Which of the following statement is NOT correct?
(A) Copper liberates hydrogen from acids.
(B) In its higher oxidation states Manganese forms stable compounds with oxygen and fluorine.
(C) \(Mn^{3+}\) and \(Co^{3+}\) are oxidizing agents in aqueous solution.
(D) \(Ti^{2+}\) and \(Cr^{2+}\) are reducing agents in an aqueous solution.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) Only
The standard electrode potential E° tells how easily a metal or ion is oxidised or reduced. A metal with a negative E°(M2+/M) can reduce H+ to H2; a high positive E°(M3+/M2+) means M3+ is easily reduced, so it is an oxidising agent.
(A) Copper liberates hydrogen from acids. — Not a correct statement. E°(Cu2+/Cu) is positive (+0.34 V), so copper cannot reduce H+ to H2. Only oxidising acids such as nitric acid react with copper, and they give oxides of nitrogen, not hydrogen.
\( \ce{3Cu + 8HNO3 (dil.) -> 3Cu(NO3)2 + 2NO + 4H2O} \)
(B) In its higher oxidation states Manganese forms stable compounds with oxygen and fluorine. — A correct statement. Oxygen and fluorine stabilise high oxidation states. The oxidation state of Mn is found from the charge balance (O = −2, F = −1):
\( \ce{Mn2O7} \): 2x + 7(−2) = 0, x = +7 \( \ce{MnF4} \): x + 4(−1) = 0, x = +4
(C) \(Mn^{3+}\) and \(Co^{3+}\) are oxidizing agents in aqueous solution. — A correct statement. Their E°(M3+/M2+) values are high and positive (+1.57 V and +1.97 V), so they readily take an electron and are the strongest oxidising agents among the M3+ ions of the series.
(D) \(Ti^{2+}\) and \(Cr^{2+}\) are reducing agents in an aqueous solution. — A correct statement. Their E°(M3+/M2+) values are negative, so they readily lose an electron. They are strong reducing agents and even liberate hydrogen from a dilute acid.
\( \ce{2Cr^{2+}(aq) + 2H^+(aq) -> 2Cr^{3+}(aq) + H2(g)} \)
NCERT (2026-27 edition):
Trends in the M2+/M Standard Electrode Potentials – “… The unique behaviour of Cu, having a positive E⊖, accounts for its inability to liberate H2 from acids. Only oxidising acids (nitric and hot concentrated sulphuric) react with Cu, the acids being reduced.”
Trends in Stability of Higher Oxidation States – “The ability of oxygen to stabilise the highest oxidation state is demonstrated in the oxides. The highest oxidation number in the oxides (Table 4.6) coincides with the group number and is attained in Sc2O3 to Mn2O7. … The ability of oxygen to stabilise these high oxidation states exceeds that of fluorine. Thus the highest Mn fluoride is MnF4 whereas the highest oxide is Mn2O7. The ability of oxygen to form multiple bonds to metals explains its superiority.”
Chemical Reactivity and E⊖ Values – “An examination of the E⊖ values for the redox couple M3+/M2+ (Table 4.2) shows that Mn3+ and Co3+ ions are the strongest oxidising agents in aqueous solutions. The ions Ti2+, V2+ and Cr2+ are strong reducing agents and will liberate hydrogen from a dilute acid, e.g.,
\( \ce{2Cr^{2+}(aq) + 2H^+(aq) -> 2Cr^{3+}(aq) + H2(g)} \)”