Electrochemistry

Notes

Chapter 2: Electrochemistry

Electrochemistry deals with the interconversion of electrical energy and chemical energy.

• Chemical energy can be converted into electrical energy.
• Electrical energy can also be converted into chemical energy.
• The device used for these conversions is called an electrochemical cell.

1. Conductors

A conductor is a substance through which electric current can pass.

Conductors are mainly of two types:

• Metallic Conductors: Metals are common examples. The charge carriers are free electrons.
• Electrolytic Conductors: Electrolyte solutions such as solutions of strong bases, acids and salts conduct electricity through ions.

Metallic Conductors

• Charge is carried by free electrons.
• With increase in temperature, resistance generally increases because vibration of atoms increases.

Electrolytic Conductors

• Charge is carried by ions.
• With increase in temperature, interionic attraction decreases and mobility of ions increases.

2. Electrolytic Conductance

Resistance (R)

Resistance is the property of an electrolytic solution which opposes the flow of electric current.

Ohm's Law:

V = IR

Therefore,

R = V/I

Unit: ohm (Ω)

Conductance (G)

Conductance is the property of an electrolytic solution which supports the flow of electric current.

It is the reciprocal of resistance.

G = 1/R

Unit: siemens (S) or Ω⁻¹

Resistivity or Specific Resistance (ρ)

For a conductor:

R ∝ l

R ∝ 1/A

Therefore,

R = ρl/A

Hence,

ρ = RA/l

where:

• R = resistance
• l = distance between the two electrodes
• A = area of cross-section of the electrodes
• ρ = resistivity or specific resistance

Unit: Ω cm

Resistivity represents the resistance of an electrolytic solution when the dimensions are taken under specified conditions.

Cell Constant

The ratio of distance between the electrodes to the area of cross-section of the electrodes is called the cell constant.

Cell constant = l/A

Unit: cm⁻¹

Conductivity or Specific Conductance (κ)

Conductivity is the reciprocal of resistivity.

κ = 1/ρ

Since:

ρ = RA/l

Therefore:

κ = l/(RA)

or

κ = G × l/A

Therefore:

κ = Conductance × Cell Constant

Unit: S cm⁻¹

Conductivity represents the conductance of a given volume of an electrolytic solution under specified geometrical conditions.

3. Molar Conductivity (Λ_m)

Molar conductivity represents the conductance of all the ions produced by one mole of an electrolyte present in a given volume of solution.

If one mole of electrolyte is present in V cm³ of solution: