AITestSeriesAITestSeries
Syllabus
🩺 NEET UG
Subject:Chemistry Syllabus

Electrochemistry

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

This topic extends redox and cell concepts to quantify solution behaviour and practical devices — conductance and molar conductivity via Kohlrausch's law, cell potential's dependence on concentration via the Nernst equation, real galvanic devices like batteries and fuel cells, and the electrochemical process of corrosion along with how it is prevented.

Chemistry Syllabus Physical Chemistry ~25% weightage
Live Animation1 / 4
15
Strong Electrolyte Λm Rise
85
Weak Electrolyte Λm Rise
100
Λm° (Limiting, Both)

1Conductance & Kohlrausch's Law

Conductance measures how easily current flows through an electrolyte solution; Kohlrausch's law shows that at infinite dilution, each ion contributes its own fixed, independent share to the total molar conductivity.

Learning Objectives

  • Define specific conductance (κ), molar conductivity (Λm), and their relationship Λm = 1000κ/C.
  • Explain why molar conductivity increases with dilution for both strong and weak electrolytes, and why the trend differs between them.
  • State Kohlrausch's law of independent migration of ions and use it to calculate Λm° for a weak electrolyte.
  • Use Λm/Λm° to calculate the degree of dissociation (α) of a weak electrolyte.

Key Terms

Specific Conductance (κ)The conductance of a 1 cm cube (unit volume) of solution; the reciprocal of specific resistance.
Molar Conductivity (Λm)The conducting power of all ions produced by one mole of electrolyte, Λm = 1000κ/C (C in mol/L).
Limiting Molar Conductivity (Λm°)The molar conductivity at infinite dilution, where ionic interactions vanish and each ion conducts fully independently.
Kohlrausch's LawAt infinite dilution, the limiting molar conductivity of an electrolyte equals the sum of the independent limiting ionic conductivities of its cation and anion, weighted by stoichiometry.

Conductance is the electrical mirror of a solution's ion content and mobility — the more ions present, and the more freely they move, the better the solution conducts.

κ = conductance × (l/A) Λm = 1000κ / C (κ in S/cm, C in mol/L, Λm in S cm² mol⁻¹)

Why Λm rises on dilution

  • Strong electrolytes (e.g. NaCl, KCl) — fully dissociated even at moderate concentration; Λm rises only slightly on dilution as inter-ionic attraction weakens, allowing greater ion mobility.
  • Weak electrolytes (e.g. CH3COOH) — degree of dissociation itself increases sharply on dilution, so Λm rises steeply and only approaches Λm° asymptotically.

Kohlrausch's law

Λm°(electrolyte) = ν+λ°+ + ν−λ°− (ν = ions per formula unit, λ° = limiting ionic conductivity)
Exam TipKohlrausch's law's biggest use is finding Λm° for a WEAK electrolyte (which can't be found by direct extrapolation) by combining Λm° values of strong electrolytes sharing its ions, e.g. Λm°(CH3COOH) = Λm°(CH3COONa) + Λm°(HCl) − Λm°(NaCl).
Common MistakeFor strong electrolytes, Λm vs √C is linear so Λm° can be found by direct extrapolation to C→0; this fails for weak electrolytes, whose Λm shoots up sharply near C=0 — always use Kohlrausch's law instead.
α = Λm / Λm° (degree of dissociation of a weak electrolyte at concentration C)