Question

Which of the following element have the least ability to react with acid to liberate hydrogen?

• A. $Zn$
• B. $Cr$
• C. $Cu$
• D. $Ni$

Answer 1

The correct option is C $Cu$

Electropositive metal will ready gives electron and reduce $H^{+}$ to $H_2$ gas.
Thus, for a metal to highly reactive with acid, it should readily give electrons i.e., it should have high negative $E_{M^{2+}/M}^0$ value.

Generally, the value of $E_{M^{2+}/M}^0$ increases from left to right in the period. Thus, the value $E_{M^{2+}/M}^0$ of 3d series becomes less negative across the 3d-series.

But Zn, Cr and nickel has more negative $E_{M^{2+}/M}^0$ value than expected. Thus, they are reactive toward acids.

In Cu, the value of $E_{M^{2+}/M}^0$ is positive which means it is stable in its metal state. This is due to the high sublimation and ionisation enthalpy and least hydration energy.
Sublimation and ionisation enthalpy is not matched by the hydration energy so it is stable in metal state.

Hence, Cu is least reactive with the acids.