Electrochemical Reactions — Where Chemistry Meets Electricity
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Electrochemical reactions interconvert chemical energy and electrical energy. In galvanic (voltaic) cells, spontaneous redox reactions generate electrical current — this is how batteries work. In electrolytic cells, an external voltage drives non-spontaneous reactions — used in electroplating, metal refining, and water splitting. Electrochemistry bridges thermodynamics, kinetics, and materials science, and is critical to energy storage, corrosion control, and sensor technology.
Reaction Mechanism
Electrochemical cells have two electrodes (anode and cathode) connected by an external circuit and an electrolyte providing ionic conduction. At the anode, oxidation occurs (electrons leave); at the cathode, reduction occurs (electrons arrive). Cell voltage (EMF) equals the difference in standard electrode potentials: E_cell = E_cathode - E_anode. The Nernst equation adjusts the voltage for non-standard concentrations. Faraday's laws quantify the relationship between charge passed and mass of substance transformed.
Everyday Examples
Lithium-ion batteries in smartphones use reversible electrochemical reactions, lasting 500-1,000 charge cycles. Car batteries use lead-acid electrochemistry dating back to 1859. Electroplated jewelry has a thin gold or silver layer deposited by electrolysis. Corrosion of iron pipes is an unwanted electrochemical reaction — cathodic protection and sacrificial anodes prevent it.
Importância industrial
Aluminum smelting (Hall-Heroult process) uses electrolysis to produce 70 million tonnes of aluminum annually, consuming about 3 percent of global electricity. Chlor-alkali electrolysis produces chlorine and sodium hydroxide — essential chemicals with combined production exceeding 80 million tonnes per year. Hydrogen production by water electrolysis is central to green energy strategies, with global electrolyzer capacity projected to reach 134-240 GW by 2030.
Safety Note
Electrolysis produces hydrogen and oxygen gases, which form explosive mixtures. Ensure adequate ventilation and eliminate ignition sources near electrolysis equipment. Electrolyte solutions can be corrosive — sulfuric acid in lead-acid batteries causes severe burns. High-current electrochemical processes pose electrical shock hazards.
Electrochemical CO₂ Reduction to CO
CO₂ + 2H⁺ + 2e⁻ → CO + H₂O
Electrochemical reduction of CO₂ at a metal cathode can selectively produce CO (on Au/Ag), formate (on Sn/Pb/Bi), or hydrocarbons (on …
Electrolysis of Water
2H₂O → 2H₂ + O₂
An electric current passed through water splits it into hydrogen gas at the cathode and oxygen gas at the anode. …
Electrolytic Production of Chlorate
NaCl + 3H₂O → NaClO₃ + 3H₂
Electrolysis of hot NaCl solution without a membrane allows chlorine produced at the anode to react with hydroxide from the …
Electrorefining of Copper
Cu (impure) → Cu²⁺ + 2e⁻ → Cu (pure)
Impure copper anodes (99.5%) dissolve electrochemically while pure copper (99.99%) deposits at the cathode from CuSO₄/H₂SO₄ electrolyte. Impurities either remain …
Alkaline Battery Discharge
Zn + 2MnO₂ + H₂O → ZnO + 2MnOOH
Alkaline batteries use zinc powder anode and MnO₂ cathode in potassium hydroxide electrolyte, producing 1.5 V. They offer 4–9 times …
Anodizing of Aluminum
2Al + 3H₂O → Al₂O₃ + 6H⁺ + 6e⁻
Anodizing grows a thick, porous aluminum oxide layer on aluminum surfaces by making the aluminum the anode in an acid …
Cathodic Protection (Zinc Sacrificial Anode)
Zn → Zn²⁺ + 2e⁻
A zinc sacrificial anode is electrically connected to a steel structure. Since zinc has a more negative electrode potential (-0.76 …
Chlor-Alkali Electrolysis
2NaCl + 2H₂O → Cl₂ + 2NaOH + H₂
Electrolysis of concentrated brine (NaCl solution) produces three essential industrial chemicals simultaneously: chlorine gas at the anode, sodium hydroxide solution …
Chrome Electroplating
Cr³⁺ + 3e⁻ → Cr
Chromium plating deposits a thin layer of chromium from a chromic acid or trivalent chromium bath. Decorative chrome is only …
Nickel-Metal Hydride Battery
MH + NiOOH → M + Ni(OH)₂
NiMH batteries use a metal hydride anode (typically a rare earth/nickel alloy like AB₅ or AB₂) and nickel oxyhydroxide cathode …
Sodium-Sulfur Battery
2Na + 3S → Na₂S₃
Sodium-sulfur batteries operate at 300–350°C with molten sodium anode, molten sulfur cathode, and a solid beta-alumina ceramic electrolyte that conducts …
Daniell Cell (Zinc-Copper Galvanic Cell)
Zn + Cu²⁺ → Zn²⁺ + Cu
The Daniell cell consists of a zinc anode in ZnSO₄ solution and a copper cathode in CuSO₄ solution, connected by …
Copper Electroplating
Cu²⁺ + 2e⁻ → Cu
Copper ions from a CuSO₄ solution are reduced at the cathode surface, depositing a thin layer of metallic copper. The …
Lithium-Ion Battery Discharge
LiC₆ + CoO₂ → C₆ + LiCoO₂
During discharge, lithium ions deintercalate from the graphite anode and intercalate into the cobalt oxide cathode through a non-aqueous electrolyte. …
Hydrogen Fuel Cell (PEM)
2H₂ + O₂ → 2H₂O
In a proton exchange membrane (PEM) fuel cell, hydrogen is oxidized at the anode and oxygen is reduced at the …
Electrochemical Machining (ECM)
Fe → Fe²⁺ + 2e⁻
Electrochemical machining dissolves metal from a workpiece (anode) in a controlled pattern using a shaped cathode tool and flowing electrolyte …
Silver Electroplating
Ag⁺ + e⁻ → Ag
Silver ions from a silver cyanide or silver nitrate bath are reduced at the cathode, depositing a thin, bright layer …
Electrolysis of Molten NaCl (Downs Process)
2NaCl → 2Na + Cl₂
The Downs process electrolyzes molten NaCl at about 600°C (lowered from 801°C by adding CaCl₂) to produce sodium metal at …
Solid Oxide Fuel Cell (SOFC)
H₂ + ½O₂ → H₂O
SOFCs operate at 600–1000°C using a solid ceramic oxide electrolyte that conducts O²⁻ ions. Oxygen is reduced at the cathode …
Zinc-Carbon Battery (Leclanché Cell)
Zn + 2MnO₂ + 2NH₄Cl → ZnCl₂ + Mn₂O₃ + 2NH₃ + H₂O
The zinc-carbon cell uses a zinc casing as anode and a carbon rod surrounded by MnO₂ paste as cathode, with …
Lead-Acid Battery Discharge
Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
During discharge, lead is oxidized to PbSO₄ at the anode while PbO₂ is reduced to PbSO₄ at the cathode, consuming …
Hall-Héroult Process (Aluminum Smelting)
2Al₂O₃ + 3C → 4Al + 3CO₂
Alumina (Al₂O₃) is dissolved in molten cryolite (Na₃AlF₆) at 960°C and electrolyzed using carbon anodes. Aluminum is deposited at the …
Electrowinning of Zinc
ZnSO₄ + H₂O → Zn + H₂SO₄ + ½O₂
Zinc electrowinning (electroextraction) deposits zinc metal from purified zinc sulfate solution onto aluminum cathodes. The cathode deposit is stripped every …
Electrolytic Manganese Dioxide Production
MnSO₄ + 2H₂O → MnO₂ + H₂SO₄ + H₂
Electrolytic manganese dioxide (EMD) is produced by electrolyzing manganese sulfate solution in sulfuric acid at 90–98°C. MnO₂ deposits on the …
Lithium Iron Phosphate Battery (LFP)
LiFePO₄ ⇌ FePO₄ + Li⁺ + e⁻
LFP batteries use lithium iron phosphate cathode and graphite anode with a cell voltage of 3.2–3.3 V and energy density …