Electrolysis plays a vital role in modern chemical processing, metallurgy, and manufacturing. Major industrial applications include electroplating, electro-refining of metals, extraction of highly reactive elements, and the chlor-alkali process.
Definition: Electroplating is the process of depositing a thin, uniform layer of a desired metal onto the surface of another conductive object using electric current.
Purposes of Electroplating:
• Protection against corrosion (e.g., plating iron with zinc, nickel, or chromium).
• Decorative enhancement (e.g., silver or gold plating of jewelry, cutlery, and ornaments).
• Increasing surface hardness and wear resistance (e.g., chromium plating on car bumpers and engine components).
Setup & Rules for Electroplating:
Example — Silver Plating a Metallic Spoon:
Ionization of electrolyte (AgNO₃): $$ AgNO₃_{(aq)} → Ag⁺_{(aq)} + NO₃⁻_{(aq)} $$ At Anode (oxidation of pure silver rod): $$ Ag_{(s)} → Ag⁺_{(aq)} + e⁻ $$ At Cathode (reduction onto the spoon): $$ Ag⁺_{(aq)} + e⁻ → Ag_{(s)} $$ Net result: Silver dissolves from the anode and plates smoothly onto the spoon at the cathode.
Notes: A steady low DC voltage is used to ensure a smooth, firm, and non-flaking coating.
Electro-refining is used to purify crude metals extracted from their ores (such as copper, gold, silver, and zinc) to achieve high-purity grades required for industrial electrical and structural applications.
Process Setup (Refining Crude Copper):
Anode: Impure copper block
Cathode: Thin sheet of 99.99% pure copper
Electrolyte: Aqueous Copper(II) Sulfate (CuSO₄) acidified with dilute H₂SO₄
Anode reaction (oxidation):
$$ Cu_{(s, impure)} → Cu²⁺_{(aq)} + 2e⁻ $$
Cathode reaction (reduction):
$$ Cu²⁺_{(aq)} + 2e⁻ → Cu_{(s, pure)} $$
Observations & Significance: The impure anode gradually dissolves into solution while pure copper deposits continuously on the cathode. Less reactive impurities (such as silver, gold, and platinum) do not oxidize and settle at the bottom as valuable anode sludge (anode slime). More reactive impurities stay dissolved in the electrolyte.
Highly reactive metals at the top of the electrochemical series (e.g., $\text{Na}$, $\text{K}$, $\text{Ca}$, $\text{Mg}$, $\text{Al}$) form extremely stable compounds that cannot be reduced chemically using carbon or hydrogen. They are extracted industrially by the electrolysis of their molten (fused) salts or oxides.
A. Extraction of Aluminium (Hall-Héroult Process):
Aluminium is extracted from purified alumina ($\text{Al}_2\text{O}_3$) dissolved in molten cryolite ($\text{Na}_3\text{AlF}_6$) at about 950 °C.
Role of Cryolite: Lowers the melting point of alumina from ~2050 °C to ~950 °C and increases electrical conductivity.
Electrodes: Carbon (graphite) blocks for both anode and cathode lining.
Cathode Reaction (reduction):
$$ Al³⁺_{(l)} + 3e⁻ → Al_{(l)} $$ (Molten aluminium collects at the bottom)
Anode Reaction (oxidation):
$$ 2O²⁻_{(l)} → O₂_{(g)} + 4e⁻ $$
Note: Oxygen gas released at the carbon anode reacts with it at high temperatures to form CO₂, requiring periodic replacement of the anodes.
B. Extraction of Sodium Metal (Downs Process):
Sodium is extracted by electrolysing a molten mixture of $\text{NaCl}$ and $\text{CaCl}_2$ (where $\text{CaCl}_2$ acts as a flux to lower $\text{NaCl}$'s melting point from 801 °C to ~600 °C).
At Cathode (cylindrical steel shell):
$$ 2Na⁺_{(l)} + 2e⁻ → 2Na_{(l)} $$
(Molten sodium floats and is collected)
At Anode (central graphite rod):
$$ 2Cl⁻_{(l)} → Cl₂_{(g)} + 2e⁻ $$ (Chlorine gas evolved as a valuable byproduct)
Electrolysis of concentrated sodium chloride solution (brine) yields three important industrial chemicals simultaneously:
Overall Cell Reaction: $$ 2NaCl_{(aq)} + 2H₂O_{(l)} → Cl₂_{(g)} + H₂_{(g)} + 2NaOH_{(aq)} $$
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Test Id : 737373 7-5-2020 ScholarpadThese are the correct answers