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Electrochemistry Overview

Jul 30, 2025

Overview

This lecture covers the essential concepts of electrochemistry, focusing on energy conversions, cell types, electrode potentials, conductivity, electrolytes, major equations (Nernst, Faraday), and key applications like electrolysis and corrosion.

Introduction to Electrochemistry

  • Electrochemistry studies the conversion of chemical energy to electrical energy and vice versa.
  • Two main processes: generating electricity from chemicals and causing chemical changes using electricity.

Types of Cells

  • Electrochemical (Galvanic/Voltaic/Daniel) cells convert chemical energy into electrical energy.
  • Electrolytic cells use electrical energy to drive chemical changes.

Electrochemical Cell Structure and Function

  • A typical cell uses copper sulfate solution, zinc rod (anode, oxidizes), and copper rod (cathode, reduces), connected via a salt bridge.
  • Salt bridge (containing KCl or KNO₃ in agar/gelatin) maintains electrical neutrality and completes the circuit.
  • Electron flow: zinc to copper; current flows: copper to zinc.

Electrode Potential

  • Electrode potential arises from charge separation between electrode and solution.
  • Standard electrode potential (E⁰): Measured at 1M concentration.
  • Cell potential: E⁰(cell) = E⁰(right, reduction) – E⁰(left, oxidation).

Standard Hydrogen Electrode (SHE)

  • SHE acts as a reference electrode with 0 volts, using H₂ gas at 1 bar and 1M HCl.
  • SHE can serve as either an anode or cathode.
  • Disadvantages: Difficult to maintain conditions, easily contaminated.

Nernst Equation

  • Used when concentrations ≠ 1M and temperature = 298K.
  • E(cell) = E⁰(cell) – (0.0591/n) × log(QC), where QC is the reaction quotient (products/reactants, ions only).
  • N = number of electron moles transferred in the balanced reaction.

Conductance and Conductivity

  • Conductance (G) = 1/Resistance (R); conductivity (κ) = 1/resistivity (ρ).
  • Cell constant (L/A) relates to the geometry of the vessel; conductivity = conductance × cell constant.

Electrolytes and Their Types

  • Electrolytes produce ions in solution.
  • Strong electrolytes: completely dissociate (e.g., NaCl).
  • Weak electrolytes: partially dissociate.

Conductivity and Molar Conductivity

  • Conductivity decreases with dilution due to fewer ions per unit volume.
  • Molar conductivity (Λm) = conductivity/molarity; increases with dilution due to increased volume.

Variation with Dilution

  • Strong electrolytes: Λm increases slightly with dilution (due to increased ion mobility).
  • Weak electrolytes: Λm increases sharply with dilution (due to increased dissociation).

Kohlrausch’s Law

  • At infinite dilution, molar conductivity is the sum of individual ion conductivities.
  • Λm⁰(electrolyte) = Λm⁰(cation) + Λm⁰(anion).

Degree of Dissociation (Alpha)

  • α = Λm (at given concentration) / Λm⁰ (infinite dilution).
  • Indicates the fraction of electrolyte dissociated.

Electrolysis and Faraday’s Laws

  • Electrolysis uses an electrolytic cell (anode: positive, cathode: negative) to drive chemical changes by electricity.
  • Product of electrolysis depends on charge supplied.
  • Faraday’s First Law: Mass deposited ∝ charge passed (W = ZIt, Z = electrochemical equivalent).
  • Faraday’s Second Law: When cells are in series, deposited mass ∝ equivalent weight.

Gibbs Free Energy and Cell Potential

  • ΔG = – nFE (E from Nernst eq.); ΔG⁰ = – nFE⁰.
  • F (Faraday constant) ≈ 96500 C/mol e⁻.

Corrosion and Rusting

  • Rusting is the deterioration of metals in presence of air and water.
  • Prevention: painting, greasing, galvanization (zinc coating as a sacrificial layer).

Key Terms & Definitions

  • Electrochemical cell — Device converting chemical energy to electrical energy.
  • Electrolytic cell — Device using electricity to drive chemical change.
  • Salt bridge — U-shaped tube with electrolyte for circuit completion and neutrality.
  • Standard Hydrogen Electrode (SHE) — Reference electrode with 0V potential.
  • Nernst equation — Calculates cell potential under non-standard conditions.
  • Conductivity (κ) — Ability of a solution to conduct electricity.
  • Molar conductivity (Λm) — Conductivity per unit molarity.
  • Kohlrausch’s Law — Molar conductivity at infinite dilution equals sum of ion conductivities.
  • Degree of dissociation (α) — Fraction of electrolyte molecules dissociated into ions.
  • Faraday’s laws — Quantitative laws of electrolysis linking charge and deposited mass.
  • Corrosion/Galvanization — Metal deterioration/prevention by coating with reactive zinc.

Action Items / Next Steps

  • Review and memorize products and reactions for electrolysis (especially from NCERT).
  • Practice calculation problems on Nernst equation, Faraday’s laws, and cell representation.
  • Study battery reactions and rusting/corrosion methods from supplementary materials or provided videos.

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