  • Forgetting that water can be reduced in aqueous electrolysis.
  • Writing electrode equations that do not balance the charge.

Last updated:

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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