Question

Concentrated nitric acid, upon long standing, turns yellow-brown due to the formation of (A) \(N O\) (B) \(\mathrm{NO}_{2}\) (C) \(N_{2} O\) (D) \(N_{2} O_{4}\)

Short answer

The yellow-brown color in concentrated nitric acid upon long standing is due to the formation of (B) NO2.

Step-by-step solution

Identify the cause of the color change

Nitric acid (HNO3) can decompose, especially upon long standing or exposure to light, releasing nitrogen oxides that can impart a yellow-brown color. The key is to identify which nitrogen oxide(s) is responsible for this coloration.

Rule out colorless gases

Among the options given, nitric oxide (NO) and nitrous oxide (N2O) are colorless gases. Since the question is asking about the cause of a yellow-brown color, these can be ruled out.

Determine the yellow-brown gas

Nitrogen dioxide (NO2) is a gas with a characteristic brown color and is known to be a decomposition product of nitric acid. It's responsible for the yellow-brown color seen in old samples of concentrated nitric acid. Dinitrogen tetroxide (N2O4) is a dimer of NO2, which is also present in equilibrium with NO2 and contributes to the color, but NO2 is the primary substance causing the observed discoloration.

Key concepts

Nitrogen Oxides

Nitrogen oxides are a family of gases and compounds composed of nitrogen and oxygen. They are common environmental pollutants and can be found in a variety of reactions, including the decomposition of nitric acid. One key member of this family is nitrogen dioxide (\textbf{NO}\(_2\)), a reddish-brown gas with a sharp, biting odor. It is well-known for contributing to the formation of photochemical smog. Another is nitric oxide (\textbf{NO}), a colorless and odorless gas that quickly reacts with oxygen to form \textbf{NO}\(_2\).
Nitrous oxide (\textbf{N}\(_2\)\textbf{O}), also known as 'laughing gas', is colorless and has significant applications in surgery and dentistry as an anaesthetic. Lastly, dinitrogen tetroxide (\textbf{N}\(_2\)\textbf{O}\(_4\)) is a chemical that is typically found in equilibrium with \textbf{NO}\(_2\) under certain conditions. Understanding the properties of these oxides, their formation, and their impact on the environment is essential. In the context of this exercise, it's the brown color of nitrogen dioxide that gives us the visual cue of nitric acid's decomposition.

Chemical Stability

Chemical stability refers to the propensity of a chemical substance to resist change and remain unchanged over time. Certain compounds are inherently more stable than others, depending on their molecular structure, environmental conditions, and energy content. In reference to our exercise, concentrated nitric acid is relatively stable under the right storage conditions; however, it can decompose under prolonged exposure to light or heat. This decomposition leads to the formation of nitrogen dioxides, which are less stable and can combine or break down further, particularly when the conditions favor such processes.
Stability is not just a chemical curiosity—it is crucial for storage, handling, and the safe use of chemicals. For students preparing for competitive exams like the JEE Advanced in Chemistry, understanding the concept of chemical stability is vital. They must recognize how factors like temperature, concentration, and catalysts can affect the stability of various chemical species.

JEE Advanced Chemistry

JEE Advanced Chemistry is a challenging and rigorous examination section encountered by students aspiring to enter premier Indian institutes for engineering and technology. It tests the depth of understanding and the application of chemical concepts. Within JEE Advanced Chemistry, topics such as chemical kinetics, reactions and their mechanisms, equilibrium, and physical chemistry principles are widely covered.
For excelling in such exams, it is not enough to simply remember facts. Students must develop a strong conceptual understanding and apply it to novel problems. In the context of our exercise, recognizing the visual change in nitric acid due to nitrogen oxides involves applying concepts like the chemical stability of compounds and their decomposition products. This practical application of theoretical knowledge is a cornerstone of the JEE Advanced Chemistry curriculum, demonstrating the synthesis of information and analytical skills required to tackle complex chemical phenomena.