JEE Chemistry 2026 Question with Solution

Given: A closed circuit Daniell cell at a fixed temperature, and plots of $E_{cell}^{\circ}$ versus time.

Find: Which plot correctly represents the behaviour of $E_{cell}^{\circ}$ with time.

The standard cell potential is a thermodynamic quantity defined under standard conditions. At a fixed temperature, it depends only on the standard electrode potentials of the two half-cells.

The instantaneous cell potential changes during cell operation according to the Nernst equation:

Here, $E_{cell}$ changes as concentrations change with time, but $E_{cell}^{\circ}$ does not change with time for a given reaction at a given temperature.

Therefore, the graph of $E_{cell}^{\circ}$ versus time is a horizontal line.

The correct option is B.

Given: The cell is a Daniell cell operating in a closed circuit.

Find: The correct time plot of the standard cell potential.

A common confusion is between $E_{cell}$ and $E_{cell}^{\circ}$.

• $E_{cell}^{\circ}$ is the standard cell potential.
• $E_{cell}$ is the actual cell potential at the current concentrations.

At constant temperature, the standard reduction potentials of the electrodes remain constant. Hence their difference also remains constant:

Although the reaction proceeds with time in a closed circuit and the concentrations of ions change, that affects only $E_{cell}$ through the reaction quotient $Q$.

So, with time:

• $E_{cell}$ may decrease,
• but $E_{cell}^{\circ}$ remains constant.

Hence the correct graph is the horizontal line, which corresponds to option B.