Chemistry · Electrochemistry · NEET
A reaction is spontaneous when E°cell is POSITIVE. Use the formula ΔG° = −nFE°cell. If E°cell is positive, then ΔG° becomes negative (because of the minus sign). A negative ΔG° means the reaction is spontaneous. So: positive E°cell → negative ΔG° → spontaneous (galvanic cell). Negative E°cell → positive ΔG° → non-spontaneous (needs outside energy, like electrolysis).
The minus sign is what makes the signs match correctly. In thermodynamics, a spontaneous reaction has NEGATIVE ΔG°. In electrochemistry, a spontaneous galvanic cell has POSITIVE E°cell. The minus sign flips the sign so both ideas agree: a positive E°cell gives a negative ΔG°. Without the minus sign the two rules would contradict each other. Just remember the sign flips.
n is the number of moles of electrons transferred in the balanced cell reaction. You find it by writing the two half-reactions and seeing how many electrons move. Example: for 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂, two electrons are transferred, so n = 2. F is the Faraday constant, 96500 C/mol. Getting n wrong is the most common mistake in NEET numericals.
Plug into ΔG° = −nFE°cell. Steps: (1) find n (electrons transferred), (2) use F = 96500 C/mol, (3) multiply. Example: E°cell = 0.24 V, n = 2 → ΔG° = −(2)(96500)(0.24) = −46320 J/mol = −46.32 kJ/mol. Note the answer comes in JOULES; divide by 1000 to get kJ. Keep the units clean, NEET options are usually in kJ/mol.
If E°cell is negative: ΔG° becomes positive (ΔG° = −nFE°cell) and Keq becomes less than 1 (ΔG° = −RT ln Keq). A positive ΔG° means ln Keq is negative, so Keq < 1. In plain words, the reaction does not go forward on its own and products are not favored. This exact case was asked in NEET 2016.
No. Both n and F are always positive numbers (n counts electrons, F = 96500 C/mol). Only E°cell decides the sign of ΔG°. So the whole question of spontaneous vs non-spontaneous depends only on whether E°cell is positive or negative. n and F only affect the SIZE of ΔG°, not its sign.
For the cell reaction 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq), E°cell = 0.24 V at 298 K. The standard Gibbs energy (ΔrG°) of the cell reaction is:
If the E°cell for a given reaction has a negative value, which of the following gives the correct relationships for the values of ΔG° and Keq?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
ΔG° = −nFE°cell, where n = moles of electrons transferred and F = 96500 C/mol. This is one of the most important formulas in the Electrochemistry chapter for NEET.
A positive ΔG° means the reaction is non-spontaneous. The cell cannot produce current on its own; you would need to supply outside electrical energy (electrolysis). It also matches a negative E°cell and Keq < 1.
Because F is in coulombs (96500 C/mol) and E° is in volts, the product nFE° is in joules. Divide by 1000 to convert to kJ. NEET options are usually in kJ/mol, so this conversion matters.
No. Both n and F are always positive. Only the sign of E°cell decides whether ΔG° is negative (spontaneous) or positive (non-spontaneous). n and F change only the magnitude.
Through ΔG°: ΔG° = −nFE°cell and ΔG° = −RT ln Keq. Combining them gives E°cell = (RT/nF) ln Keq. A positive E°cell gives Keq > 1 (products favored). This link is covered in the next concept, finding Keq from E°cell.
