From the given standard reduction potentials, arrange the metals in decreasing order of their reducing power.
(A) \( E^0_{Zn^{2+}/Zn} = -0.76V \)
(B) \( E^0_{Mg^{2+}/Mg}= -2.37 V \)
(C) \( E^0_{Al^{3+}/Al} = -1.67 V \)
(D) \( E^0_{Ag^+/Ag}= +0.80 V \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B), (C), (A), (D)
A standard reduction potential measures the tendency of the ion to be reduced. The more negative the E°, the weaker the tendency of the ion to take electrons and the greater the tendency of the metal to give them up, that is, to be oxidised. So the metal with the most negative E° is the strongest reducing agent:
\( \displaystyle \ce{M -> M^{n+} + ne^-} \quad \text{(easier when } E^\circ \text{ is more negative)} \)
Arranging the given potentials from the most negative to the most positive:
\( \displaystyle \text{Mg}\ (-2.37\ \mathrm{V}) \lt \text{Al}\ (-1.67\ \mathrm{V}) \lt \text{Zn}\ (-0.76\ \mathrm{V}) \lt \text{Ag}\ (+0.80\ \mathrm{V}) \)
So the decreasing order of reducing power is:
\( \displaystyle \text{Mg} \gt \text{Al} \gt \text{Zn} \gt \text{Ag} \)
Silver, with a positive E°, is a weaker reducing agent than hydrogen, while \( \ce{Ag^+} \) is a good oxidising agent.
Shortcut: Mg has the most negative E° and Ag the only positive one, so the order starts with (B) and ends with (D); only Option (4) does both.
NCERT (2026-27 edition):
Measurement of Electrode Potential – “… Lithium has the lowest electrode potential indicating that lithium ion is the weakest oxidising agent while lithium metal is the most powerful reducing agent in an aqueous solution. It may be seen that as we go from top to bottom in Table 2.1 the standard electrode potential decreases and with this, decreases the oxidising power of the species on the left and increases the reducing power of the species on the right hand side of the reaction.”