Which of the following is not true of electrode potentials?

They are independent of temperature

The correct answer is option 3. They are independent of temperature.

Let us evaluate each statement regarding electrode potentials to identify which one is not true:

1. They vary with concentration:

The statement is true. Electrode potentials can vary with the concentration of ions involved in the half-reaction. This is because the Nernst equation accounts for the dependence of electrode potential on ion activities, which are influenced by concentration changes.

2. They vary from electrolyte to electrolyte:

The statement is true. Electrode potentials are specific to the redox couple and the electrolyte in which the electrode is immersed. Different electrolytes can have different ion activities and solvation effects, leading to variations in electrode potentials.

3. They are independent of temperature:

The statement is false. Electrode potentials are temperature-dependent. As temperature changes, the standard electrode potentials (E°) listed at 25°C can be adjusted using the Nernst equation to account for the temperature dependence of the reaction equilibrium constants.

4. They are dependent on the number of electrons involved in the reaction:

The statement is true. Electrode potentials are influenced by the number of electrons exchanged in the redox reaction. The standard electrode potential (E°) is directly related to the Gibbs free energy change (ΔG°), which in turn depends on the number of electrons transferred (n) in the half-reaction.

Among the statements provided, statement (3) is not true: (3) They are independent of temperature

This statement is false because electrode potentials do vary with temperature. The Nernst equation explicitly includes a term (RT/nF)log(Q) to adjust the standard electrode potential (E°) for non-standard conditions, which includes temperature changes. Therefore, the correct answer is: (3) They are independent of temperature