Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{Zn}\), on reacting with cold dil. \(\mathrm{HNO}_{3}\) forms (a) \(\mathrm{ZnNO}\) (b) \(\mathrm{NO}_{2}\) (c) \(\mathrm{NH}_{4} \mathrm{NO}_{3}^{3}\) (d) \(\mathrm{NO}\)

Step-by-Step Solution

Answer

(d)  ext{NO} is formed.

Step 1: Analyze the reaction conditions

The reaction involves zinc ( ext{Zn}) and dilute cold nitric acid ( ext{dil. HNO}_3). Cold dilute nitric acid primarily acts as a mild oxidizing agent.

Step 2: Predict the possible reaction products

Zinc being a metal will dissolve in acid to typically form  ext{Zn}^{2+} ions along with the reduction of  ext{HNO}_3 which generally produces nitrogen oxides or ammonium compounds, given the dilute conditions.

Step 3: Identify possible nitrogen oxides formed

In the presence of cold, dilute nitric acid, nitrogen monoxide ( ext{NO}) is a common product.  ext{NO}_2 is usually formed in more concentrated conditions, and  ext{NH}_4 ext{NO}_3 or similar compounds are formed in very concentrated conditions.

Step 4: Conclude based on reaction type

Given these reaction conditions (cold and dilute acid),  ext{Zn} reacting with  ext{HNO}_3 will most likely produce  ext{NO}.

Key Concepts

Oxidation-reduction reactions

In chemistry, oxidation-reduction reactions, often referred to as redox reactions, are essential processes where electrons are transferred between substances. In the case of zinc reacting with dilute nitric acid, zinc undergoes oxidation and loses electrons. This is represented by zinc going from a neutral state to a positively charged ion, \(\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-\). The electrons lost by zinc are used to reduce nitric acid. This makes zinc the reducing agent.During this reaction, the nitric acid acts as an oxidizing agent, accepting electrons from zinc. The process of reduction changes the nitrogen in nitric acid to form various nitrogen oxides, depending on the conditions:

• In concentrated conditions: \(\text{NO}_2\)
• In dilute conditions: \(\text{NO}\)
• In very concentrated conditions: ammonium nitrate compounds might form

Understanding redox reactions involves keeping track of oxidation numbers and knowing the tendency of substances to either donate or accept electrons.
Reducing and oxidizing agents play their roles in these dynamic exchanges, making redox reactions central to many chemical processes.

Nitrogen oxide formation

When zinc reacts with dilute, cold nitric acid, specifically under the given conditions, the primary nitrogen oxide formed is nitrogen monoxide (NO). This occurs because dilute nitric acid under mild temperature conditions preferentially facilitates the formation of less oxidized nitrogen species. The reaction can be simplified as follows:\[\text{3 Zn} + \text{8 HNO}_3 \rightarrow \text{3 Zn}^{2+} + \text{2 NO} + \text{4 H}_2\text{O} + \text{6 NO}_3^-\]In contrast, different conditions can lead to varying nitrogen oxides:

• In more concentrated acid conditions, nitrogen dioxide \(\text{NO}_2\) is more likely to form, characterized by its brownish color and distinct properties.
• Ammonium nitrate forms primarily under highly concentrated nitric acid conditions.

The formation of these compounds depends critically on the concentration of the acid and the temperature of the reaction environment.

Chemical reaction conditions

Chemical reactions are highly dependent on their specific conditions, such as concentration and temperature. These elements dictate the pathway the reaction takes and the products that are formed. For the zinc and nitric acid reaction, the conditions are cold temperature and dilute acid.Dilute acid suggests that the amount of nitrate ions available is lower, which doesn't favor forming the higher oxidized nitrogen products like \(\text{NO}_2\). Instead, we usually see the production of \(\text{NO}\) because these conditions reduce the oxidizing power of the acid.Temperature also plays a crucial role in chemical reactions. In this case, a colder environment slows down molecular movement, reducing the energy available for the reaction, thereby influencing what products are formed without shifting to higher oxidation states.Conditions such as pressure, solvent, and presence of catalysts can further impact reactions, but in the zinc and nitric acid scenario, concentration and temperature are the primary influences, determining the feasibility and direction of the reaction.