Which of the following statements is not correct?

The correct answer is Option (3) → Cu liberates hydrogen from acids

A metal can displace hydrogen from a dilute acid only if its standard electrode potential \( E^\circ(\ce{M^2+}/\ce{M}) \) is negative, that is, if it is a better reducing agent than hydrogen. Copper is the one 3d metal with a positive value:

    \( \displaystyle E^\circ(\ce{Cu^2+}/\ce{Cu}) = +0.34\ \mathrm{V} \)

So copper cannot liberate hydrogen from acids such as dilute HCl. It reacts only with oxidising acids (nitric acid, hot concentrated sulphuric acid), and then the acid itself is reduced instead of hydrogen being given off:

    \( \displaystyle \ce{Cu + 2H2SO4} \xrightarrow{\text{hot, conc.}} \ce{CuSO4 + SO2 + 2H2O} \)

• Option (1) \(Mn^{3+}\) and \(Co^{3+}\) are oxidizing agents in aqueous solution — A correct statement. They have the highest \( E^\circ(\ce{M^3+}/\ce{M^2+}) \) values of the series. \( \ce{Mn^3+} \) (\( \mathrm{3d^{4}} \)) gains an electron to reach the stable half-filled \( \mathrm{3d^{5}} \) of \( \ce{Mn^2+} \).

• Option (2) \(Ti^{2+}\) and \(Cr^{2+}\) are reducing agents in aqueous solution — A correct statement. They lose an electron readily; \( \ce{Cr^2+} \) (\( \mathrm{3d^{4}} \)) becomes \( \ce{Cr^3+} \) (\( \mathrm{3d^{3}} \)), which has a half-filled \( t_{2g} \) level, and can even liberate hydrogen: \( \ce{2Cr^2+ + 2H+ -> 2Cr^3+ + H2} \).

• Option (3) Cu liberates hydrogen from acids — Not a correct statement (Cu does not liberate H₂ from acids). Its \( E^\circ(\ce{Cu^2+}/\ce{Cu}) \) is positive, +0.34 V.

• Option (4) in its higher oxidation states, Mn forms stable compounds with oxygen and fluorine. — A correct statement. Oxygen and fluorine are small and highly electronegative; Mn shows +7 in \( \ce{Mn2O7} \) (oxygen can also form multiple bonds to the metal) and +4 in \( \ce{MnF4} \).

NCERT (2026-27 edition):
Trends in the M²⁺/M Standard Electrode Potentials – …

Chemical Reactivity and E^o Values – “… An examination of the E^o 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.,

    \( \ce{2Cr^2+(aq) + 2H^+(aq) -> 2Cr^3+(aq) + H2(g)} \)”