Arrange the following species in increasing order of their reducing p…

The correct answer is Option (1) → (A), (B), (D), (C)

A reducing agent gives away electrons and is itself oxidised. The standard reduction potential E° of a couple measures how easily the oxidised form takes electrons. So:

• a high (positive) E°: the oxidised form is a strong oxidising agent, and the reduced form is a weak reducing agent;

• a low (negative) E°: the reduced form gives up electrons easily and is a strong reducing agent.

So reducing power increases as E° decreases. The given species are the reduced forms of their couples:

• (A) \( \ce{Cl^{-}} \): E°(\( \ce{Cl2} \)/\( \ce{Cl^{-}} \)) = +1.36 V, the highest, so the weakest reducing agent.

• (B) \( \ce{I^{-}} \): E°(\( \ce{I2} \)/\( \ce{I^{-}} \)) = +0.54 V.

• (D) Ni: E°(\( \ce{Ni^{2+}} \)/Ni) = −0.25 V.

• (C) Cr: E°(\( \ce{Cr^{3+}} \)/Cr) = −0.74 V, the lowest, so the strongest reducing agent.

Shortcut: \( \ce{Cl^{-}} \) (highest E°) must come first and Cr (lowest E°) last. Only Option (1) starts with (A) and ends with (C).

NCERT (2026-27 edition):
Measurement of Electrode Potential – “… 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.”