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આ MCQ મોડ્યુલ આના પર આધારિત છે: Batteries Fuel Cells Corrosion
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Batteries Fuel Cells Corrosion
🎓 Class 12
Chemistry
CBSE
Theory
Ch 2 – Electrochemistry
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Batteries Fuel Cells Corrosion
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Batteries Fuel Cells Corrosion
2.6 Batteries — Electrochemistry in Your Pocket
A battery is simply a galvanic cell — or several joined in series — packaged to deliver usable current for a long time. Commercial batteries must satisfy three demands at once: a reasonably high cell voltage, a constant voltage during discharge (not a slow fade), and a compact, leak-proof design. They fall into two broad groups.
2.6.1 Primary Batteries (Single-Use)
In a primary battery, the redox reactants run to completion and cannot be regenerated from within — once exhausted, the cell is dead and must be discarded.
(a) Dry cell / Leclanché cell (Zn–C)
The common 1.5 V "dry cell" powers TV remotes and wall clocks. The metal casing is the zinc anode; the central graphite rod surrounded by a MnO₂ + carbon paste is the cathode. A moist paste of NH₄Cl + ZnCl₂ acts as the electrolyte.
\(\text{Anode: Zn}(s) \to \text{Zn}^{2+} + 2e^{-}\)
\(\text{Cathode: MnO}_{2}+\text{NH}_{4}^{+}+e^{-} \to \text{MnO(OH)} + \text{NH}_{3}\)
EMF ≈ 1.5 V. The voltage slowly falls during use as NH₃ coats the cathode and product salts accumulate.
(b) Mercury cell
A miniature button cell used in hearing aids, pacemakers and wrist-watches. Anode: Zn amalgam (Zn–Hg). Cathode: HgO paste with carbon. Electrolyte: KOH with ZnO paste.
\(\text{Anode: Zn(Hg)} + 2\text{OH}^{-} \to \text{ZnO}(s) + \text{H}_{2}\text{O} + 2e^{-}\)
\(\text{Cathode: HgO}(s) + \text{H}_{2}\text{O} + 2e^{-} \to \text{Hg}(\ell) + 2\text{OH}^{-}\)
Net: Zn(Hg) + HgO → ZnO + Hg. EMF ≈ 1.35 V, and it stays remarkably constant until the cell is almost fully discharged — vital for medical devices.
2.6.2 Secondary Batteries (Rechargeable)
A secondary battery stores electricity chemically — on charging, the current forces the reactions to run backward, restoring the original reactants.
(a) Lead storage battery
The familiar 12 V car battery. Six cells of 2 V each are stacked in series; the electrolyte is 38% H₂SO₄.
- Anode (oxidation during discharge): Pb(s) + SO₄²⁻ → PbSO₄(s) + 2e⁻
- Cathode (reduction during discharge): PbO₂(s) + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄(s) + 2H₂O
- Net discharge: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
On charging, an external source pushes the reaction backward: 2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄. EMF per cell ≈ 2 V; the density of the acid falls during discharge (old cars used a float hydrometer to tell). Used in cars, inverters, emergency power.
(b) Nickel–cadmium (NiCd) cell
Longer cycle life than lead-acid and higher current. The net discharge reaction is:
\(\text{Cd}(s) + 2\text{Ni(OH)}_{3}(s) \to \text{CdO}(s) + 2\text{Ni(OH)}_{2}(s) + \text{H}_{2}\text{O}\)
Used in power tools, older cordless phones. More expensive; cadmium is toxic — largely superseded by Li-ion today.
(c) Lithium-ion batteries
The reigning champion for phones, laptops and electric vehicles. A layered graphite anode and a layered metal-oxide (e.g., LiCoO₂) cathode shuttle Li⁺ ions through a non-aqueous electrolyte. Extremely high energy density (≈ 150 Wh kg⁻¹), very low self-discharge, and 1000+ charge cycles make them the workhorse of modern electronics and EVs.
2.7 Fuel Cells
A fuel cell is a galvanic cell in which the reactants are continuously supplied from outside rather than stored inside. As long as fuel flows in, electricity flows out. The classic example is the H₂–O₂ fuel cell developed for NASA's Apollo missions and now revived as a clean-energy option for vehicles and stationary power.
\(\text{Anode: 2H}_{2}(g) + 4\text{OH}^{-} \to 4\text{H}_{2}\text{O}(\ell) + 4e^{-}\)
\(\text{Cathode: O}_{2}(g) + 2\text{H}_{2}\text{O}(\ell) + 4e^{-} \to 4\text{OH}^{-}\)
\(\text{Net: 2H}_{2}(g) + \text{O}_{2}(g) \to 2\text{H}_{2}\text{O}(\ell) + \text{electricity}\)
The electrodes are porous carbon plates impregnated with Pt or Pd catalyst; the electrolyte is concentrated KOH. Efficiency: ~70%, compared with ~40% for a coal-fired thermal plant. The only product is pure water (drunk by the Apollo astronauts!). Because no combustion occurs, no NOx, SOx or particulates form — fuel cells are central to the proposed Hydrogen Economy: hydrogen generated by electrolysis of water using solar/wind power could be stored, transported, and used in fuel cells to power vehicles, with water as the sole exhaust.
2.8 Corrosion — When Electrochemistry Works Against Us
Iron pillars rust, silver spoons tarnish, copper domes go green. All these are corrosion — the slow, spontaneous oxidation of a metal by its environment. In India alone, corrosion destroys material worth billions of rupees each year. The remedy begins with understanding the chemistry.
2.8.1 Electrochemical Theory of Rusting
A drop of slightly acidic (CO₂-containing) water sitting on an iron surface sets up a miniature galvanic cell. At the anode region (usually the centre of the drop or an impurity site), iron oxidises:
\(\text{Anode: Fe}(s) \to \text{Fe}^{2+} + 2e^{-}\)
The electrons travel through the metal itself to the edge of the drop, where atmospheric O₂ plus H⁺ ions act as the cathode:
\(\text{Cathode: O}_{2} + 4\text{H}^{+} + 4e^{-} \to 2\text{H}_{2}\text{O}\)
The dissolved Fe²⁺ is then further oxidised by atmospheric O₂ to Fe³⁺, which combines with water to form the reddish-brown hydrated oxide called rust:
\(4\text{Fe}^{2+} + \text{O}_{2} + (4+2x)\text{H}_{2}\text{O} \to 2\text{Fe}_{2}\text{O}_{3}\!\cdot\!x\text{H}_{2}\text{O} + 8\text{H}^{+}\)
2.8.2 Prevention of Corrosion
- Barrier protection: paint, grease, oil, lacquer, or a polymer coating keeps oxygen and water away. Effective only so long as the film is intact.
- Galvanisation: iron sheets and nails are coated with a thin layer of zinc (E° = −0.76 V, below Fe's −0.44 V). Zn oxidises in preference to Fe, and even when the coat is scratched, Fe does not rust as long as Zn remains.
- Alloying: stainless steel (Fe + ~18% Cr + ~8% Ni) forms a self-repairing Cr₂O₃ passive layer; it resists rust indefinitely in normal atmospheres.
- Sacrificial anode (cathodic protection): a block of Mg or Zn is bolted to an iron pipeline, ship's hull, or water tank. The more reactive metal corrodes first (is "sacrificed"), protecting the iron that lies downstream in the galvanic series. Replace the sacrificial anode every few years.
- Electroplating with a less reactive but durable metal (chromium, nickel, tin): used on car bumpers and on the inside of tin cans.
Worked Examples — Batteries, Fuel Cells & Corrosion
Example 2.16 — Cell EMF of lead storage battery
Given E°(PbO₂/PbSO₄) = +1.685 V and E°(PbSO₄/Pb) = −0.356 V, compute the EMF of a single lead-acid cell.
\(E^{\circ}_{cell} = E^{\circ}_{cathode} - E^{\circ}_{anode} = 1.685 - (-0.356) = 2.041\,\text{V} \approx 2\,\text{V}\)
Answer: ≈ 2 V per cell; six cells in series give the nominal 12 V of a car battery.
Example 2.17 — Charge needed to plate 10 g of Al
What quantity of electric charge is required to deposit 10.0 g of Al from molten Al₂O₃?
n(Al) = 10/27 = 0.370 mol; Al³⁺ needs 3 e⁻ per ion → mol e⁻ = 1.111.
\(Q = 1.111 \times 96500 = 1.072 \times 10^{5}\,\text{C}\)
Answer: ≈ 1.07 × 10⁵ C.
Example 2.18 — Rate of corrosion from mass loss
A steel nail lost 0.012 g of Fe in 30 days. The Fe²⁺/Fe couple has n = 2. What average current corresponds to this rusting?
mol Fe lost = 0.012/56 = 2.143 × 10⁻⁴ mol; mol e⁻ = 4.286 × 10⁻⁴; Q = 4.286 × 10⁻⁴ × 96500 = 41.36 C.
Time = 30 × 24 × 3600 = 2.592 × 10⁶ s. I = Q/t = 41.36/2.592×10⁶ = 1.60 × 10⁻⁵ A (16 μA).
Answer: About 16 μA of "corrosion current" continuously trickles through the nail — invisible, but over years it will destroy it.
Example 2.19 — Why the mercury cell is stable
Explain quantitatively why the mercury cell keeps a nearly constant 1.35 V throughout its life, whereas a dry cell drops steadily from 1.5 V.
In the mercury cell the net reaction Zn + HgO → ZnO + Hg involves only solids and a liquid metal — none of the species' concentrations change during discharge. Applying the Nernst equation, Q stays essentially equal to 1, so E = E° throughout. In the dry cell, NH₄⁺ is consumed and NH₃/ZnCl₂ accumulate, so Q increases steadily and the Nernst correction (0.0591/n) log Q pulls the voltage down.
Answer: Constant activities of reactants/products → constant EMF.
Example 2.20 — ΔG° for H₂–O₂ fuel cell
The H₂–O₂ fuel cell delivers E°cell = 1.23 V. Compute ΔG° for 2H₂ + O₂ → 2H₂O (n = 4 per unit reaction).
\(\Delta G^{\circ} = -nFE^{\circ} = -(4)(96500)(1.23) = -474\,780\,\text{J} = -474.8\,\text{kJ}\)
Answer: ΔG° ≈ −475 kJ per 2 mol H₂ burned — very close to the enthalpy of combustion, showing the near-100% theoretical efficiency of the fuel cell.
Activity 2.3 — Demonstrate Sacrificial Protection with Iron Nails
L3 Apply
Predict: If you keep one clean iron nail alone in salt water, another wrapped in zinc foil, and a third wrapped in copper foil, which nail will rust fastest? Which will be protected?
- Take three test-tubes half-filled with salt water mixed with a few drops of potassium ferricyanide (a pink/blue indicator for Fe²⁺).
- Drop a clean iron nail into tube 1.
- Wrap another iron nail with a strip of zinc foil and drop into tube 2.
- Wrap the third iron nail with copper foil and drop into tube 3.
- Observe for 30 minutes — where does the blue colour appear first and most intensely?
Expected: Tube 1 — moderate blue colour (rust forms). Tube 2 — no blue colour near the iron; Zn (E° = −0.76 V) oxidises preferentially and protects the iron cathodically. Tube 3 — intense blue colour; Cu (E° = +0.34 V) actually accelerates Fe's oxidation because the Fe/Cu couple makes Fe an even better anode. Lesson: always choose a sacrificial metal more reactive than iron, never less.
Interactive: Battery Comparison L2 Understand
Select a battery type to see its EMF, rechargeability, and typical use.
Battery:
Select a battery and press Show.
Competency-Based Questions
A car's 12 V lead-acid battery has six 2 V cells in series. The electrolyte is 38% H₂SO₄. Elsewhere, a space agency is comparing the H₂–O₂ fuel cell (E° = 1.23 V) with the lithium-ion battery for an interplanetary mission. An engineer is also designing a buried iron pipeline protected by a magnesium block (E°(Mg²⁺/Mg) = −2.37 V).
Q1. L1 Remember The overall discharge reaction of the lead storage battery is:
Answer: B. Both electrodes end up as PbSO₄, and water is formed — hence the dilution of the acid during discharge.
Q2. L3 Apply Why does a fuel cell have a higher efficiency than a thermal power plant burning the same amount of hydrogen? (3 marks)
A thermal plant converts heat → mechanical → electrical, limited by the Carnot efficiency (~40%). A fuel cell converts chemical energy directly to electrical energy (ΔG pathway), bypassing Carnot's ceiling and reaching ~70% efficiency.
Q3. L3 Apply For the fuel cell reaction 2H₂ + O₂ → 2H₂O with E° = 1.23 V and n = 4, compute ΔG° in kJ. (2 marks)
ΔG° = −(4)(96500)(1.23) = −474.8 kJ.
Q4. L4 Analyse A student suggests protecting an iron pipeline with a copper block bolted to it. Evaluate this proposal. (3 marks)
Bad idea — Cu (E° = +0.34 V) is less reactive than Fe (E° = −0.44 V). In a Fe–Cu couple, Fe becomes the anode and rusts faster, not slower. A magnesium or zinc sacrificial anode is needed; both have more negative E° than iron and will preferentially oxidise.
Q5. L2 Understand List two reasons why the mercury button cell is preferred for hearing-aid batteries. (2 marks)
(i) Constant EMF (~1.35 V) throughout life — vital for steady audio output. (ii) Small compact size, high energy density, and long shelf life. (A third: no gassing inside the sealed case.)
Assertion-Reason Questions
Assertion (A): Galvanised iron does not rust even if its zinc coating is scratched.
Reason (R): Zinc has a more negative standard reduction potential than iron, so it oxidises preferentially and protects the iron cathodically.
Answer: A. Zn (−0.76 V) sits below Fe (−0.44 V) in the reduction series, so it is the anode in the Zn–Fe galvanic couple.
Assertion (A): Fuel cells are more environmentally friendly than internal combustion engines.
Reason (R): Fuel cells convert chemical energy directly into electrical energy without combustion, so no NOx, SOx, or particulate pollutants form.
Answer: A. Only water (drinkable by astronauts!) is produced by an H₂–O₂ fuel cell.
Assertion (A): The mercury cell maintains a constant voltage throughout its useful life.
Reason (R): The net cell reaction involves only solid and liquid species at constant activity, so the Nernst correction term is zero.
Answer: A. Activities of pure solids and pure liquids are 1, so log Q = 0 and E = E° throughout.
Frequently Asked Questions - Batteries Fuel Cells Corrosion
What is the main concept covered in Batteries Fuel Cells Corrosion?
In NCERT Class 12 Chemistry Chapter 2 (Electrochemistry), "Batteries Fuel Cells Corrosion" covers the core chemistry principles and reactions students need for board exam success. The MyAiSchool lesson explains the topic with definitions, structural diagrams, reaction mechanisms, worked examples, and interactive simulations. Key reactions, IUPAC names, and chemical reasoning are highlighted throughout aligned with CBSE 2025-26 syllabus.
How is Batteries Fuel Cells Corrosion useful in real-life or applied chemistry?
Real-life applications of "Batteries Fuel Cells Corrosion" from NCERT Class 12 Chemistry Chapter 2 include drug design, polymer industry, food chemistry, electrochemical cells, fuel cells, dyes/pigments, agrochemicals, and biochemistry. The MyAiSchool lesson links every concept to a tangible industrial or biological example so students see chemistry as a problem-solving framework for the molecular world.
What are the key reactions students should memorize for Batteries Fuel Cells Corrosion?
Key reactions in "Batteries Fuel Cells Corrosion" (NCERT Class 12 Chemistry Chapter 2 Electrochemistry) are tabulated in the MyAiSchool reaction map. Students should memorize each reaction with its reagent, conditions, mechanism class (SN1/SN2/E1/E2/electrophilic addition/etc), product, and stereochemistry. The Summary section provides a quick-reference reaction chart for last-minute revision.
How does this part connect to other parts of Chapter 2?
NCERT Class 12 Chemistry Chapter 2 (Electrochemistry) is structured so each part builds chemical understanding sequentially. "Batteries Fuel Cells Corrosion" connects to neighbouring parts via shared functional groups, reaction mechanisms, and structural concepts. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Batteries Fuel Cells Corrosion?
CBSE board questions from "Batteries Fuel Cells Corrosion" typically include: (1) 1-mark MCQs on definitions and IUPAC naming, (2) 2-mark short-answer reactions/products, (3) 3-mark mechanism questions, (4) 5-mark long-answer combining mechanism + product + stereochemistry + application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Batteries Fuel Cells Corrosion" lesson allows students to explore reaction outcomes, predict products, or compare reaction conditions, with live visual feedback. To use it effectively: (1) try every option/configuration, (2) compare with the analytical reasoning, (3) check IUPAC names and structural correctness, (4) test edge cases from worked examples. The simulation reinforces conceptual intuition that pure mechanism memorisation cannot provide.
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