Standard Electrode Potential (E°) of following redox couples are given below at 298K. What will be the correct increasing order of the reducing strength of the metals?
(A) Mg2+/Mg = -2.36 V
(B) Al3+/Al = -1.66 V
(C) Cr3+/Cr = - 0.74 V
(D) Ni2+/Ni = - 0.25 V
Choose the correct order from the options given below.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (D), (C), (B), (A)
The standard electrode potential (E°) is written as a reduction potential. It measures how readily the ion accepts electrons:
A more negative E° means the ion is reluctant to accept electrons. So the metal readily loses electrons and is a stronger reducing agent.
A more positive E° means the ion readily accepts electrons, so it is a stronger oxidising agent.
Arranging the metals from least negative to most negative E° gives increasing reducing strength:
\( \displaystyle \mathrm{Ni}\ (-0.25\ \mathrm{V}) < \mathrm{Cr}\ (-0.74\ \mathrm{V}) < \mathrm{Al}\ (-1.66\ \mathrm{V}) < \mathrm{Mg}\ (-2.36\ \mathrm{V}) \)
Shortcut: Mg has the most negative E°, so it must come last in the increasing order, and Ni has the least negative E°, so it must come first. Only Option (3) starts with (D) and ends with (A).
NCERT (2026-27 edition):
Measurement of Electrode Potential – …
