Aluminium does not react with either dilute or concentrated trioxonitrate (V) acid because

Analysis of the Question

The question asks why aluminium, a reactive metal, remains unreactive when placed in trioxonitrate(V) acid ($HNO_3$), regardless of whether the acid is dilute or concentrated.

Concept Explanation: Passivity

In chemistry, passivity refers to a phenomenon where a metal becomes "passive" or unreactive despite its position in the reactivity series. This happens when a thin, tough, and non-porous layer of an oxide forms on the surface of the metal. This layer acts as a protective barrier, preventing the underlying metal from coming into contact with the acid, thereby stopping any further chemical reaction.

In the case of aluminium, when it is exposed to concentrated trioxonitrate(V) acid, a thin, tenacious layer of aluminium oxide ($Al_2O_3$) forms on its surface. This layer is insoluble in the acid and adheres tightly to the metal, effectively "passivating" it.

Step-by-Step Solution

1. Evaluate the Reactivity: Aluminium is high up in the electrochemical series (above hydrogen), so it should theoretically react with acids to displace hydrogen. However, the observation shows it does not.
2. Analyze the Role of $HNO_3$: Trioxonitrate(V) acid is a strong oxidizing agent. When aluminium comes into contact with it, the acid oxidizes the surface of the aluminium to form aluminium oxide.
3. Understand the Protective Layer: Unlike the oxides of some other metals, the aluminium oxide layer is very stable, non-porous, and insoluble in the acid.
4. Conclusion: Because this oxide layer covers the entire surface of the aluminium, the acid molecules cannot reach the metal atoms beneath the layer to continue the reaction. This state is known as passivation.

Evaluating the Options

• A: Incorrect. Aluminium is not inherently "insoluble" in the sense of a physical mixture; it is the chemical reaction that is prevented by the oxide layer.
• B: Incorrect. Aluminium is actually above hydrogen in the electrochemical series.
• C: Incorrect. Activation energy is not the limiting factor here; the physical barrier (the oxide layer) is.
• D: Correct. The formation of an insoluble oxide layer on the surface renders the metal passive.

Exam Tip

Whenever you see a question about why Aluminium, Iron, or Chromium do not react with concentrated trioxonitrate(V) acid, always look for the keyword "passivity" or "oxide layer." This is a classic JAMB Chemistry question that tests your understanding of metal surface chemistry!