Question

ASSERTION :Reactivity of aluminium decreases when it is dipped in nitric acid.
REASON: A protective layer of aluminium nitrate is formed when aluminium is dipped in nitric acid.

A. both assertion and reason are correct and reason is the correct explanation for assertion.
B. both assertion and reason are correct and reason is not the correct explanation for assertion.
C. assertion is correct but reason is incorrect.
D. both assertion and reason are incorrect.

Answer 1

: To determine the answer we should know the properties of nitric acid reactivity of metal. The metals whose oxidation potential is less than the water can be easily oxidized by water or moisture. So, the metal dissolves in water. The less oxidation potential means reactivity is high.

Complete step by step solution: The reduction potential of aluminium is $- 1.66$ volt and the reduction potential of water is $- 0.83$ volt. So, the reduction potential of aluminium is less than the reduction potential of water thus aluminium gets oxidised by the environment. This process of oxidation of aluminium is known as corrosion. The corrosion surface gets rough. So, aluminium has high reactivity.

The chemical formula of nitric acid is ${\text{HN}}{{\text{O}}_{\text{2}}}$. The nitric acid is a good oxidizing agent. So, it oxidises the aluminium. The reaction of oxidation of aluminium is as follows:

${\text{Al}}\,\mathop \to \limits^{{\text{HN}}{{\text{O}}_{\text{2}}}} {\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$

By the oxidation of aluminium, aluminium oxide forms. Aluminium oxide does not get oxidised by water so, assertion, Reactivity of aluminium decreases when it is dipped in nitric acid, is true.

The surface is covered by this aluminium oxide, so the rest aluminium which is present below the surface does not get oxidised hence reactivity of aluminium decreases. So, the reason, a protective layer of aluminium nitrate is formed when aluminium is dipped in nitric acid is also correct.

Therefore, option (A), both assertion and reason are correct and reason is the correct explanation for assertion, is correct.

Note: The species which accept electrons is known as reduced and the process is known as reduction. The opposite process in which the species release electrons is known as oxidation. The electrochemical series is a series of elements, in which elements are arranged according to their reduction potential. The reduction potential of an element tells the ease of reduction of an element.