JEE Main 2026 February: The mixed electrochemical cell
The problem looked like a standard cell until the concentration of one half-cell was not 1 M. That single twist converts a textbook answer into a Nernst application, and the trap is to grab the standard potential and forget the concentration.
Full solution for the JEE Main February 2026 chemistry question combining standard electrode potentials, a non-standard concentration and the Nernst equation.
How this actually works
Recognising the cell type
The question described a cell with one half-cell at standard conditions and one at a stated, non-standard concentration. The naive move is to look up the standard potentials in the data booklet, subtract them, and report 2.5 V or whatever the table says. The concentration term is not optional — it is the entire reason the question was worth four marks.
Writing the Nernst term
For the half-reaction with the non-standard concentration, the electrode potential is E = E° − (0.059/n) log Q, where Q is the reaction quotient at that electrode and n is the number of electrons in that half-reaction. Two things go wrong here. The first is using the overall cell n instead of the half-reaction n. The second is writing Q for the whole cell when the concentration term belongs to one electrode only.
The sign convention trap
Several candidates reported a negative cell potential because they applied the same sign to both electrodes. The convention is that the spontaneous direction makes E positive, so identify the higher-potential half-reaction as the cathode before plugging in numbers. The 2026 question was built so that the concentration term almost exactly cancelled the standard-potential gap, which is why it rewarded reading and penalised memorised numbers.
What the question was actually testing
Underneath the electrochemistry is a discipline question: can you hold four things in mind at once — which electrode, which standard potential, which reaction quotient, which sign convention? Electrochemistry questions in JEE are rarely hard because of the mathematics. They are hard because the setup has to be right before any arithmetic happens, and a candidate working backwards from the answer will not notice the setup was wrong.
JEE Main 2026 February: The mixed electrochemical cell: the facts to keep
| Question type | Electrochemistry, standard and non-standard conditions in one cell |
|---|---|
| The first move | Identify which half-reaction departs from standard conditions before writing any equation |
| The Nernst form | E = E degree - (0.059 / n) log Q, with n taken from the half-reaction, not the whole cell |
| Sign convention | The spontaneous direction makes E positive, so the higher-potential half-reaction is the cathode |
| The trap | Reporting a negative cell potential by applying one sign to both electrodes |
| Sanity check | When both half-cells are at 1 M and 1 bar, every logarithm is zero and the Nernst term disappears |
Questions students ask about this
Why is the concentration term sometimes positive and sometimes negative?
It depends on whether Q is greater or less than 1, which depends on the stoichiometry of the half-reaction as written. For a reduction where the product ion is concentrated, Q > 1 and the term reduces E; where it is diluted, Q < 1 and the term raises E.
Do I need the Nernst equation for a cell with both half-cells at 1 M?
No. At 1 M and 1 bar, every activity is unity, every Q is 1, and every logarithm is zero. The cell potential is simply the difference of the standard potentials. The Nernst equation is only needed once something departs from standard conditions.