Question

A copper rod and Al rod are placed in conc. $HN{O_3}$ in separate containers. Which of the following observations are found after some time?
(A) Copper rod and aluminium rod continue to deteriorate at the same rate.
(B)Copper rod continues to deteriorate while aluminium rod deteriorates initially then stops.
(C)There is no deterioration in either copper rod or aluminium rod
(D)Aluminium rod continues to deteriorate while copper rod deteriorates initially and then stops.

Answer 1

Hint: We should be aware of the role of concentrated nitric acid that is, whether it is a good oxidizing agent, reducing agent or nitrating agent. Writing chemical equations of copper and aluminum will help us come to a conclusion easily.

Complete step by step answer:
The reaction of copper rod in concentrated nitric acid:
\[Cu + HN{O_3} \to Cu{(N{O_3})_2} + N{O_2} + {H_2}O\]…………………………..REACTION 1
The reaction of aluminium rod in concentrated nitric acid:
\[Al + 6HN{O_3} \to Al{(N{O_3})_3} + 3N{O_2} + 3{H_2}O\]……………………….REACTION 2
Nitric acid is a good oxidizing agent. The formation of a protective layer in metal rods that are prevented from undergoing corrosion is known as passivation. The metals such as aluminum, titanium, silicon etc. undergo the process of passivation. The coatings formed by this process have resistance against nitric acid since it is concentrated in the aluminium oxide layer and becomes thinner as an already certain amount undergoes reaction 2.
Copper does not form copper oxide, that is, it does not form the passivation layer. Therefore, it undergoes reaction as shown in reaction 1.
Option A is “Copper rod and aluminium rod continues to deteriorate at the same rate”. From the above discussion we know that Al forms a passive coating which prevents corrosion that is deterioration therefore this option is the wrong answer.
Option B is “copper rod continues to deteriorate while aluminium rod deteriorates initially then stops”. This statement is true since the nitric acid is concentrated and the passive coating becomes thin, but not deteriorated fully therefore the remaining layer stops further deterioration. The copper doesn’t form a passive layer thus option B is the correct answer.
Option C is ” There is no deterioration in either copper rod or aluminium rod”. Metal rod undergoes deterioration in the first place. Thus, the option C is the wrong answer.
Option D is “Aluminium rod continues to deteriorate while copper rod deteriorates initially and then stops”. This statement is exactly contradictory to the above discussion. Thus, this option is the wrong answer.

Note: The formation of a protective layer in metal rods that are prevented from undergoing corrosion is known as passivation. Nitric acid is a good oxidizing agent. In case of Al since, the nitric acid is concentrated and the passive coating becomes thin, but not deteriorated fully therefore the remaining layer stops further deterioration. These are the main points to remember.