Class 12 Chemistry Electrochemistry

Class 12 Chemistry Electrochemistry

2.1 Electrochemical Cells

An electrochemical cell is a device that converts chemical energy into electrical energy through redox reactions.

Main Components

  • Anode: Oxidation occurs
  • Cathode: Reduction occurs
  • Electrolyte: Conducts ions
  • External circuit: Conducts electrons

✔ Oxidation always occurs at anode
✔ Reduction always occurs at cathode

2.2 Galvanic (Voltaic) Cells

A galvanic cell produces electrical energy from a spontaneous redox reaction.

Example: Daniell Cell

  • Anode: Zinc rod in ZnSO₄
  • Cathode: Copper rod in CuSO₄

Cell Representation

Zn | Zn2+∣∣Cu2+∣Cu\text{Zn | Zn}^{2+} || \text{Cu}^{2+} | \text{Cu}Zn | Zn2+∣∣Cu2+∣Cu

Cell Reaction

  • Oxidation:

Zn→Zn2++2e−\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-Zn→Zn2++2e−

  • Reduction:

Cu2++2e−→Cu\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}Cu2++2e−→Cu

2.3 Nernst Equation

The Nernst equation relates cell potential to concentration.

General Form

E=E∘−0.0591nlog⁡QE = E^\circ – \frac{0.0591}{n} \log QE=E∘−n0.0591​logQ

Where:

  • E∘E^\circE∘ = Standard electrode potential
  • n = Number of electrons transferred
  • Q = Reaction quotient

At Equilibrium

E=0E = 0E=0

This equation is used to calculate:

  • Cell potential
  • Equilibrium constant
  • pH of solutions

2.4 Conductance of Electrolytic Solutions

Electrical Conductance (G)

Ability of solution to conduct electricity.G=1RG = \frac{1}{R}G=R1​

Specific Conductance (κ)

Conductance of solution of unit length and unit area.

Molar Conductance (Λₘ)

Λm=κ×1000M\Lambda_m = \frac{\kappa \times 1000}{M}Λm​=Mκ×1000​

Variation with Concentration

  • Increases with dilution
  • Strong electrolytes: Gradual increase
  • Weak electrolytes: Sharp increase

2.5 Electrolytic Cells and Electrolysis

An electrolytic cell uses electrical energy to drive a non-spontaneous reaction.

Electrolysis

Chemical decomposition of electrolyte by passing electric current.

Faraday’s Laws of Electrolysis

First Law

Mass deposited ∝ Quantity of electricity passed.

Second Law

Mass deposited ∝ Equivalent weight of substance.m=QF×Em = \frac{Q}{F} \times Em=FQ​×E

Where:

  • F = Faraday constant (96500 C)

2.6 Batteries

A battery is a combination of electrochemical cells.

Primary Batteries

  • Cannot be recharged
  • Example: Dry cell

Secondary Batteries

  • Rechargeable
  • Example: Lead-acid battery

Lead-Acid Battery

  • Used in automobiles
  • Electrodes: Lead and lead dioxide
  • Electrolyte: Dilute sulphuric acid

2.7 Fuel Cells

A fuel cell converts chemical energy directly into electrical energy.

Hydrogen-Oxygen Fuel Cell

  • Fuel: Hydrogen
  • Oxidant: Oxygen
  • Product: Water

Advantages

  • High efficiency
  • Environment friendly
  • Continuous operation

2.8 Corrosion

Corrosion is the gradual destruction of metals due to chemical or electrochemical reactions.

Example

Rusting of iron:4Fe+3O2+6H2O→4Fe(OH)34Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_34Fe+3O2​+6H2​O→4Fe(OH)3​

Prevention of Corrosion

  • Painting
  • Galvanization
  • Cathodic protection
  • Alloy formation