Problem 21 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Concentrated nitric acid, upon long standing, turns yellow brown due to the formation of [Adv. 2013] (a) NO (b) \(\mathrm{NO}_{2}\) (c) \(\mathrm{N}_{2} \mathrm{O}\) (d) \(\mathrm{N}_{2} \mathrm{O}_{4}\)

Step-by-Step Solution

Answer

(b) \\mathrm{NO}_{2}

Step 1: Identify the Problem

The exercise requires identifying the compound responsible for the color change in concentrated nitric acid when it turns yellow-brown over time.

Step 2: Understand Nitric Acid Behavior

Know that concentrated nitric acid can decompose over time. This decomposition happens because nitric acid dissociates and reacts to produce nitrogen oxides.

Step 3: Determine the Possible Compounds

The compounds that can form from nitric acid decomposition include NO, NO₂, N₂O, and N₂O₄, which are all nitrogen oxides. Analyze which of these is likely responsible for the observed color.

Step 4: Assess the Color Change

Nitrogen dioxide ( ext{NO}_{2}) is known to be a reddish-brown gas; it is most likely to impart a yellow-brown coloration to the nitric acid. Other nitrogen oxides like NO, N₂O, and N₂O₄ don't produce this color.

Step 5: Select the Correct Answer

Since NO₂ causes a yellow-brown color change due to its reddish-brown hue, the correct answer to the exercise is that concentrated nitric acid turns yellow-brown due to the formation of NO₂.

Key Concepts

Nitrogen Oxides

Nitrogen oxides are a diverse group of gases composed of nitrogen and oxygen. They play significant roles in various chemical processes in the atmosphere and industrial applications. When it comes to concentrated nitric acid, there are four main nitrogen oxides you should know:

• NO (Nitric oxide)
• NO₂ (Nitrogen dioxide)
• N₂O (Nitrous oxide)
• N₂O₄ (Dinitrogen tetroxide)

Each of these compounds has distinct properties and effects in chemical reactions. NO is a colorless gas, while NO₂ is reddish-brown. Nitrous oxide is also known as "laughing gas" and is colorless. Dinitrogen tetroxide is a dimer of NO₂ and appears as a colorless gas at lower temperatures, but in equilibrium with NO₂, it may also have a slight brownish color at higher temperatures.

Understanding these compounds is crucial for identifying which among them is responsible for specific color changes when nitric acid decomposes. Their formation and effects can be attributed to various conditions such as temperature and concentration.

Color Change in Nitric Acid

Concentrated nitric acid is initially a colorless or slightly yellow liquid. However, over time, especially when exposed to light and air, it can undergo a color change, turning yellow-brown. This occurs due to the chemical decomposition of nitric acid, resulting in the production of nitrogen oxides.

This process begins when nitric acid decomposes slowly, giving rise to nitrogen dioxide (NO₂). NO₂ is a prominent nitrogen oxide responsible for such a noticeable color change.

• The nitric acid decomposes, producing NO and NO₂.
• NO quickly reacts with oxygen in the air to form more NO₂.
• The presence of NO₂ in solution gives a yellow-brown hue.

This color change not only indicates chemical reactivity but also suggests the gradual breakdown of nitric acid's stability in certain conditions. During this process, safety precautions become necessary due to NO₂ being a hazardous and corrosive gas.

NO₂ Formation

The formation of NO₂ in concentrated nitric acid is a result of its decomposition process. This can be summarized by the following reaction steps:

• In an initial stage, concentrated nitric acid undergoes dissociation.
• It releases NO, which is a colorless gas.
• This NO further reacts with oxygen available in the atmosphere.
• The reaction between NO and oxygen produces nitrogen dioxide (NO₂).

The presence of NO₂ gives the nitric acid a characteristic yellow-brown coloration, which can serve as an indicator of the acid’s ongoing decomposition.

Chemically, this process can be depicted by these simplified equations:

• The slow reaction of nitric acid:
  \( 4 ext{HNO}_3 ightarrow 4 ext{NO}_2 + 2 ext{H}_2 ext{O} + ext{O}_2 \)
• NO₂ formation from NO:
  \( 2 ext{NO} + ext{O}_2 ightarrow 2 ext{NO}_2 \)

Through these reactions, the once clear nitric acid begins to reflect the distinct color characteristic of nitrogen dioxide, illustrating the chemical dynamics at play.