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 \( \mathrm{H^{+}_{(aq)}+e^{-}\rightarrow \frac{1}{2}H_2(g)} \)
On the basis of the standard electrode potential values,
Sn2+/ Sn = −0.14 V, Br2/ Br− = +1.09 V, Zn2+/ Zn = −0.76 V, O2/ H2O = +1.23 V.
which of the following is the strongest oxidizing agent?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → O2
A standard electrode potential written for a reduction (oxidised form/reduced form) measures how readily the oxidised form accepts electrons. The more positive the E°, the stronger the oxidised form is as an oxidising agent. The more negative the E°, the stronger the reduced form is as a reducing agent.
The oxidising agents in the options are the oxidised forms of the given couples:
\( \displaystyle \ce{O2}\ (+1.23\ \text{V}) > \ce{Br2}\ (+1.09\ \text{V}) > \ce{Sn^2+}\ (-0.14\ \text{V}) > \ce{Zn^2+}\ (-0.76\ \text{V}) \)
\( \ce{O2} \)/\( \ce{H2O} \) has the highest reduction potential, so O2 is the strongest oxidising agent.
Why the other options are incorrect
Option (1) Sn2+ — Incorrect. Its E° (−0.14 V) is negative, so it is a weak oxidising agent.
Option (2) Br2 — Incorrect. It is a good oxidising agent (+1.09 V), but weaker than \( \ce{O2} \) (+1.23 V).
Option (3) Zn2+ — Incorrect. It has the most negative E° (−0.76 V), so it is the weakest oxidising agent of the four.
NCERT (2026-27 edition):
Measurement of Electrode Potential – …
