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) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (d) \(\mathrm{NO}\)

Step-by-Step Solution

Answer

The reaction forms NO (d).

Step 1: Understand the reaction

When zinc (Zn) reacts with cold dilute nitric acid (HNO_3), a redox reaction occurs. Zinc is oxidized, and nitrogen from the nitric acid is reduced.

Step 2: Reduction products of nitric acid

In the reaction of metals with dilute HNO_3, common reduction products are NO or NH_4NO_3. The specific product depends on the conditions, including temperature and concentration.

Step 3: Identify the condition

The problem states the condition of cold dilute HNO_3. Under these conditions, NO is a typical nitrogen reduction product.

Step 4: Determine the correct product

Considering the common reduction product under cold dilute conditions, and the provided options, NO corresponds to option (d).

Key Concepts

Oxidation States

In a redox reaction, which includes both reduction and oxidation processes, substances exchange electrons. This is where the concept of oxidation states becomes crucial. The oxidation state, also known as oxidation number, is a number assigned to an element in a compound or reaction which indicates its degree of oxidation or reduction.
For example, consider a zinc atom, represented by Zn. In its elemental form, it has an oxidation state of 0. When zinc reacts with an acid like HNO₃, it undergoes oxidation. The oxidation state increases as electrons are lost. Specifically, zinc serves as a reducing agent, providing electrons for the reduction process.

• Oxidation involves an increase in oxidation state, e.g., Zn → Zn²⁺ with an oxidation state of +2.
• Reduction involves a decrease in oxidation state as it gains electrons.

During such reactions, it is important to track these states to understand how electrons move. Ultimately, oxidation states help determine which substances are oxidized or reduced in a chemical reaction, guiding us toward the products formed.

Reduction Products

Reduction products refer to the substances formed when a reactant gains electrons in a chemical reaction. In the context of zinc reacting with nitric acid, zinc itself is oxidized while the nitric acid is reduced.
When cold dilute HNO₃ reacts with metals, it is typically reduced to form either nitrogen monoxide (NO) or ammonium nitrate (NH₄NO₃). The choice between these two depends on various factors like temperature, acid concentration, and presence of additional reactants.

• For zinc and cold dilute HNO₃, NO is typically formed as the reduction product.
• Elements like nitrogen gain electrons, causing their oxidation state to decrease.

Understanding reduction products in different conditions helps predict the outcome of a reaction. This concept is vital, especially when dealing with multiple choice problems, as knowing likely products can lead to the correct answer.

Dilute HNO3 Reaction

Dilute HNO₃, or dilute nitric acid, is a weaker form of nitric acid. The term "dilute" refers to the concentration of acid in the solution, indicating that much of the solution consists of water. Reactions involving dilute nitric acid are typically less exothermic compared to those with concentrated acids, given the lower acid content. When zinc is introduced primarily to cold dilute nitric acid, it simplifies the redox reaction dynamics, wherein nitric acid serves as an oxidizing agent.
This particular reaction is noteworthy because the temperature and dilution primarily influence the reduction potential outcomes:

• Cold conditions limit the energy available for the reaction, leading to the reduction of HNO₃ into less nitrogen oxide forms.
• Therefore, HNO₃ at low concentrations yields simpler nitrogen oxides, with NO being a typical reduction product.

Understanding the conditions, such as dilution and temperature, is essential, as they decisively influence the type of nitrogen oxide formed. This insight aids in accurately predicting the reaction products, enhancing problem-solving skills in redox reactions.