Which of the following has least tendency to liberate $$H_2$$ from mineral acids?

The ability of a metal to liberate $$H_2$$ from mineral acids depends on its reduction potential. A metal can liberate hydrogen from acids only if its standard reduction potential is less than that of hydrogen ($$E^\circ = 0$$ V).

The standard reduction potentials of the given metals are:

• $$Zn^{2+}/Zn$$: $$E^\circ = -0.76$$ V
• $$Mn^{2+}/Mn$$: $$E^\circ = -1.18$$ V
• $$Ni^{2+}/Ni$$: $$E^\circ = -0.25$$ V
• $$Cu^{2+}/Cu$$: $$E^\circ = +0.34$$ V

Among these, Cu has a positive standard reduction potential, meaning it is a weaker reducing agent than hydrogen. Copper cannot displace hydrogen from dilute mineral acids like $$HCl$$ or dilute $$H_2SO_4$$.

All the other metals (Mn, Ni, Zn) have negative reduction potentials and can liberate $$H_2$$ from mineral acids.

Therefore, Cu has the least tendency to liberate $$H_2$$ from mineral acids.

The correct answer is Option A.