Simulation · Chemistry · Class 12

Why does molar conductivity rise on dilution?

From the lesson Molar conductivity and dilution in Electrochemistry. Change the values and watch what happens.

The idea behind it

NCERT §2.4.2

Molar conductivity Λm = κ/c. With κ in S m⁻¹ and c in mol m⁻³, Λm is in S m² mol⁻¹; with κ in S cm⁻¹, Λm (S cm² mol⁻¹) = κ × 1000/molarity. 1 S cm² mol⁻¹ = 10⁻⁴ S m² mol⁻¹, and 1 mol L⁻¹ = 1000 mol m⁻³.
Worked: a cell reads 100 Ω with 0.1 M KCl (κ = 1.29 S m⁻¹), so G* = 129 m⁻¹; with 0.02 M KCl it reads 520 Ω, so κ = 0.248 S m⁻¹ and Λm = 0.248/20 = 124 × 10⁻⁴ S m² mol⁻¹.
On dilution κ always falls, for strong and weak electrolytes alike, because fewer ions are left in each unit volume.
Λm rises on dilution. Λm is the conductance of the whole volume holding 1 mol of electrolyte, set between electrodes 1 unit apart; that volume grows faster than κ shrinks.
The limit of Λm as c → 0 is the limiting molar conductivity, Λ°m.
Strong electrolytes: Λm rises slowly and Λm = Λ°m − A c^½, a straight line against √c with intercept Λ°m and slope −A. A depends on the solvent, temperature and electrolyte type (NaCl 1-1, CaCl₂ 2-1, MgSO₄ 2-2), and is the same for all electrolytes of one type.
Worked (KCl): the Λm–√c line gives Λ°m = 150.0 S cm² mol⁻¹ and A = 87.46 S cm² mol⁻¹ (mol L⁻¹)^−½.
Weak electrolytes such as acetic acid: Λm stays low at ordinary concentrations and shoots up steeply near zero, because the degree of dissociation grows on dilution. The curve cannot be extrapolated to find Λ°m.

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