Question

Which of the following metal do not liberate hydrogen from dilute hydrochloric acid
A. Zn
B. Mg
C. Fe
D. Au

Answer 1

Hint: When the metal is dipped in a solution of acid, let’s take hydrochloric acid (HCl), the metal starts degrading and it gets smaller over time and we can see bubbles and a gas coming out of the solution with a precipitate in the solution. The reactivity of all the metals depends on their electron affinity. Electron affinity is a tendency of an atom, ion, or molecule to attract electrons or any negatively charged species.

Complete Step by Step Answer:
When a metal reacts with acid they produce salt and hydrogen gas. So when we dip a metal rod in acid the bubbles we see are formed due to the release of the hydrogen gas from the solution and precipitate like solid particles which can be of any colour white, black, blue, etc. is the salt which is formed due to the reaction and the depends on the metal and its reactivity.

The reactions of the given metals are as follows:
$Zn+2HCl\rightarrow\ Zn{\rm Cl}_2+H_2\uparrow$
$2Mg+2HCl\rightarrow\ 2MgCl+H_2\uparrow$
$Fe+2HCl\rightarrow\ Fe{Cl}_2+H_2\uparrow$
$Au+HCl\rightarrow\ no\ reaction$

As we know, the reaction of metal with acids depends on the reactivity of metals, gold (Au) comes under the category of least reactive metals. It does not even corrode in presence of oxygen or water or any alkali or acid. Gold is also called a noble metal. Its reactivity is the least because its electron affinity is less than hydrogen so it is unable to replace the hydrogen ion of the acid in the reaction. Thus, Au (gold) will not liberate hydrogen gas.
Thus, Option (D) is correct

Note: Only acidic solution that can dissolve gold is aqua-regia, is a mixture of hydrochloric acid with nitric acid in a ratio of 3∶1 (HNO₃ + 3HCl) and which is due to the good oxidising property of nitric acid and this solution can dissolve gold.