Problem 218 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of \(\mathrm{Zn}\) with dil and conc \(\mathrm{HNO}_{3}\) respectively produces. (a) \(\mathrm{NO}_{2}\) and \(\mathrm{NO}\) (b) \(\mathrm{NO}\) and \(\mathrm{N}_{2} \mathrm{O}\) (c) \(\mathrm{NO}_{2}\) and \(\mathrm{N}_{2} \mathrm{O}\) (c) \(\mathrm{N}_{2} \mathrm{O}\) and \(\mathrm{NO}_{2}\)

Step-by-Step Solution

Answer

Option (a): NO2 and NO.

Step 1: Understanding the Reaction with Dilute HNO3

When zinc (Zn) reacts with dilute nitric acid (HNO3), the reaction results in the production of nitric oxide (NO) gas, a colorless gas that acts as the reduction product of nitrate ion (NO3^-). The reaction can be represented as follows: \[ \text{4 Zn} + \text{10 HNO}_3 \rightarrow \text{4 Zn(NO}_3\text{)}_2 + \text{5 H}_2\text{O} + \text{2 NO} \]

Step 2: Observing Reaction with Concentrated HNO3

In the case of zinc reacting with concentrated nitric acid, nitrogen dioxide (NO2), a brown gas, is usually produced as the nitrate ion reduction product. The equation is written as: \[ \text{Zn} + \text{4 HNO}_3 \rightarrow \text{Zn(NO}_3\text{)}_2 + \text{2 NO}_2 + \text{2 H}_2\text{O} \]

Step 3: Summary of Reaction Products

From the observations of the above reactions, it is noted that dilute HNO3 produces NO gas while concentrated HNO3 produces NO2 gas during its reaction with Zn. Thus, for Zn with dilute HNO3, NO is the primary nitrogen product, and with concentrated HNO3, it's NO2.

Step 4: Identify the Correct Option

Based on the reaction descriptions, the correct option can be identified. Dilute HNO3 produces NO and concentrated HNO3 produces NO2. Referring to the options provided: **(a) NO2 and NO** shows concentrated producing NO2 and dilute producing NO, which matches our observations.

Key Concepts

Reactions with Nitric Acid

When zinc (Zn) interacts with nitric acid (HNO53), interesting reactions occur that are largely dependent on the concentration of the acid. In the case of dilute nitric acid, zinc reacts to produce nitric oxide (NO), a colorless gas. This process is a reductive reaction where the nitrate ions (503^-) in the acid are reduced to form NO gas. The chemical equation representing this is:
\[\text{4 Zn} + \text{10 HNO}_3 \rightarrow \text{4 Zn(NO}_3\text{)}_2 + \text{5 H}_2\text{O} + \text{2 NO}\]For concentrated nitric acid, the reaction differs significantly. Here, zinc oxidizes to form zinc nitrate, releasing nitrogen dioxide (NO502), a brown gas. This indicates a different reduction product for the nitrate ions under more acidic conditions. The equation governing this reaction is:
\[\text{Zn} + \text{4 HNO}_3 \rightarrow \text{Zn(NO}_3\text{)}_2 + \text{2 NO}_2 + \text{2 H}_2\text{O}\]Understanding these reactions is crucial as they demonstrate how the concentration of the acid can control the type of nitrogen oxide produced.

Nitrogen Oxides Formation

The formation of nitrogen oxides (NO and NO502) in reactions involving nitric acid and zinc is fascinating. In chemistry, nitrogen oxides derive from the reduction of nitrate ions during these types of reactions.

• Nitric oxide ( NO) emerges from reactions with dilute nitric acid.
• Nitrogen dioxide ( NO502) forms when zinc reacts with concentrated nitric acid.

These gases are significant due to their distinct physical properties and implications. Nitric oxide is colorless and, upon exposure to air, can rapidly oxidize to form nitrogen dioxide, which is notably brown in color. This transformation showcases how chemicals can change under different environmental conditions. It's crucial to note that these oxides play an integral role in atmospheric chemistry, contributing to phenomena like smog.

Dilute vs Concentrated Acid Reactions

Understanding the differences between reactions involving dilute and concentrated acids is pivotal in chemistry. With dilute nitric acid ( HNO53), the environment is less acidic, promoting the formation of nitric oxide ( NO) gas.

• The zinc acts as a reducing agent, turning nitrate ions into NO gas.
• In a dilute solution, acid molecules are spread out, decreasing their overall effect.

Conversely, with concentrated HNO53:

• The higher acidity facilitates zinc's further oxidation, producing nitrogen dioxide ( NO502).
• In a concentrated solution, acid molecules are more plentiful, amplifying their effects.

These contrasting reactions underscore a critical concept in chemistry: the concentration of reactants can influence product formation. The concept helps chemists predict and control reaction outcomes.

Zinc Reaction Products

The products formed when zinc reacts with nitric acid depend on the concentration of the acid. With both concentrated and dilute nitric acid, zinc serves primarily as a reducing agent. For dilute HNO53 :

• Zinc reacts to form zinc nitrate, water, and nitric oxide ( NO) gas.
• Resulting from a gentler reaction, NO emerges as the major nitrogenous product.

In reactions with concentrated HNO53:

• The output includes zinc nitrate, water, and nitrogen dioxide ( NO502) gas.
• Due to stronger acidic conditions, NO502 is generated.

These reaction outputs not only demonstrate how chemical environments dictate product types but also serve as a practical model for understanding reaction dynamics in chemistry.