JEE Mains Chemistry · Teaching notes

Electrochemistry — JEE Mains Chemistry

Electrochemistry has 125 past-year questions from 2021 to 2026, and more than half of them ask for a number rather than a choice of option. The Nernst equation alone carries about a quarter of the chapter, so the sign of its log term must be automatic. Most of the numericals need only a handful of relations: E°cell as cathode minus anode, the Nernst correction, ΔG° = −nFE°, Λm = 1000κ/c with Kohlrausch's law, and Faraday's m = MIt/nF. The rest is recall: the electrochemical series, what forms at each electrode, and the named batteries, whose questions are all multiple choice.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

All 125 past questions JEE Mains Chemistry strategy JEE Mains home

Subtopic notes

Formula & revision sheet

11 formulas · 5 reference tables · 33 gotchas across all subtopics — the exam-eve cheat-sheet

Galvanic Cells and Electrode Potentials

Formulas (1)

Setting up a galvanic cell · Standard cell potential $E^\circ_{cell}=E^\circ_{cathode}-E^\circ_{anode}$

Reference tables (1)

The electrochemical series18 rows

Couple	E° at 298 K (V)	What it tells you
$\mathrm{Li^+/Li}$	$-3.05$	Li is the strongest reducing agent in water
$\mathrm{Na^+/Na}$	$-2.71$	Na ionises more easily than Li, yet its E° is higher
$\mathrm{Mg^{2+}/Mg}$	$-2.37$	Mg displaces almost every metal ion from water
$\mathrm{Al^{3+}/Al}$	$-1.66$	Al is a strong reducing agent
$\mathrm{Zn^{2+}/Zn}$	$-0.76$	Zn is the anode of the Daniell cell
$\mathrm{Cr^{3+}/Cr}$	$-0.74$	Cr is a reducing agent close to Zn
$\mathrm{Fe^{2+}/Fe}$	$-0.44$	Fe dissolves in dilute acid and gives H₂
$\mathrm{H^+/\tfrac12 H_2}$	$0.00$	The zero of the scale, by definition
$\mathrm{Cu^{2+}/Cu}$	$+0.34$	Cu does not release H₂ from dilute acid
$\mathrm{I_2/I^-}$	$+0.54$	I⁻ is a fairly good reducing agent
$\mathrm{Fe^{3+}/Fe^{2+}}$	$+0.77$	Fe³⁺ oxidises I⁻ to I₂
$\mathrm{Ag^+/Ag}$	$+0.80$	Ag is oxidised by nitric acid
$\mathrm{NO_3^-/NO}$	$+0.97$	Nitrate in acid oxidises Ag but not Au
$\mathrm{Cr_2O_7^{2-}/Cr^{3+}}$	$+1.33$	Dichromate in acid oxidises Ag and Fe²⁺
$\mathrm{Cl_2/Cl^-}$	$+1.36$	Cl₂ oxidises Br⁻ and I⁻
$\mathrm{Au^{3+}/Au}$	$+1.40$	Au resists every common oxidant here
$\mathrm{MnO_4^-/Mn^{2+}}$	$+1.51$	Permanganate in acid oxidises Cl⁻
$\mathrm{F_2/F^-}$	$+2.87$	F₂ is the strongest oxidising agent

Read down the table for stronger oxidising agents (the left-hand species); read up for stronger reducing agents (the right-hand species).

Watch out for (4)

Picking the oxidised form as the reducing agent
→ The electrochemical series
Reading the sign backwards
→ The electrochemical series
Adding the two potentials
→ Setting up a galvanic cell
Scaling E° with the equation
→ Setting up a galvanic cell

Nernst Equation and Concentration Effects

Formulas (3)

Cell emf from the Nernst equation · Nernst equation at 298 K $E_{cell}=E^\circ_{cell}-\frac{0.059}{n}\log Q$
Solving the Nernst equation for an unknown · Nernst equation, rearranged $\log Q=\frac{n\left(E^\circ_{cell}-E_{cell}\right)}{0.059}$
Electrodes that depend on pH · Hydrogen electrode $E_{H^+/H_2}=-0.059\,\mathrm{pH}-\frac{0.059}{2}\log p_{H_2}$

Watch out for (6)

Dropping the powers in Q
→ Cell emf from the Nernst equation
Q upside down
→ Cell emf from the Nernst equation
Losing the sign of the log
→ Solving the Nernst equation for an unknown
Square root forgotten
→ Solving the Nernst equation for an unknown
Electrode potential is not the cell emf
→ Electrodes that depend on pH
Forgetting the pressure term
→ Electrodes that depend on pH

Gibbs Energy, Equilibrium Constant and Combining Potentials

Formulas (2)

Gibbs energy, K and work from E° · Gibbs energy and equilibrium constant $\Delta G^\circ=-nFE^\circ_{cell},\qquad \log K=\frac{nE^\circ_{cell}}{0.059}$
Combining electrode potentials · Combining two steps $n_3E^\circ_3=n_1E^\circ_1\pm n_2E^\circ_2$

Watch out for (4)

Work as charge divided by potential
→ Gibbs energy, K and work from E°
Joules against kilojoules
→ Gibbs energy, K and work from E°
Subtracting potentials directly
→ Combining electrode potentials
Averaging a Latimer diagram
→ Combining electrode potentials

Conductivity, Cell Constant and Molar Conductivity

Formulas (1)

Cell constant, conductivity and molar conductivity · From resistance to molar conductivity $\kappa=\frac{G^*}{R},\qquad \Lambda_m=\frac{1000\,\kappa}{c}$

Reference tables (1)

What conductance depends on11 rows

Ion	λ° at 298 K (S cm² mol⁻¹)	Why
$\mathrm{H^+}$	$349.6$	Proton hopping along hydrogen bonds
$\mathrm{OH^-}$	$199.1$	Proton hopping, in reverse
$\mathrm{SO_4^{2-}}$	$160.0$	Double charge carries twice the current
$\mathrm{Ca^{2+}}$	$119.0$	Double charge
$\mathrm{Mg^{2+}}$	$106.0$	Double charge, but a smaller ion is more hydrated than Ca²⁺
$\mathrm{Br^-}$	$78.1$	Large anion, lightly hydrated
$\mathrm{Cl^-}$	$76.3$	Close to K⁺, which is why KCl is the standard
$\mathrm{K^+}$	$73.5$	Least hydrated of Li⁺, Na⁺, K⁺
$\mathrm{Na^+}$	$50.1$	More hydrated than K⁺
$\mathrm{CH_3COO^-}$	$40.9$	Large, bulky organic anion
$\mathrm{Li^+}$	$38.7$	Smallest bare ion, largest hydrated ion

Values are per mole of the ion as written.

Watch out for (4)

Mixing unit systems
→ Cell constant, conductivity and molar conductivity
Resistivity used as conductivity
→ Cell constant, conductivity and molar conductivity
Bare size against hydrated size
→ What conductance depends on
κ and Λm move opposite ways
→ What conductance depends on

Molar Conductivity, Dilution and Kohlrausch's Law

Formulas (3)

Strong and weak electrolytes on dilution · Debye–Hückel–Onsager (strong electrolytes) $\Lambda_m=\Lambda_m^\circ-A\sqrt c$
Kohlrausch's law of independent migration · Kohlrausch's law $\Lambda_m^\circ=\nu_+\lambda_+^\circ+\nu_-\lambda_-^\circ$
Degree of dissociation, Ka and solubility · Degree of dissociation $\alpha=\frac{\Lambda_m}{\Lambda_m^\circ},\qquad K_a=\frac{c\alpha^2}{1-\alpha}$

Watch out for (6)

Extrapolating a weak electrolyte
→ Strong and weak electrolytes on dilution
Plotting against c instead of √c
→ Strong and weak electrolytes on dilution
Doubling a divalent salt
→ Kohlrausch's law of independent migration
Leaving an ion uncancelled
→ Kohlrausch's law of independent migration
Dropping the 1000
→ Degree of dissociation, Ka and solubility
The wrong Ksp expression
→ Degree of dissociation, Ka and solubility

Electrolysis and Faraday's Laws

Formulas (1)

Faraday's laws of electrolysis · Faraday's first law $m=\frac{M\,I\,t}{n\,F}$

Reference tables (1)

Products of electrolysis8 rows

Electrolyte	Electrodes	Cathode	Anode
Molten NaCl	Inert	Na	$\mathrm{Cl_2}$
Aqueous NaCl (brine)	Inert	$\mathrm{H_2}$ , with $\mathrm{OH^-}$ left in solution	$\mathrm{Cl_2}$
Aqueous $\mathrm{AgNO_3}$	Pt	Ag	$\mathrm{O_2}$
Aqueous $\mathrm{AgNO_3}$	Ag	Ag	Ag dissolves as $\mathrm{Ag^+}$
Aqueous $\mathrm{CuSO_4}$	Pt	Cu	$\mathrm{O_2}$
Aqueous $\mathrm{CuSO_4}$	Cu	Cu	Cu dissolves as $\mathrm{Cu^{2+}}$
Dilute $\mathrm{H_2SO_4}$	Pt	$\mathrm{H_2}$	$\mathrm{O_2}$
Concentrated $\mathrm{H_2SO_4}$	Pt	$\mathrm{H_2}$	$\mathrm{S_2O_8^{2-}}$

An active anode dissolves; an inert anode oxidises an anion or water.

Watch out for (5)

Four electrons for oxygen
→ Faraday's laws of electrolysis
Minutes left as minutes
→ Faraday's laws of electrolysis
The charge on a complex ion's metal
→ Faraday's laws of electrolysis
Depositing sodium from water
→ Products of electrolysis
Forgetting an active anode
→ Products of electrolysis

Batteries, Fuel Cells and Corrosion

Reference tables (2)

Primary and secondary batteries5 rows

Cell	Anode	Cathode	Electrolyte	Type and use
Dry (Leclanché) cell	Zn container	Graphite rod in $\mathrm{MnO_2}$ and carbon	Paste of $\mathrm{NH_4Cl}$ and $\mathrm{ZnCl_2}$	Primary; clocks, transistors, torches
Mercury cell	Zn–Hg amalgam	Paste of HgO and carbon	Paste of KOH and ZnO	Primary; hearing aids, watches; steady voltage
Lead storage battery	Pb	$\mathrm{PbO_2}$ packed on a lead grid	About 38% $\mathrm{H_2SO_4}$	Secondary; cars and inverters
Nickel–cadmium cell	Cd	$\mathrm{Ni(OH)_3}$	KOH	Secondary; long life, rechargeable devices
$\mathrm{H_2}$ – $\mathrm{O_2}$ fuel cell	Porous carbon with $\mathrm{H_2}$ fed in	Porous carbon with $\mathrm{O_2}$ fed in	Concentrated aqueous NaOH or KOH	Continuous feed; Apollo space programme

Primary: used once. Secondary: recharged. Fuel cell: reactants fed in continuously.

Fuel cells and corrosion10 rows

Statement	Verdict	Reason
The H₂–O₂ fuel cell was used in the Apollo space programme	True	Its water was drunk by the crew
The H₂–O₂ fuel cell is about 40% efficient	False	About 70%, far above a thermal power plant
Its electrodes use aluminium as a catalyst	False	Finely divided Pt or Pd on porous carbon
Reactants are fed in at one go	False	They are fed in continuously
A fuel cell is a galvanic cell	True	A spontaneous reaction gives electricity
In a methanol fuel cell, methanol is oxidised at the anode	True	The fuel is always the anode's reactant
Rusting is an electrochemical process	True	Anodic and cathodic spots on one piece of iron
Rusting is faster in alkaline water than in acid	False	$\mathrm{H^+}$ drives the cathode reaction; above pH 9 to 10 rusting stops
A tin coat protects iron even after it peels	False	Iron is below tin in the series, so exposed iron corrodes faster
A scratched zinc coat still protects iron	True	Zinc is oxidised first, as a sacrificial anode

Watch out for (4)

Renaming the plates on charging
→ Primary and secondary batteries
Why the mercury cell is steady
→ Primary and secondary batteries
Oxygen at the anode
→ Fuel cells and corrosion
Tin protects like zinc
→ Fuel cells and corrosion

PYQ weightage by concept

16 concepts · 125 PYQs — where the marks actually sit, so you know what to drill first

Galvanic Cells and Electrode Potentials14 PYQs · 11%

Concept	PYQs	Share
Setting up a galvanic cell	8	6%
The electrochemical series	6	5%

Nernst Equation and Concentration Effects30 PYQs · 24%

Concept	PYQs	Share
Cell emf from the Nernst equation	11	9%
Electrodes that depend on pH	10	8%
Solving the Nernst equation for an unknown	9	7%

Gibbs Energy, Equilibrium Constant and Combining Potentials16 PYQs · 13%

Concept	PYQs	Share
Gibbs energy, K and work from E°	9	7%
Combining electrode potentials	7	6%

Conductivity, Cell Constant and Molar Conductivity14 PYQs · 11%

Concept	PYQs	Share
Cell constant, conductivity and molar conductivity	10	8%
What conductance depends on	4	3%

Molar Conductivity, Dilution and Kohlrausch's Law21 PYQs · 17%

Concept	PYQs	Share
Strong and weak electrolytes on dilution	9	7%
Degree of dissociation, Ka and solubility	7	6%
Kohlrausch's law of independent migration	5	4%

Electrolysis and Faraday's Laws19 PYQs · 15%

Concept	PYQs	Share
Faraday's laws of electrolysis	15	12%
Products of electrolysis	4	3%

Batteries, Fuel Cells and Corrosion11 PYQs · 9%

Concept	PYQs	Share
Primary and secondary batteries	7	6%
Fuel cells and corrosion	4	3%

Test yourself on Electrochemistry

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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Galvanic Cells and Electrode Potentials

Nernst Equation and Concentration Effects

Gibbs Energy, Equilibrium Constant and Combining Potentials

Conductivity, Cell Constant and Molar Conductivity

Molar Conductivity, Dilution and Kohlrausch's Law

Electrolysis and Faraday's Laws

Batteries, Fuel Cells and Corrosion

Formula & revision sheet

PYQ weightage by concept

Test yourself on Electrochemistry

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