FAD1018 L4-L5 — Electrolytic Cell

Electrolytic Cell Part 3 for FAD1018 Basic Chemistry II. Source file: L4 L5 EC2526 V2- Stdn copy.pdf (50 pages, PowerPoint slides).

[!note] Direct Image Processing Content reconstructed from direct visual processing of all 50 slide images.

Learning Outcomes

  • Differentiate between galvanic and electrolytic cells
  • Predict products of electrolysis for molten and aqueous electrolytes
  • Explain the factors affecting selective discharge of ions
  • Apply Faraday's laws to quantitative electrolysis problems
  • Describe industrial applications of electrolysis (Downs cell, Hall process)

1. Electrolytic Cells

An electrolytic cell uses electrical energy to drive a non-spontaneous chemical reaction.

Feature Galvanic Cell Electrolytic Cell
Energy Chemical → Electrical Electrical → Chemical
Anode Negative Positive
Cathode Positive Negative
Process Spontaneous Non-spontaneous

Electrolysis of CuSO₄ with Copper Electrodes

  • Anode (impure copper): Cu oxidises and dissolves: $\text{Cu}(s) \rightarrow \text{Cu}^{2+}(aq) + 2e^-$
  • Cathode (pure copper): Cu²⁺ reduces and deposits: $\text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s)$
  • Result: Pure copper deposited on cathode; impurity falls as anode mud
[Cu+2]

2. Factors Affecting Electrolysis Products of Aqueous Solutions

When more than one type of cation and anion are present in the electrolyte, selective discharge occurs. Three factors determine which ions are preferentially discharged:

Factor 1: Standard Electrode Potential (Electrochemical Series)

The standard electrode potential of the ion determines its ease of discharge.

Cations (discharged at cathode): ease of discharge increases down the series $$\text{K}^+ < \text{Na}^+ < \text{Ca}^{2+} < \text{Mg}^{2+} < \text{Al}^{3+} < \text{Zn}^{2+} < \text{Fe}^{2+} < \text{Sn}^{2+} < \text{Pb}^{2+} < \text{H}^+ < \text{Cu}^{2+} < \text{Ag}^+$$

Anions (discharged at anode): ease of discharge increases down the series $$\text{F}^- < \text{SO}_4^{2-} < \text{NO}_3^- < \text{Cl}^- < \text{Br}^- < \text{I}^- < \text{OH}^-$$

Factor 2: Concentration of Ion in the Electrolyte

Higher concentration favours discharge of that ion, even if it is not the easiest to discharge according to the electrochemical series.

Factor 3: Type of Electrode Used

  • Active electrode: Participates in the reaction (e.g., Cu electrodes in CuSO₄ electrolysis)
  • Inert electrode: Does not participate (e.g., Pt, graphite); only provides surface for reaction

3. Overpotential (Overvoltage) Effect

The products predicted by comparing standard electrode potentials alone are not always accurate due to overpotential (or overvoltage).

Oxygen Overvoltage is High

  • Producing O₂ at the anode requires significantly more voltage than its theoretical value
  • This is due to slow reaction kinetics and bubble formation
  • Water oxidation requires an additional 0.4–0.6 V overpotential
  • Effective potential for water oxidation: 1.22–1.42 V

Consequence

Overpotential modifies the effective potential required for electrolysis, enabling anions like Cl⁻, Br⁻, and I⁻ to oxidise preferentially over water despite unfavourable standard potentials.

Example in Dilute NaCl:

  • Platinum anode: overvoltage for O₂ is ~0.5 V
  • Cl₂ has minimal overvoltage
  • Chloride oxidises at Cl₂ (total potential ≈ -1.36 V)
  • Water would require ≈ (-1.23 V) + (-0.6 V) = -1.73 V
  • Therefore, Cl⁻ oxidises before water

4. Industrial Implementation

The Downs Cell

  • Process: Electrolysis of molten NaCl
  • Mixture: NaCl and CaCl₂ (ratio 2:3) to lower melting point from 801°C to ~580°C
  • Products: Liquid Na metal (cathode) and Cl₂ gas (anode)

The Hall Process

  • Process: Extraction of aluminium from alumina (Al₂O₃)
  • Conditions: High temperatures (~940–980°C)
  • Electrolyte: Alumina dissolves in molten cryolite, forming a conductive electrolyte
  • Cell components: Carbon anodes, iron vessel, molten aluminium collected at bottom

5. Exercises from Lecture

Exercise 3: Draw an Electrolytic Cell

Draw a diagram of an electrolytic cell used to electrolyse molten Lead(II) bromide, PbBr₂.

Key components to label:

  • Power source (battery)
  • Anode and cathode
  • Molten PbBr₂
  • Direction of ion migration (Pb²⁺ → cathode, Br⁻ → anode)
  • Half-equations

Key Concepts

  • Electrochemistry — Concept page
  • Electrolytic Cell — Non-spontaneous electrochemical cell
  • Electrochemical Series — Selective discharge of ions
  • Overpotential — Overvoltage effect in electrolysis
  • Faraday's Laws — Quantitative electrolysis
  • Industrial Electrolysis — Downs cell, Hall process

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