General Chemistry
Electrochemistry
Chemistry of electron transfer — batteries, electrolysis, corrosion
Electrochemistry studies chemical reactions involving electron transfer across electrodes. Two main branches: (1) Galvanic (voltaic) cells — spontaneous redox produces electricity (batteries). (2) Electrolytic cells — applied current drives non-spontaneous reactions (electroplating, electrolysis of water). Components: anode (oxidation), cathode (reduction), salt bridge (ion flow). Cell potential E°cell = E°cathode - E°anode. Applications: batteries (portable energy), fuel cells, corrosion prevention, metal refining, sensors, biology (nerve signals).
- Galvanic cellSpontaneous redox; produces voltage
- Electrolytic cellExternal voltage drives non-spontaneous redox
- AnodeOxidation occurs (negative in galvanic; positive in electrolytic)
- CathodeReduction occurs (opposite of anode)
- Standard reduction potentialF₂/F⁻ = +2.87 V (highest); Li⁺/Li = -3.04 V
- Faraday constantF = 96,485 C/mol
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Why electrochemistry matters
- Energy storage. Batteries; renewable grid integration.
- Manufacturing. Electroplating, electrorefining.
- Industrial. Cl₂, NaOH, Al production.
- Corrosion prevention. Galvanizing, cathodic protection.
- Sensors. pH meters, biosensors.
- Biology. Nerve impulses, ATP synthesis.
- Future energy. Fuel cells, hydrogen economy.
Common misconceptions
- Anode is positive always. Polarity reverses for galvanic vs electrolytic.
- Higher voltage means more energy. Energy = voltage × charge × time.
- Redox doesn't apply. Foundation of all electrochemistry.
- Cell potential is constant. Depends on concentrations (Nernst).
- Batteries store electricity directly. Store chemical energy; release as electricity.
- Corrosion is just oxidation. Specifically electrochemical with cathodic site.
Frequently asked questions
How does a battery work?
Galvanic cell. Anode: spontaneous oxidation (e.g., Zn → Zn²⁺ + 2e⁻). Cathode: spontaneous reduction (e.g., Cu²⁺ + 2e⁻ → Cu). Electrons flow through external circuit (current). Salt bridge maintains ion balance — prevents charge buildup. Voltage from difference in reduction potentials. Daniell cell: Zn + Cu²⁺ → Zn²⁺ + Cu, E° = +1.10 V.
What's electrolysis?
Non-spontaneous redox driven by external voltage. Battery applied; opposite of galvanic operation. Examples: (1) Water electrolysis: 2H₂O → 2H₂ + O₂ (V_min = 1.23 V). (2) Electroplating: deposit metal onto object. (3) Aluminum production: Al³⁺ + 3e⁻ → Al (Hall-Héroult process). (4) Chlor-alkali: NaCl(aq) → NaOH + Cl₂ + H₂.
How is cell potential calculated?
E°cell = E°cathode - E°anode (using standard reduction potentials). Positive E°cell: spontaneous (galvanic). Negative: non-spontaneous (electrolytic). Standard reduction potentials measured against SHE (Standard Hydrogen Electrode, E° = 0). Tables: F₂/F⁻ = +2.87 (best oxidizer), Li⁺/Li = -3.04 V (best reducer).
What's the Nernst equation?
Relates cell potential to concentrations. E = E° - (RT/nF) ln(Q). Where E° is standard potential, n is electrons transferred, F is Faraday constant, Q is reaction quotient. At 25°C: E = E° - (0.0592/n) log Q. Used to: predict voltage at non-standard conditions, design pH meters, ion-selective electrodes.
What's a fuel cell?
Electrochemical cell where reactants continuously supplied. H₂ fuel cell: H₂ + ½O₂ → H₂O + electricity. No combustion (more efficient). Used in: spacecraft, vehicles (Toyota Mirai), backup power. Different types: PEM (low T, transportation), SOFC (high T, stationary). Promise: clean energy if H₂ from renewable sources.
How is corrosion electrochemical?
Iron corrodes in moist air via redox. Anode (different parts of metal): Fe → Fe²⁺ + 2e⁻ (oxidation). Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (reduction). Forms Fe(OH)₂ → Fe₂O₃·xH₂O (rust). Prevention: galvanizing (zinc coating, sacrificial), painting, alloying (stainless steel), cathodic protection.
What's electroplating?
Electrolytic deposition of metal layer onto object. Object connected to cathode (reduction). Anode is metal to be deposited. Solution contains metal ions. Apply voltage; metal ions reduce on object. Examples: chrome plating, gold plating, silverware, copper coating circuit boards. Controls thickness, smoothness.