Question

State reasons for the following:
(a) Zinc oxide is an amphoteric oxide.
(b) Sodium metal is stored in bottle filled with kerosene oil.
(c) In the reactions of nitric acid with metals, generally hydrogen gas is not evolved.

Answer 1

Step 1: Reason for Zinc oxide being an Amphoteric Oxide:
Zinc oxide (ZnO) is an amphoteric oxide because it exhibits both acidic and basic properties. It reacts with acids to form salts and water, and it also reacts with bases to form zincates.
1. When it reacts with an acid (e.g., hydrochloric acid), it forms zinc chloride and water: \[ \text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O} \] 2. When it reacts with a base (e.g., sodium hydroxide), it forms sodium zincate and water: \[ \text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2\text{ZnO}_2 + 2\text{H}_2\text{O} \] This ability to react with both acids and bases makes zinc oxide amphoteric.

Step 2: Reason for Storing Sodium Metal in Kerosene Oil:
Sodium metal is highly reactive, especially with water and oxygen in the air. It reacts vigorously with moisture to form sodium hydroxide and hydrogen gas, which can be dangerous.
Therefore, sodium metal is stored in a bottle filled with kerosene oil to prevent its contact with air and moisture. Kerosene acts as a protective barrier, preventing the sodium from reacting with water and oxygen.

Step 3: Reason for No Hydrogen Gas Evolution in Nitric Acid Reactions with Metals:
In reactions of nitric acid (HNO₃) with metals, hydrogen gas is generally not evolved because nitric acid is a strong oxidizing agent. Instead of releasing hydrogen gas, nitric acid oxidizes the metal to form metal nitrates and releases nitrogen oxides such as nitrogen dioxide (NO₂), nitric oxide (NO), or nitrous oxide (N₂O) as by-products. For example, when zinc reacts with nitric acid, it forms zinc nitrate and nitrogen dioxide gas instead of hydrogen gas: \[ \text{Zn} + 2\text{HNO}_3 \rightarrow \text{Zn(NO}_3)_2 + \text{NO}_2 + \text{H}_2\text{O} \] Thus, hydrogen gas is not released due to the oxidizing nature of nitric acid.

Step 4: Conclusion:
- Zinc oxide is an amphoteric oxide because it reacts with both acids and bases.
- Sodium metal is stored in kerosene oil to prevent it from reacting with moisture and air.
- In reactions of nitric acid with metals, hydrogen gas is not evolved because nitric acid acts as an oxidizing agent, producing nitrogen oxides instead.