Products of Electrolysis
Predicting Reaction Outcomes at Electrodes
1. Factors Affecting Products
The products obtained during electrolysis depend on:
- Nature of Electrolyte: Molten vs. Aqueous.
- Type of Electrode: Inert (Pt, Graphite) vs. Active (Cu, Ag).
- Standard Electrode Potentials ($E^\circ$): Determines which species reacts first.
- Overpotential: Kinetic barrier for gases like $O_2$.
2. Prediction at Cathode (Reduction)
The cathode is the site of reduction (gain of electrons). Cations move here.
Rule: The species with the Higher Standard Reduction Potential ($E^\circ_{red}$) is preferentially reduced.
Competition in Aqueous Solutions:
- Active Metals (Group 1, 2, 13): $E^\circ$ is very low (e.g., $Na^+ = -2.71V$). Water has higher reduction potential ($E^\circ = -0.83V$).
$\Rightarrow$ Hydrogen gas ($H_2$) is produced from water.
$$ 2H_2O + 2e^- \rightarrow H_2(g) + 2OH^- $$ - Less Active Metals (Cu, Ag, Au): $E^\circ$ is positive (higher than water).
$\Rightarrow$ Metal deposits at the cathode.
$$ Cu^{2+} + 2e^- \rightarrow Cu(s) $$
3. Prediction at Anode (Oxidation)
The anode is the site of oxidation (loss of electrons). Anions move here.
Rule: The species with the Lower Reduction Potential (or Higher Oxidation Potential) is preferentially oxidized.
A. With Inert Electrodes (Pt, C)
- Simple Ions ($Cl^-, Br^-, I^-$): Oxidize to form gas ($Cl_2, Br_2, I_2$).
Exception: For $Cl^-$, water has a slightly lower oxidation potential, but due to Overpotential of oxygen formation, $Cl^-$ oxidizes preferentially in concentrated solutions. - Polyatomic Ions ($SO_4^{2-}, NO_3^-$): Very stable, high decomposition potential. Water oxidizes instead.
$\Rightarrow$ Oxygen gas ($O_2$) is produced.
$$ 2H_2O \rightarrow O_2(g) + 4H^+ + 4e^- $$
B. With Active Electrodes (e.g., Cu, Ag)
The metal electrode itself oxidizes because metal oxidation requires less energy than oxidizing water or anions.
$$ M(s) \rightarrow M^{n+} + ne^- $$The anode dissolves.
4. Summary of Common Electrolytes
| Electrolyte | Electrodes | Product at Cathode | Product at Anode | Change in Solution |
|---|---|---|---|---|
| Molten NaCl | Pt/C | Sodium (Na) | Chlorine ($Cl_2$) | None |
| Aq. NaCl (Conc.) | Pt/C | Hydrogen ($H_2$) | Chlorine ($Cl_2$) | Forms $NaOH$ (pH increases) |
| Aq. NaCl (Dilute) | Pt/C | Hydrogen ($H_2$) | Oxygen ($O_2$) | Concentration increases |
| Aq. $CuSO_4$ | Inert (Pt) | Copper (Cu) | Oxygen ($O_2$) | Forms $H_2SO_4$ (Acidic) |
| Aq. $CuSO_4$ | Active (Cu) | Copper (Cu) | Cu Anode Dissolves | No change in conc. |
| Acidulated Water | Pt | Hydrogen ($H_2$) | Oxygen ($O_2$) | Volume decreases |
Practice Quiz
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