By Kokil Jain, Chemistry faculty at Pramana Institute (IIT Kanpur, 14 years teaching)
Electrochemistry is a short chapter that pays well: questions on the Nernst equation, cell EMF and conductivity come up almost every year in both JEE Main and NEET. Almost every one of them is solved by picking the right formula and keeping the signs and units straight. This page puts every formula you need in one table, with the mistakes that cost marks most often.
Galvanic cells and thermodynamics
| What you need | Formula | Notes |
|---|---|---|
| Standard cell EMF | E°cell = E°cathode − E°anode | Use standard reduction potentials for both electrodes |
| Gibbs energy and EMF | ΔG° = −nFE°cell, and ΔG = −nFEcell | n = electrons transferred in the balanced reaction |
| Nernst equation | E = E° − (RT/nF) ln Q | General form |
| Nernst at 298 K | E = E° − (0.0591/n) log Q | The most-used form in exams |
| EMF and equilibrium | E°cell = (0.0591/n) log K at 298 K | From E = 0 at equilibrium |
| ΔG° and K | ΔG° = −RT ln K = −2.303 RT log K | Links this chapter to equilibrium |
| Hydrogen electrode | E = −0.0591 pH at 298 K | For H⁺/H₂ at 1 bar |
Conductance
| What you need | Formula | Units |
|---|---|---|
| Conductance | G = 1/R | S (siemens) |
| Cell constant | G* = l/A | cm⁻¹ or m⁻¹ |
| Conductivity | κ = G × G* = G × (l/A) | S cm⁻¹ or S m⁻¹ |
| Molar conductivity | Λm = (1000 × κ)/M, with κ in S cm⁻¹ and M in mol L⁻¹ | S cm² mol⁻¹ |
| Strong electrolyte (dilution) | Λm = Λ°m − A√c | A depends on the electrolyte type |
| Kohlrausch’s law | Λ°m = ν₊λ°₊ + ν₋λ°₋ | Limiting molar conductivities of the ions |
| Weak electrolyte | α = Λm/Λ°m, and Ka = cα²/(1 − α) | α = degree of dissociation |
Electrolysis
| What you need | Formula | Notes |
|---|---|---|
| Charge | Q = I × t | Coulombs, with t in seconds |
| Faraday’s first law | m = (M × I × t)/(n × F) | M = molar mass; n = electrons per ion discharged |
| Faraday’s second law | m₁/m₂ = E₁/E₂ | E = equivalent mass, for the same charge passed |
| Faraday constant | 1 F ≈ 96,500 C mol⁻¹ (96,485 exact) | Charge on 1 mole of electrons |
Worked example
For the Daniell cell, E°(Cu²⁺/Cu) = +0.34 V and E°(Zn²⁺/Zn) = −0.76 V.
E°cell = 0.34 − (−0.76) = 1.10 V. With n = 2, ΔG° = −2 × 96,485 × 1.10 = −212,267 J ≈ −212.3 kJ mol⁻¹.
Three mistakes that cost marks
- Multiplying E° by the coefficients. Electrode potential is an intensive property and doesn’t change when you multiply a half-reaction by 2. ΔG° does scale, which is why you go through ΔG when combining half-reactions.
- Missing the 1000 in Λm. With κ in S cm⁻¹ and concentration in mol L⁻¹, Λm = 1000κ/M. Leaving it out gives an answer 1000 times too small.
- Writing Q upside down. Q is products over reactants, written for the cell reaction exactly as balanced, with pure solids left out.
FAQs
What is the value of 2.303RT/F at 298 K? About 0.0591 V, which is why it appears in the short form of the Nernst equation.
Is the Nernst equation in the NEET syllabus? Yes. Electrochemistry, including the Nernst equation and conductance, is part of NEET and JEE Main Chemistry.
Why does molar conductivity increase on dilution? For weak electrolytes, dilution increases dissociation (α). For strong electrolytes, it reduces the attraction between ions.
Revise this chapter in a batch of at most 40 at Pramana, Geeta Bhawan Square, Indore. See programs and fees or book free counselling.
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