Standard electrode potentials of four metals (Mn+/M) are given below. Arrange them in decreasing order of their reducing power.
(A) Zn (-0.76V)
(B) Fe (-0.44V)
(C) Cu (0.33V)
(D) Ca (-2.8 V)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (D), (A), (B), (C)
The standard electrode (reduction) potential E°(Mn+/M) measures how easily the metal ion is reduced. The lower (more negative) the value, the harder the ion is to reduce and the easier the metal is to oxidise. A metal that loses electrons easily is a strong reducing agent:
more negative E° ⇒ stronger reducing power
Arranging the given metals from the most negative to the most positive potential:
(D) Ca: −2.8 V (strongest reducing agent)
(A) Zn: −0.76 V
(B) Fe: −0.44 V
(C) Cu: +0.33 V (weakest reducing agent)
Shortcut: Calcium (D), with the most negative potential, is the strongest reducing agent. Options (2) and (3) both start with (D). Zinc is more negative than iron, so (A) comes before (B), and only Option (2) remains.
NCERT (2026-27 edition):
Measurement of Electrode Potential – …
