Electrochemistry

Concept overview

Electrochemistry connects chemical reactions to electrical energy. Galvanic (voltaic) cells convert spontaneous redox reactions into electric current; electrolytic cells drive non-spontaneous reactions using an external current. The two operate with opposite signs of ΔG but share the same underlying redox framework.

The Standard Hydrogen Electrode (SHE) sits at 0 V by convention, and every other half-cell is measured relative to it. Standard electrode potentials E° form a periodic table of oxidising/reducing strength — species with high E° are strong oxidants (F₂, MnO₄⁻, Cr₂O₇²⁻) and species with low E° are strong reductants (Li, Na, K). Given any two half-cells, the one with higher E° will be the cathode; the difference E°_cell = E°_cathode − E°_anode gives the cell EMF.

The Nernst equation extends this to non-standard conditions: E = E° − (RT/nF) ln Q, or at 25°C, E = E° − (0.0591/n) log Q. This is one of the most tested formulas in JEE Chemistry — expect at least one direct Nernst problem every year.

Electrolysis links current, time and mass through Faraday's laws. Mass deposited = (M × I × t) / (n × F), where F = 96,485 C/mol is the Faraday constant. Applications include electroplating, aluminium extraction, chlor-alkali processes and modern rechargeable batteries. Conductance, molar conductivity Λ_m and its variation with dilution (Kohlrausch's law) complete the chapter.

Key formulas & concepts

• Standard cell EMF: E°_cell = E°_cathode − E°_anode
• Nernst equation (25°C): E = E° − (0.0591/n) log Q
• ΔG and E: ΔG° = −nFE°_cell
• ΔG° and K: ΔG° = −RT ln K → E°_cell = (0.0591/n) log K
• Faraday's first law: m = ZIt, where Z = M/(nF)
• Molar conductivity: Λ_m = κ / C (units: S cm² mol⁻¹)
• Kohlrausch's law: Λ°_m = ν₊ λ°₊ + ν₋ λ°₋

Common mistakes to avoid

• Getting cathode/anode confused between galvanic and electrolytic cells — in a galvanic cell the cathode is the positive terminal; in an electrolytic cell it's connected to the negative terminal of the external source.
• Using the wrong sign of ΔG when computing E — ΔG = −nFE, not +nFE.
• Applying Nernst with concentrations in mol/L when Q needs partial pressures for gas half-cells.
• Forgetting that Faraday's law requires the number of electrons transferred per formula unit — silver plating uses n=1, copper deposition uses n=2, aluminium uses n=3.

JEE Main relevance

JEE Main asks 2 electrochemistry problems every paper: typically one Nernst-equation cell-EMF calculation and one Faraday's-law electroplating numerical. Conductivity problems appear less often but with predictable Kohlrausch's-law setups.

JEE Advanced relevance

Advanced problems combine cell EMF with equilibrium (linking E° to K) or with thermodynamics (linking E° to ΔG° and ΔS° through the temperature coefficient of EMF). Concentration cells — two half-cells of the same species at different concentrations — are also a frequent Advanced pattern.

Previous-year questions (PYQ)

Electrochemistry PYQs concentrate into three archetypes: EMF calculation, Nernst-shift-with-concentration, and electrolysis-mass-deposition. Drill archetype-by-archetype, then combine.