Chapter 2 / 2.6 Batteries

🔋 What is a Battery?

A battery is one or more galvanic cells wired in series so the chemical energy of a redox reaction changes straight into electrical energy. Good batteries stay light, compact, and keep a steady voltage while they work. 😊

Two Big Families of Batteries

Primary batteries – the redox reaction runs just once, so you toss the battery after it discharges. ⚡
Secondary batteries – you can push current the opposite way to recharge them again and again. 🔄

🟢 Primary Batteries

Dry Cell (Leclanché Cell)

Setup: a zinc can (anode) surrounds a graphite rod mixed with MnO₂ and carbon (cathode). A moist paste of NH₄Cl + ZnCl₂ fills the gap.
Anode reaction: \( \mathrm{Zn(s) \rightarrow Zn^{2+} + 2e^-} \)
Cathode reaction: \( \mathrm{MnO_2 + NH_4^{+} + e^- \rightarrow MnO(OH) + NH_3} \)
NH₃ ties up Zn²⁺ as \( \mathrm{[Zn(NH_3)_4]^{2+}} \), keeping the voltage steady.
Typical voltage ≈ 1.5 V. 🔋

Mercury Cell

Perfect for tiny, low-current gadgets like watches and hearing aids.
Anode: \( \mathrm{Zn(Hg) + 2OH^- \rightarrow ZnO + H_2O + 2e^-} \)
Cathode: \( \mathrm{HgO + H_2O + 2e^- \rightarrow Hg(l) + 2OH^-} \)
Overall: \( \mathrm{Zn(Hg) + HgO \rightarrow ZnO + Hg(l)} \)
Voltage holds at about 1.35 V throughout its life. 😊

🔵 Secondary Batteries

Lead Storage Battery (Car Battery)

Electrodes: lead grid (anode) and a lead-dioxide-pasted grid (cathode) dipped in 38 % H₂SO₄.
Discharge – Anode: \( \mathrm{Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^-} \)
Discharge – Cathode: \( \mathrm{PbO_2 + SO_4^{2-} + 4H^{+} + 2e^- \rightarrow PbSO_4 + 2H_2O} \)
Overall during use: \( \mathrm{Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O} \)
Charging flips these reactions so PbSO₄ turns back into Pb and PbO₂. 🔁

Nickel–Cadmium (Ni–Cd) Cell

Lasts longer than lead cells but costs more.
Overall discharge: \( \mathrm{Cd + 2Ni(OH)_3 \rightarrow CdO + 2Ni(OH)_2 + H_2O} \)

Quick Recap ⚡

Batteries convert chemical energy straight into electrical energy.
Primary cells (dry cell, mercury cell) are single-use; secondary cells (lead, Ni–Cd) are rechargeable.
Voltage stays steady in a mercury cell because no ions in solution change concentration.
In a lead cell, sulfuric-acid concentration falls as PbSO₄ forms during discharge—and rises again on charging.

Important Concepts for NEET ✅

Identify anode and cathode reactions in dry, mercury, and lead storage cells quickly in MCQs.
Recall standard voltages: dry cell ≈ 1.5 V, mercury cell ≈ 1.35 V, each lead cell unit ≈ 2 V.
Explain why secondary batteries recharge: the redox steps are fully reversible.
Link NH₃ complex formation in the dry cell to removal of Zn²⁺ and stable voltage.