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NCERT Class 12
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1 Solutions 2 Electrochemistry 3 Chemical Kinetics 4 The d-and f-Block Elements 5 Coordination Compounds 6 Haloalkanes and Haloarenes 7 Alcohols, Phenols and Ethers 8 Aldehydes, Ketones and Carboxylic Acids 9 Amines 10 Biomolecules

1 Solutions

1.1 Types of Solutions 1.2 Expressing Concentration of Solutions 1.3 Solubility 1.4 Vapour Pressure of Liquid Solutions 1.5 Ideal and Non-ideal Solutions 1.6 Colligative Properties and Determination of Molar Mass 1.7 Abnormal Molar Masses 2.1 Electrochemical Cells 2.2 Galvanic Cells 2.3 Nernst Equation 2.4 Conductance of Electrolytic Solutions 2.5 Electrolytic Cells and Electrolysis 2.6 Batteries 2.7 Fuel Cells 2.8 Corrosion

Batteries

NCERT Reference: Chapter 2 – Electrochemistry – Pages 50–51

Quick Notes

  • Batteries are devices consisting of one or more galvanic cells used to generate electrical energy.
  • Primary batteries: Non-rechargeable; used once (e.g. dry cell).
  • Secondary batteries: Rechargeable; can be reused multiple times (e.g. lead storage battery).

Full Notes

A battery is essentially a device that converts chemical energy into electrical energy through redox reactions.

It consists of one or more electrochemical cells connected in series or parallel. Batteries are broadly classified into two types: primary (non-rechargeable) and secondary (rechargeable).

Primary Batteries

Primary batteries are designed for single-use applications. Once the chemical reaction completes and all reactants are used up, they cannot be recharged. The most familiar example is the dry cell.

Dry Cell (Leclanché Cell)

Chemistry NCERT Class 12 dry cell diagram showing zinc anode can, NH4Cl and ZnCl2 paste electrolyte, and MnO2-carbon cathode mix around a carbon rod.

Construction:

  • Zinc container: Acts as the anode.
  • A paste of NH4Cl and ZnCl2: Acts as the electrolyte.
  • A carbon rod surrounded by manganese dioxide (MnO2) and carbon powder: Cathode.

Reactions:

  • Anode: Zn → Zn2+ + 2e−
  • Cathode: MnO2 + NH4+ + e− → MnO(OH) + NH3

Features:

  • Inexpensive.
  • Portable.
  • Cannot be recharged.
  • Used in flashlights, clocks, and remotes.

Mercury Cell (Used in hearing aids, watches)

Chemistry NCERT Class 12 mercury cell schematic with zinc amalgam anode, KOH–ZnO paste electrolyte, and HgO cathode giving a near-constant EMF.
  • Electrolyte: Paste of KOH–ZnO
  • Anode: Zinc amalgam — Zn(Hg) → Zn2+ + 2e−
  • Cathode: HgO + H2O + 2e− → Hg(l) + 2OH−
  • EMF: ~1.35 V (constant throughout use)

Secondary Batteries

Secondary (or rechargeable) cells are electrochemical cells in which the redox reactions are reversible.

The cell can be recharged by passing an electric current in the opposite direction to when the cell is in normal use. Common examples include lead–acid car batteries and lithium-ion batteries (such as used in mobile phones and laptops).

Discharge vs. Charging Reactions

To work out a charging reaction:

Example: Lead–Acid Battery

Chemistry NCERT Class 12 lead–acid battery diagram with Pb anode, PbO2 cathode, and H2SO4 electrolyte indicating discharge and charge processes.

Discharge reactions:

  • Anode (oxidation): Pb(s) + SO42−(aq) → PbSO4(s) + 2e−
  • Cathode (reduction): PbO2(s) + 4H+(aq) + SO42−(aq) + 2e− → PbSO4(s) + 2H2O(l)

Charging reactions:
Reverse both reactions using an external voltage source.

Nickel–Cadmium (Ni–Cd) Battery

Chemistry NCERT Class 12 Ni–Cd battery layout with Cd anode, NiO(OH) cathode and KOH electrolyte showing overall discharge reaction.
  • Anode: Cadmium (Cd)
  • Cathode: Nickel(III) oxide (NiO(OH))
  • Electrolyte: KOH
  • Discharge reaction: Cd + 2NiO(OH) + 2H2O → Cd(OH)2 + 2Ni(OH)2
  • Rechargeable, but more expensive than lead–acid.
  • Used in: Cameras, mobile devices (older models).

Summary