Problem 16 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The reaction of zinc with dilute and concentrated nitric acid, respectively, produces: (a) \(\mathrm{NO}\) and \(\mathrm{N}_{2} \mathrm{O}\) (b) \(\mathrm{NO}_{2}\) and \(\mathrm{N}_{2} \mathrm{O}\) (c) \(\mathrm{N}_{2} \mathrm{O}\) and \(\mathrm{NO}_{2}\) (d) \(\mathrm{NO}_{2}\) and \(\mathrm{NO}\)

Step-by-Step Solution

Answer

Option (d) NO₂ and NO.

Step 1: Understanding the Reaction of Zinc with Nitric Acid

The reaction of zinc (Zn) with nitric acid (HNO₃) depends on the concentration of the acid. Typically, dilute nitric acid acts as an oxidizing agent producing nitrogen monoxide (NO), while concentrated nitric acid usually leads to the production of nitrogen dioxide (NO₂). Both reactions produce zinc nitrate and other by-products like water.

Step 2: Examining the Reaction of Zinc with Dilute Nitric Acid

When zinc reacts with dilute nitric acid, the primary oxidation product formed is nitrogen monoxide (NO). The chemical equation for this reaction is: \[ 3 ext{Zn} + 8 ext{HNO}_3 ightarrow 3 ext{Zn(NO}_3)_2 + 4 ext{H}_2 ext{O} + 2 ext{NO} \] This indicates that NO is the major gas evolved in this reaction.

Step 3: Examining the Reaction of Zinc with Concentrated Nitric Acid

In this case, zinc reacts with concentrated nitric acid to form nitrogen dioxide (NO₂) as the main nitrogen oxide product. The equation is: \[ ext{Zn} + 4 ext{HNO}_3 ightarrow ext{Zn(NO}_3)_2 + 2 ext{H}_2 ext{O} + 2 ext{NO}_2 \] This indicates that NO₂ is the major gas evolved in this reaction.

Step 4: Matching Reactions with Given Options

From the analysis above, with dilute nitric acid, the product is NO, and with concentrated nitric acid, the product is NO₂. Looking at the options, the reaction that matches these products is: (d) NO₂ and NO.

Key Concepts

Oxidizing Agents

An oxidizing agent is a chemical substance that gains electrons and, in the process, causes another substance to lose electrons. In the reaction between zinc and nitric acid, nitric acid acts as an oxidizing agent. This means that the nitric acid takes electrons away from zinc, allowing zinc to be oxidized. Zinc, being a metal, is keen to lose electrons and readily donates them to the nitric acid. This electron transfer results in changes for both substances involved:

• The zinc (Zn) is converted into zinc ions (Zn²⁺), which integrate into the zinc nitrate (Zn(NO₃)₂) compound.
• The nitric acid (HNO₃) is reduced and forms by-products, such as nitrogen oxides, which can vary depending on the concentration of the acid.

Understanding the role of oxidizing agents is crucial, as it explains why the products change based on the different conditions of nitric acid concentration.

Nitric Acid Concentration

The concentration of nitric acid is a key factor in determining the outcome of its reaction with zinc. Nitric acid can be found in either a dilute or concentrated form, and each produces distinct products when reacting with metals like zinc. When dilute nitric acid is used, the less aggressive environment leads to the production of nitrogen monoxide ( O). This less oxidizing condition does not fully oxidize nitrogen in the nitric acid. In contrast, concentrated nitric acid has a greater oxidizing ability. This results in the formation of nitrogen dioxide ( NO₂), which is a reddish-brown gas. The differences in products highlight how merely altering the concentration of an oxidizing agent like nitric acid can result in different types of nitrogen oxides. This is not just an academic point; it's essential for industrial and laboratory applications where specific reactions are desired.

Nitrogen Oxides Formation

Nitrogen oxides are compounds consisting of nitrogen and oxygen, and they are a significant outcome in reactions involving nitric acid. In the context of zinc reacting with nitric acid, we typically observe two nitrogen oxides: nitrogen monoxide ( O) and nitrogen dioxide ( NO₂).

• In the presence of dilute nitric acid, nitrogen monoxide ( O) is formed. This colorless gas can be further oxidized in the air to form nitrogen dioxide.
• With concentrated nitric acid, nitrogen dioxide ( NO₂) is directly formed. This gas is notable for its brownish color and is commonly associated with photochemical smog.

These nitrogen oxides have various applications but are also environmental pollutants, with potential health impacts. Their formation in chemical reactions highlights the intricate chemistry and the role of oxidation states in determining the reaction's outcome.