Question

The metal which cannot displace hydrogen from acid is:
A. iron
B. copper
C. sodium
D. zinc

Answer 1

Hint: In the electrochemical series, it is placed below the hydrogen and less reactive than hydrogen. Also, its electrode potential is less than the hydrogen. It is an alkaline earth metal.

Complete answer: In the electrochemical series, the metals, which are placed above the hydrogen are more reactive than hydrogen. So, they have the ability to displace hydrogen. For example, some of these elements are sodium, $\left( {Na} \right)$, iron $\left( {Fe} \right)$ , zinc $\left( {Zn} \right)$, etc.
On the other hand, in the electrochemical series, the metals, which are placed below the hydrogen are less reactive than hydrogen. So, they cannot displace hydrogen. For example, copper $\left( {Cu} \right)$, mercury $\left( {Hg} \right)$, silver $\left( {Ag} \right)$, gold $\left( {Au} \right)$, etc.
So, clearly, the correct answer is copper.
So, the correct answer is “Option d”.

Additional Information:
Copper cannot displace hydrogen from the reaction with dilute hydrochloric acid. The reason is that in electrochemical series copper is placed below the hydrogen. So, its electrode potential is less than the hydrogen. Electrochemical series is also sometimes called an activity series. It is a list, which explains the arrangement of elements in the order of their increasing electrode potential values. The series is established by measuring the potential of various electrodes versus SHE (standard hydrogen electrode). So, a series of electrodes that are arranged in order of their increasing standard oxidation potentials or in the decreasing order of their standard reduction potentials is called an electrochemical series.

Note:
The metal that can give electrons to hydrogen ions $\left( {{H^ + }} \right)$ present in dilute acids for reduction, produces hydrogen from dilute acids.
Oxidation half-reaction will be: $Mn \to M{n^{n + }} + n{e^ - }$ and
Reduction half-reaction will be: \[2{H^ + } + 2{e^ - } \to {H_2}\]
So, the metal, which has negative values of reduction potential, has the tendency of losing electron(s).
So if the reduction potential of metal is less than that of Hydrogen, the metal will provide electrons to ${H^ + }$ of acids. Hence, the metal can displace Hydrogen gas. It is to be noted that generally the alkali and alkaline earth metals have lesser reduction potential than that of Hydrogen.