# Electrochemistry

> Electrochemistry for JEE Chemistry: Nernst equation, EMF calculations, conductance, Kohlrausch law, electrolysis and Faraday's laws.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/electrochemistry
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Electrochemistry", PrepElephant, https://prepelephant.com/topics/jee/chemistry/electrochemistry

## Direct answer

Electrochemistry converts chemical energy to electrical energy in a galvanic cell, whose driving force is the EMF: E°cell = E°cathode - E°anode using standard reduction potentials, and a positive E°cell marks a spontaneous cell because delta G° = -n F E°cell. The Nernst equation, E_cell = E°cell - (0.059/n) log Q at 25 °C, tracks EMF as concentrations change, while conductance and Faraday's laws handle electrolysis, where an external current drives a non-spontaneous change.

## What you must remember

- E°cell = E°cathode - E°anode; for the zinc-copper Daniell cell, E°cell = 0.34 - (-0.76) = 1.10 V; galvanic cells require a positive E°cell.
- delta G° = -n F E°cell with F = 96500 C per mole, and delta G° = -2.303 R T log K, linking EMF to the equilibrium constant.
- Nernst at 25 °C: E_cell = E°cell - (0.059/n) log Q; for a metal electrode M^n+ + ne → M, E = E° + (0.059/n) log[M^n+]; concentration cells give E = (0.059/n) log(c2/c1).
- Molar conductivity lambda_m = 1000 × kappa / c; on dilution kappa falls but lambda_m rises; for a weak electrolyte alpha = lambda_m / lambda°_m.
- Kohlrausch's law: lambda°_m = nu+ lambda°+ + nu- lambda°-, assembling limiting molar conductivities (as for acetic acid) from strong-electrolyte data.
- Faraday's first law: mass deposited = (equivalent mass × I × t)/96500; one faraday deposits one mole of a monovalent metal.
- Lead-acid battery: Pb and PbO2 in H2SO4; discharge forms PbSO4 on both plates and consumes acid, so density falls; about 2 V per cell. Dry cell is primary; Ni-Cd and lead-acid secondary; the H2-O2 fuel cell gives electricity directly with water as product.
- Corrosion is electrochemical: iron dissolves at anodic spots while oxygen is reduced nearby; galvanising or a sacrificial magnesium anode protects it.

## Common confusion

The commonest error is electrode bookkeeping: a galvanic cell's anode is negative and cathode positive, an electrolytic cell's anode is positive — oxidation always sits at the anode either way. Students invert the Nernst log or drop its minus sign, and mix the dilution trends: conductivity falls (fewer ions per volume) while molar conductivity rises. In Faraday problems, substituting molar for equivalent mass is the standard slip.

## Exam-focused takeaway

JEE Main tests the toolkit numerically: EMF from standard potentials, Nernst calculations, mass deposited by a current, conductivity conversions. JEE Advanced prefers reasoning on the same laws — concentration cells, Kohlrausch applications to weak electrolytes, electrolysis of brine or copper sulphate with products at both electrodes, and corrosion written as paired half-reactions. Write both half-reactions and the electron direction before computing.

## Frequently asked questions

### What does the Nernst equation calculate?

The cell EMF under non-standard conditions: E_cell = E°cell - (0.059/n) log Q at 25 °C.

### How are E°cell and K related?

Through delta G° = -nFE°cell = -2.303 RT log K, so a large positive EMF means a huge equilibrium constant.

### Why does molar conductivity increase on dilution?

Weak electrolytes ionise more completely and interionic attraction weakens, so conducting power per mole rises even as solution conductivity falls.

### What is Kohlrausch's law used for?

Adding limiting ionic molar conductivities of the constituent ions gives lambda°_m of any electrolyte — the indirect route for weak electrolytes like acetic acid.

### What are the products of electrolysis of aqueous NaCl?

Hydrogen at the cathode, chlorine at the anode, sodium hydroxide left behind — the chlor-alkali outcome.

### Why is the anode negative in a galvanic cell but positive in electrolysis?

Both anodes host oxidation; a galvanic anode feeds electrons to the circuit, while in electrolysis the supply pulls electrons out of it.