Problem 41 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is obtained when Fe reacts with dilute HNO \(_{3}\) ? (a) \(\mathrm{NO}\) (b) \(\mathrm{N}_{2} \mathrm{O}\) (c) \(\mathrm{NO}_{2}\) (d) none of these

Step-by-Step Solution

Answer

The product is nitric oxide: (a) \(\text{NO}\).

Step 1: Understanding the Reaction Context

Iron (Fe) reacts differently with various concentrations of nitric acid (\(\text{HNO}_3\)). With dilute \(\text{HNO}_3\), the typical gases produced include nitric oxide (\(\text{NO}\)) and nitrogen gas. We need to identify what is primarily produced in this specific reaction scenario.

Step 2: Identify the Common Reaction Products

Generally, reactions of metals with dilute \(\text{HNO}_3\) result in the reduction of nitric acid to nitric oxide (\(\text{NO}\)). More concentrated solutions may produce other nitrogen oxides, such as \(\text{NO}_2\), but dilute solutions seldom do.

Step 3: Conclusion Based on Chemical Knowledge

Given that the question specifies the reaction with dilute nitric acid, the most likely nitrogen compound formed is nitric oxide (\(\text{NO}\)). Therefore, from the options provided, \(\text{NO}\) is the expected product.

Key Concepts

Iron and Nitric Acid Reaction

When iron, a common metal, comes into contact with nitric acid, an interesting chemical reaction takes place. This reaction is categorized in the field of chemistry as a metal-acid reaction, producing a gas as one of its primary products. In this context, when iron (Fe) reacts with dilute nitric acid (HNO_3), it primarily results in the formation of nitric oxide (\(\text{NO}\)).
Dilute nitric acid facilitates a reaction that isn't as aggressive as its concentrated counterpart. In a more concentrated state, nitric acid tends to yield nitrogen dioxide (\(\text{NO}_2\)), identifiable as a brown gas. However, with a diluted version, the reactivity results favor the formation of nitric oxide, a colorless gas.
It's crucial to note that nitric oxide is rapidly oxidized to nitrogen dioxide when exposed to air, so this reaction is intriguing both in a controlled laboratory setting or in practical industrial applications.

Metal Reactivity with Acids

Metals like iron react uniquely with acids due to their chemical properties. The reactivity of a metal with acid usually depends on the metal's position in the reactivity series, a list ordering metals from most reactive to least reactive.
Iron is moderately reactive, and it can easily react with acids like nitric acid. When a metal reacts with an acid, it typically results in the production of hydrogen gas alongside a salt. However, in the case of nitric acid, which is a strong oxidizing agent, the typical hydrogen gas production seen with reactions involving other acids like hydrochloric acid ( HCl ) can be replaced by nitrogen oxides.
As a result, mediation between oxidizing effects of nitric acid leads to unique outcomes such as producing nitric oxide or other nitrogen compounds. The activity and nature of the metal, the concentration of the acid, and the specific reaction conditions all influence the specific outcome of these chemical interactions.

Oxidation-Reduction Reactions

Chemical reactions often involve the transfer of electrons between substances, described in terms of oxidation and reduction. In the reaction between iron and nitric acid, the nitric acid undergoes a reduction process while the iron is oxidized.
Here's how it works:

• **Oxidation**: Iron loses electrons and increases its oxidation state, forming iron ions.
• **Reduction**: Nitric acid takes in the electrons released by iron, reducing it to nitric oxide.

When these reactions occur, they are often called redox reactions – a shorthand for reduction-oxidation reactions. Redox reactions are vital in numerous chemical processes, from the functioning of batteries to biological systems like cellular respiration. Understanding these types of reactions involves looking at the electron movements that lead to changes in energy and compositions of substances involved.
In the case of iron and nitric acid, the transformation highlights the interesting interplay of oxidizing and reducing agents, with nitric acid serving as a potent oxidizer, facilitating these fascinating chemical changes.