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

Question

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

Step-by-Step Solution

Answer

Zn reacts with cold dil. \( \mathrm{HNO}_{3} \) to produce \( \mathrm{Zn(NO}_{3})_{2} \) and \( \mathrm{NO} \) (Option d).

Step 1: Analyzing the Reaction Context

When zinc (Zn) reacts with cold dilute nitric acid ( 2 4), it typically undergoes a redox reaction. In this scenario, zinc is oxidized from its elemental state to a +2 oxidation state, forming zinc nitrate ( 2(n

Key Concepts

Oxidation States

Oxidation states are vital for understanding redox reactions. They help us track electron transfer. An oxidation state is a number assigned to an element that represents the total number of electrons lost or gained, based on the element's behavior in compounds.

The oxidation state of a free element, like elemental zinc (Zn), is always zero. However, when Zn becomes part of a compound, its oxidation state can change. For example, in its reaction with nitric acid, zinc transitions from an oxidation state of 0 to +2. This shift indicates that zinc loses two electrons, and thus is oxidized. Recognizing these changes is essential for identifying which part of a redox reaction is being oxidized or reduced.

Having a grasp on oxidation states not only aids in balancing chemical equations but also helps predict the products of a reaction. In summary, this concept is the cornerstone of defining and understanding redox reactions.

Zinc and Nitric Acid Reaction

When zinc reacts with cold, dilute nitric acid, an interesting redox reaction takes place. Here, nitric acid ( HNO_3 ) serves as an oxidizing agent. This means it helps another substance lose electrons while it gets reduced. In this reaction:

• Zinc starts in a 0 oxidation state and ends up as Zn²⁺, showing it loses electrons, or oxidizes.
• The nitric acid is reduced, commonly resulting in nitrogen monoxide ( NO ) as one of the products, instead of releasing hydrogen gas, which is typical for reactions with acids like hydrochloric acid or sulfuric acid.

As a result, the typical products you will observe are zinc nitrate ( Zn(NO_3)_2 ), nitrogen monoxide ( NO ), and water. The specific products formed can vary depending on the concentration of HNO_3 and reaction conditions. Understanding these conditions and outcomes is key to mastering reactions involving metals and acids.

Chemical Equation Balancing

Balancing chemical equations is a crucial step in representing chemical reactions accurately. It ensures that the same number of each type of atom exists on both sides of the equation, reflecting the law of conservation of mass.

To balance a redox reaction like the one between zinc and nitric acid, start by writing the unbalanced chemical equation. Next, identify the oxidation and reduction halves. For our reaction:

• Zn(s) + HNO₃(aq) → Zn(NO₃)₂(aq) + NO(g) + H₂O(l)

The zinc oxidation half-reaction: Zn → Zn²⁺ + 2e^-
The nitric acid reduction half-reaction varies but an example is: 4H⁺ + NO₃^- + 3e^- → NO + 2H₂O
Balance electrons lost and gained, and adjust coefficients to ensure equal atoms for each element. This may take trial adjustments. Eventually, you reinforce both matter and charge balances, an essential skill in chemistry. By mastering this skill, you'll find it much simpler to predict the outcomes of various chemical reactions.