ELECTROCHEMICAL PROCESSES
The standard electrode potentials are very important and we can extract a lot of useful information from them. If the standard electrode potential of an electrode is greater than zero then its reduced form is more stable compared to hydrogen gas. Similarly, if the standard electrode potential is negative then hydrogen gas is more stable than the reduced form of the species. Electrochemical cells are extensively used for determining the pH of solutions, solubility product, equilibrium constant, thermodynamic properties and for potentiometric titrations. The potential of individual half-cell cannot be measured. We can measure only the difference between the two half-cell potentials that gives the emf of the cell. If we arbitrarily choose the potential of one electrode (half-cell) then that of the other can be determined with respect to this. According to convention, a half-cell called the Standard Hydrogen Electrode (SHE) is assigned a zero potential at all temperatures corresponding to the reaction \( \mathrm{H^{+}_{(aq)}+e^{-}\rightarrow \frac{1}{2}H_2(g)} \)
Which statement given below is not true for the reaction ?
Fe3+ + e− → Fe2+
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Both Fe2+ and Fe3+ are anions here.
In a redox half-reaction, reduction is the gain of electrons, which lowers the oxidation state. A species that is reduced takes electrons from something else, so it acts as the oxidising agent. Ions are named by their charge: positive ions are cations and negative ions are anions.
\( \displaystyle \ce{Fe^3+ + e^- -> Fe^2+} \)
Here \( \ce{Fe^{3+}} \) gains one electron and its oxidation state falls from +3 to +2. Both \( \ce{Fe^{3+}} \) and \( \ce{Fe^{2+}} \) carry a positive charge, so both are cations, not anions.
Option (1) Fe3+ is being reduced. — True for this reaction (Fe3+ gains an electron). Gain of an electron is reduction.
Option (2) The oxidation state of Fe has changed. — True (+3 to +2). It changes from +3 in \( \ce{Fe^{3+}} \) to +2 in \( \ce{Fe^{2+}} \).
Option (3) Fe3+ could be referred to as an oxidizing agent in this reaction. — True (Fe3+ accepts electrons). It oxidises the species that supplies the electron.
Option (4) Both Fe2+ and Fe3+ are anions here. — Not true: both are cations. Both carry a positive charge.