Corrosion, Galvanic Cells and Electroplating

• Understand how a galvanic cell converts chemical energy to electrical energy
• Calculate cell EMF from standard electrode potentials
• Apply the Nernst equation to find EMF under non-standard conditions
• Use Faraday's laws to calculate mass deposited in electrolysis
• Explain corrosion as an electrochemical process and methods to prevent it
• Solve JEE-level numerical problems on EMF, Nernst equation, and electrolysis

Level 1 — Foundations

Galvanic (Voltaic) cell:

• Converts chemical energy from a spontaneous redox reaction into electrical energy
• Anode (−): oxidation occurs (metal dissolves: M → Mⁿ⁺ + ne⁻)
• Cathode (+): reduction occurs (ions plate out: Mⁿ⁺ + ne⁻ → M)
• Salt bridge: maintains electrical neutrality by allowing ion flow
• Electrons flow from anode to cathode through external circuit

Cell notation: Anode | anode solution || cathode solution | cathode Example: Zn | Zn²⁺(1M) || Cu²⁺(1M) | Cu (the Daniell cell)

Electroplating:

• Uses external current (electrolytic cell) — non-spontaneous reaction
• Object to be plated = cathode; plating metal = anode
• Example: silver plating uses Ag anode, object as cathode, AgNO₃ solution

Level 2 — JEE depth

Standard Electrode Potential E°: Measured relative to Standard Hydrogen Electrode (SHE), E° = 0.00 V. More positive E°: stronger oxidising agent (higher tendency to be reduced). More negative E°: stronger reducing agent (higher tendency to be oxidised).

Cell EMF: E°_cell = E°_cathode − E°_anode = E°_reduction(cathode) − E°_reduction(anode)

Relationship to Gibbs energy: ΔG° = −nFE°_cell Spontaneous cell: E°_cell > 0, ΔG° < 0

Relationship to equilibrium constant: ΔG° = −RT ln K → nFE° = RT ln K → log K = nE°/0.0592 (at 25°C)

Nernst Equation: At non-standard concentrations/pressures:

E = E° − (RT/nF) ln Q = E° − (0.0592/n) log Q at 25°C

where:

• n = number of electrons transferred in the balanced cell reaction
• Q = reaction quotient (same expression as Kc for cell reaction)
• F = Faraday constant = 96500 C/mol = 96485 C/mol

As reaction proceeds, Q increases → E decreases → cell reaches equilibrium when E = 0 and Q = K.

Concentration cell: Both electrodes are of the same metal but in solutions of different concentrations. E = (0.0592/n) log(C_cathode/C_anode) at 25°C (Higher concentration = cathode)

Faraday's Laws of Electrolysis:

First law: mass deposited (m) ∝ charge passed (Q = It) Second law: for same charge, masses deposited ∝ equivalent weight (M/n)

Combined formula: m = (M × I × t) / (n × F)

where:

• M = molar mass (g/mol)
• I = current (A)
• t = time (seconds)
• n = number of electrons per ion (valency)
• F = 96500 C/mol

Corrosion as electrochemical process: Iron rusting example:

• Anode (Fe surface): Fe → Fe²⁺ + 2e⁻ (oxidation)
• Cathode (O₂-rich area): O₂ + 4H⁺ + 4e⁻ → 2H₂O (or O₂ + 2H₂O + 4e⁻ → 4OH⁻ in neutral)
• Fe²⁺ further oxidises to Fe³⁺ → forms Fe₂O₃·xH₂O (rust)
• Electrolyte (moisture + dissolved CO₂/salts) completes the circuit

Prevention of corrosion:

JEE trap: In Nernst equation, n is electrons in the BALANCED cell equation, not just a half-reaction. For Cu/Zn cell: Zn + Cu²⁺ → Zn²⁺ + Cu, n = 2.

JEE trap: In electrolysis time calculations, always convert minutes to seconds. t(s) = t(min) × 60.

Cu²⁺/Cu (E° = +0.34 V) vs Zn²⁺/Zn (E° = −0.76 V) — find cell EMF

Identify anode and cathode:
More negative E° = anode (oxidation): Zn²⁺/Zn, E° = −0.76 V
More positive E° = cathode (reduction): Cu²⁺/Cu, E° = +0.34 V

Cell reaction:
Anode:   Zn → Zn²⁺ + 2e⁻
Cathode: Cu²⁺ + 2e⁻ → Cu
Overall: Zn + Cu²⁺ → Zn²⁺ + Cu

E°_cell = E°_cathode − E°_anode
        = (+0.34) − (−0.76)
        = +0.34 + 0.76
        = +1.10 V

ΔG° = −nFE° = −2 × 96500 × 1.10 = −212,300 J = −212.3 kJ

Answer: E°_cell = +1.10 V; ΔG° = −212.3 kJ (spontaneous) ✓

How long to deposit 5 g of Ag from AgNO₃ with 2 A current? (M = 108, n = 1, F = 96500)

Using Faraday's first law:
m = (M × I × t) / (n × F)

Rearrange for t:
t = (m × n × F) / (M × I)
  = (5 × 1 × 96500) / (108 × 2)
  = 482500 / 216
  = 2233 seconds

Convert to minutes: 2233 / 60 = 37.2 minutes

Verify: charge = I × t = 2 × 2233 = 4466 C
Moles of e⁻ = 4466/96500 = 0.04628 mol
Moles of Ag = 0.04628 mol (n=1, so 1 mol e⁻ per mol Ag)
Mass = 0.04628 × 108 = 5.00 g ✓

Answer: t ≈ 2233 s ≈ 37.2 minutes

• Q1: E°(Ag⁺/Ag) = +0.80 V, E°(Cu²⁺/Cu) = +0.34 V. Which is cathode and which anode? Find E°_cell.
• Q2: For Zn/Cu cell at standard conditions, what is log K at 25°C if E°_cell = +1.10 V and n = 2?
• Q3: Using Nernst equation, find E for Zn/Cu cell when [Zn²⁺] = 0.1 M and [Cu²⁺] = 0.01 M (E° = 1.10 V, n = 2).
• Q4: How many grams of Cu are deposited when 0.5 A passes through CuSO₄ for 1 hour? (M_Cu = 64, n = 2)
• Stretch: Q5: A galvanic cell has E = 0 at equilibrium. For the cell Zn | Zn²⁺ || Cu²⁺ | Cu (E° = 1.10 V, n = 2), at what ratio [Zn²⁺]/[Cu²⁺] does the cell reach equilibrium? Calculate K.

NCERT Chapter 8 link: Chapter 8 (Class 11) — "Redox Reactions" provides the foundation. The detailed electrochemistry (Nernst equation, Faraday's laws, cell EMF) is in Chapter 3 of Class 12 ("Electrochemistry"). Both are essential for JEE. The JEE Foundation course bridges these — master the concepts here, then deepen with Class 12 Chapter 3.

Exam connections: JEE Mains tests standard cell EMF calculations, Nernst equation numericals (common: find E when Q = 10 or 100), and Faraday's law mass calculations. JEE Advanced tests cell reactions with concentration cells, ΔG° ↔ E° ↔ K inter-conversions, and multi-step electrolysis problems.

Study strategy: Build a one-page reference with: (1) cell EMF formula, (2) ΔG = −nFE, (3) Nernst equation, (4) log K formula, (5) Faraday's law. Practise converting between all four (EMF, K, ΔG, concentration effect) in a single problem — JEE Advanced loves these connections.

• Use the platform-native live simulation or PhET-style tool for this topic.
• Mirror / body / home activity: make a lemon/potato battery using two different metals (zinc nail and copper coin), measure voltage with a multimeter, and explain which is anode and cathode.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.