6.2.7Additional lessons
Electrolytic Cells — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 90 of 98
Section 1 of 6Electrolysis Basics
Remember RED CATreduction at the cathode, in both voltaic and electrolytic cells.
Electrolytic cell
- An electrolytic cell uses electrical energy to drive a non-spontaneous redox reaction.
Electrodes
- Reduction occurs at the cathode (negative) and oxidation at the anode (positive).
Molten ionic compounds
- Ions are free to move only when the compound is molten or dissolved, so solids do not conduct.
Ion movement
- Cations move to the cathode and anions to the anode, carrying the current through the electrolyte.
Section 2 of 6Electrolysis of Molten Salts
Electrolysis of Molten Salts
Molten NaCl
- Cathode
- Na⁺ + e⁻ → Na(l).
- Anode: 2Cl⁻ → Cl₂(g) + 2e⁻.
Molten PbBr₂
- Cathode
- Pb²⁺ + 2e⁻ → Pb(l).
- Anode: 2Br⁻ → Br₂(g) + 2e⁻.
Molten MgCl₂
- The overall change is the decomposition MgCl₂(l) → Mg(l) + Cl₂(g).
Inert electrodes
- Graphite or platinum electrodes do not react and only supply or accept electrons.
Section 3 of 6Aqueous Solutions and Comparison
Concentrated NaCl(aq)
- Cathode
- 2H₂O + 2e⁻ → H₂ + 2OH⁻.
- Anode: 2Cl⁻ → Cl₂ + 2e⁻, leaving NaOH(aq).
Water competes
- In aqueous electrolysis water can also be oxidised or reduced, so the products depend on reactivity and concentration.
Uses
- Electrolysis extracts reactive metals such as aluminium and produces chlorine and sodium hydroxide.
| Feature | Voltaic cell | Electrolytic cell |
|---|---|---|
| Energy change | Chemical → electrical | Electrical → chemical |
| Reaction | Spontaneous | Non-spontaneous |
| Anode polarity | Negative | Positive |
| Cathode polarity | Positive | Negative |
Section 4 of 6Choosing Products in Aqueous Electrolysis
Choosing Products in Aqueous Electrolysis
Less reactive metals
- Ions of unreactive metals such as copper are reduced to the metal at the cathode in preference to water.
Reactive metals
- Ions such as Na⁺ stay in solution and hydrogen forms at the cathode from water.
Halide ions
- Concentrated chloride ions are oxidised to chlorine at the anode, while dilute solutions tend to give oxygen.
Electrode material
- Reactive electrodes such as copper can dissolve at the anode, which is how copper is purified.
Section 5 of 6Worked Example
Question Write the electrode equations and overall equation for the electrolysis of molten magnesium chloride using inert electrodes.
- Mg²⁺ ions move to the cathode and gain electrons: Mg²⁺ + 2e⁻ → Mg(l).
- Cl⁻ ions move to the anode and lose electrons: 2Cl⁻ → Cl₂(g) + 2e⁻.
- Two electrons are transferred in each half equation, so add them directly.
Answer MgCl₂(l) → Mg(l) + Cl₂(g).
Section 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.
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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.