Is it correct to say: Interchanging cathode and anode in the cell notation?

Convention: anode | anode-solution || cathode-solution | cathode. Anode on LEFT.

Is it correct to say: Using natural log in Nernst formula with 0.0591 prefactor?

0.0591 comes with base-10 log. Prefactor for ln is (RT/nF) = 0.0257 V at 298 K.

Is it correct to say: Writing Q with concentrations of solids or pure liquids?

Activities of pure solids and pure liquids are 1. Only solutes and gases appear in Q.

Is it correct to say: Applying Ostwald's dilution law K_a = cα² for strong electrolytes?

Ostwald's law is valid only for WEAK electrolytes (α ≪ 1). Strong ones do not obey it.

Is it correct to say: Confusing conductivity (κ) with molar conductivity (Λ_m)?

κ is intrinsic (S/cm); Λ_m = κ × 1000/c depends on concentration and dilution.

Is it correct to say: Using Faraday constant as 96500 J/mol instead of C/mol?

F = 96500 C/mol (charge). ΔG° = −nFE° needs F in coulombs.

Electrochemistry

Galvanic cells, EMF, Nernst equation, conductance, Kohlrausch's law, batteries — Maharashtra HSC Chemistry Ch 5

📐 12 formulas✏️ 3 examples🎯 5 practice⚖️ 5 marks🏫 Maharashtra State Board📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→Nernst equation for a single electrode and for the cell (E_cell = E°_cell − (0.0591/n) log Q) is a 3-mark HSC favourite.
→Derivation and numerical use of Kohlrausch's law of independent migration of ions is a 3-marker in Maharashtra Board.
→Distinguish clearly between galvanic (spontaneous) and electrolytic (non-spontaneous) cells — often asked as a 2-marker.
→Faraday's laws of electrolysis + numerical (mass deposited = ZIt) is asked almost every year in HSC.
→For Daniell cell write the electrode notation Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s) with correct phase boundaries. Board is strict about notation.

📐 Formulas(12)

Cell EMF from Electrode Potentials★ Board fav

E_{ce l l}^{\circ} = E_{c a t h o d e}^{\circ} - E_{an o d e}^{\circ}

Gibbs Energy — EMF Relation★ Board fav

Δ G^{\circ} = - n F E_{ce l l}^{\circ}

Nernst Equation (electrode)★ Board fav

E = E^{\circ} - \frac{0.0591}{n} lo g \frac{[ oxidised ]}{[ reduced ]} (T = 298 K)

Nernst Equation (cell)★ Board fav

E_{ce l l} = E_{ce l l}^{\circ} - \frac{0.0591}{n} lo g Q Q = reaction quotient

Equilibrium Constant from E°

lo g K = \frac{n E _{ce l l}^{\circ}}{0.0591}

Faraday's First Law

w = Z I t

Faraday's Second Law

\frac{w _{1}}{w _{2}} = \frac{E _{1}}{E _{2}}

Conductivity and Molar Conductivity

κ = \frac{1}{ρ}, Λ_{m} = \frac{κ \times 1000}{c}

Symbol	Meaning
$κ$	Specific conductivity (S/m)
$Λ_{m}$	Molar conductivity (S·cm²·mol⁻¹)
$c$	Concentration (mol/L)

Kohlrausch's Law★ Board fav

Λ_{m}^{\infty} = λ_{+}^{\infty} + λ_{-}^{\infty}

Degree of Dissociation from Λ

α = \frac{Λ _{m}}{Λ _{m}^{\infty}}

Ostwald's Dilution Law

K_{a} = \frac{c α ^{2}}{1 - α}

Cell Constant

G^{*} = \frac{l}{A} = κ \cdot R

✏️ Solved Examples

1Solved Exampleeasy2 steps

Calculate the standard EMF of the Daniell cell. Given: E°(Cu²⁺/Cu) = +0.34 V and E°(Zn²⁺/Zn) = −0.76 V.

Identify cathode (higher reduction potential) and anode.

Cathode: Cu²⁺/Cu (+0.34 V); Anode: Zn²⁺/Zn (-0.76 V)

2Solved Exampleboard4 steps

Calculate the EMF of the cell Zn(s) | Zn²⁺ (0.01 M) || Cu²⁺ (0.1 M) | Cu(s) at 25 °C. E°_cell = 1.10 V.

Cell reaction: Zn + Cu²⁺ → Zn²⁺ + Cu; n = 2.

Q = \frac{[ Z n ^{2 +} ]}{[ C u ^{2 +} ]} = \frac{0.01}{0.1} = 0.1

3Solved ExampleHOTS4 steps

For a 0.01 M solution of acetic acid, molar conductivity is 5.20 S·cm²/mol. Λ°_m for CH₃COOH = 390.5 S·cm²/mol. Find the degree of dissociation and dissociation constant.

Degree of dissociation.

α = \frac{Λ _{m}}{Λ _{m}^{\infty}} = \frac{5.20}{390.5} \approx 0.0133

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Interchanging cathode and anode in the cell notation
✓Convention: anode | anode-solution || cathode-solution | cathode. Anode on LEFT.
2
Using natural log in Nernst formula with 0.0591 prefactor
✓0.0591 comes with base-10 log. Prefactor for ln is (RT/nF) = 0.0257 V at 298 K.
3
Writing Q with concentrations of solids or pure liquids
✓Activities of pure solids and pure liquids are 1. Only solutes and gases appear in Q.
4
Applying Ostwald's dilution law K_a = cα² for strong electrolytes
✓Ostwald's law is valid only for WEAK electrolytes (α ≪ 1). Strong ones do not obey it.
5
Confusing conductivity (κ) with molar conductivity (Λ_m)
✓κ is intrinsic (S/cm); Λ_m = κ × 1000/c depends on concentration and dilution.
6
Using Faraday constant as 96500 J/mol instead of C/mol
✓F = 96500 C/mol (charge). ΔG° = −nFE° needs F in coulombs.

🎯 Practice Yourself

🎯Practice Yourself5 questions

Q1
Find the standard EMF of a cell: E°(Ag⁺/Ag) = +0.80, E°(Fe²⁺/Fe) = −0.44.
Q2
How many grams of Cu (M = 63.5) are deposited by passing 0.5 A current for 30 min through CuSO₄ solution?
Q3
Calculate K for the cell with E°_cell = 0.30 V and n = 2 at 25 °C.
Q4
The specific conductance of 0.02 M KCl is 2.77×10⁻³ S/cm. Find the cell constant if the observed resistance is 250 Ω.
Q5
State Kohlrausch's law and use it to compute Λ°_m for acetic acid given Λ°_m(HCl), Λ°_m(NaCl), Λ°_m(CH₃COONa).

📝 Notes

Electrochemistry — Maharashtra HSC Overview

Chapter 5 of the Balbharati Class 12 Chemistry textbook covers electrochemical cells, EMF, Nernst equation, conductance, and industrial applications like batteries and corrosion.

Maharashtra syllabus specifics

Nernst equation derivation from Gibbs energy — a formal step-by-step ΔG = ΔG° + RT ln Q → E = E° − (0.0591/n) log Q — is asked as a 3-marker in Maharashtra Board.
Kohlrausch's law with numerical application for weak electrolytes (compute Λ°_m of acetic acid using strong electrolytes) is a Maharashtra HSC favourite.
Ostwald's dilution law (K_a = cα²/(1−α)) with derivation and numerical is treated more formally in Maharashtra Board.
Primary and secondary batteries (Leclanché cell, mercury cell, lead storage battery, nickel–cadmium cell) and fuel cells — Balbharati asks these regularly as 2-mark descriptive questions.
Corrosion of iron and its electrochemical mechanism is treated as a separate section in Balbharati.

Numerical tips

Use 0.0591 V (base-10 log form) unless the question specifies temperature ≠ 298 K.
For any galvanic cell number of electrons n comes from the balanced ionic equation, not from arbitrary guess.
Faraday constant F = 96,485 C/mol; use 96,500 in board answers for simple arithmetic.