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ELECTROLYTES AND IONIC CONDUCTION

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Electrolytes and ionic conduction

Debye-Hückel theory

• Interaction between ions (++ / + ̶ / ̶ ̶ )

• Debye-Hückel theory

Ionic conductivity

• Ionic conductivity / transport number

• Change of conductivity with concentration

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Introduction

Electrolyte: medium of ionic movement

: electrolyte solution: ionic salts dissolved by solvents

: ions in a electrolyte solution moves when electric field is applied

: ionic mobility of the electrolyte is a key factor of battery performance

Activity

: aj=γjcj: activity coefficient (γj) is 1 in a infinitely dilute

solution, decreases as the ionic concentration increases, and increases again

Interactions in an ionic solution

: ion-ion interaction, ion-solvent interaction

: electrostatic attraction or repulsion among ions

: ion-dipole interaction between ion and solvent

Degree of dissociation (α): ionization ratio of an electrolyte ex) MX → M+ + X- in a aqueous solution

: α = [M+] / [MX]initial

→ strong / weak electrolyte

1. Electrolyte

2. Activity coefficient

3. Interactions in electrolyte solution

4. Dissociation rate

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Debye Hückel theory

Debye Hückel theory

: decrease of activity coefficient of ions by ion-ion interaction (stabilization of ions by + ̶ ionic interaction)

Debye length

: radius of a sphere in which charge of an ion is completely compensated by neighboring opposite ions

: Debye length is inversely proportional to ionic strength

Ionic strength is defined as

: I=1/2Σzj2cj

: increase of ionic strength – decrease of Debye length – increase of attractive interaction – stabilization of ions – decrease of activity coefficient

Ion – dipole(solvelt) interaction

ex) hydration by water molecules in an aqueous solution

: this also stabilizes ions, and moves together

: ion-dipole interaction between ion and solvent

In a solution, ions of opposite charge coexist in general

: mean activity coefficient is defined and often used

1. Debye Hückel theory

2. Debye length

3. Ionic strength

4. Effect of solvent

5. Mean activity coefficient

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Ionic conductivity

Conduction in an electrolyte solution

: ions carry charge (electron can’t flow)

: both of ions (+,-) contribute to the conduction

Ionic conductivity

Transport number

: “contributory portion” of an ion to conduction

Relaxation effect

: ionic movement in a field is hindered by opposite ions

: this increases with increase of ionic concentration

Viscosity of solution affects ionic mobility

: viscosity increases with concentration in general

As concentration of ions increases, number of carrier increases but ionic mobility decreases by relaxation effect and increase of viscosity

1. Conduction of charge in an electrolyte solution

2. Ionic conductivity

3. Transport number

4. Relaxation effect / viscosity

5. Change of ionic conductivity with concentration

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Electrolyte and interface Electrolyte: medium of ionic movement

: ions in a electrolyte solution moves when electric field is applied

: ionic mobility of the electrolyte is a key factor of battery performance

Degree of dissociation (α): ionization ratio of an electrolyte ex) MX → M+ + X- in a aqueous solution

: α = [M+] / [MX]initial

equivalent conductance: Λ[S cm2 eq-1]

Conductance at infinite dilution: Λ∞

→ α at a given concentration= Λ / Λ∞

Λ∞ = Λ+∞ + Λ-

∞ (independent motion of ions in a infinitely dilute electrolyte solution)

Electrostatic interaction exists in actual solutions

→ activity, instead of concentration is used as an effective factor: a = γc where γ → 1 for infinite dilution

1. Dissociation rate

2. Ionic mobility / conductivity

3. Ionic activity / Ionic strength