Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

For making \(\mathrm{H}_{2} \mathrm{O}_{2}\) in the laboratory (a) \(\mathrm{Na}_{2} \mathrm{O}_{2}\) is added to boiling water (b) \(\mathrm{PbO}_{2}\) is added to an acidified solution of \(\mathrm{KMnO}_{4}\) (c) \(\mathrm{BaO}_{2}\) is added to \(\mathrm{CO}_{2}\), bubbling through cold water (d) \(\mathrm{MnO}_{2}\) is added to dilute cold \(\mathrm{H}_{3} \mathrm{PO}_{4}\)

Step-by-Step Solution

Answer

(a) or (c)

Step 1: Analyze the Reaction Involving \( \mathrm{Na}_{2} \mathrm{O}_{2} \)

The addition of \( \mathrm{Na}_{2} \mathrm{O}_{2} \) to boiling water produces \( \mathrm{H}_{2} \mathrm{O}_{2} \). The reaction is as follows:\[\mathrm{Na}_{2} \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} + \mathrm{H}_{2} \mathrm{O}_{2}\]This reaction directly produces hydrogen peroxide from sodium peroxide and water.

Step 2: Analyze the Reaction Involving \( \mathrm{PbO}_{2} \) and Acidified \( \mathrm{KMnO}_{4} \)

\( \mathrm{PbO}_{2} \) interacts with acidified \( \mathrm{KMnO}_{4} \), but this reaction does not lead to the formation of \( \mathrm{H}_{2} \mathrm{O}_{2} \). Instead, it generally involves redox reactions where manganese changes its oxidation state.

Step 3: Analyze the Reaction Involving \( \mathrm{BaO}_{2} \) and \( \mathrm{CO}_{2} \) in Cold Water

When \( \mathrm{BaO}_{2} \) is added to \( \mathrm{CO}_{2} \) and bubbled through cold water, it forms \( \mathrm{H}_{2} \mathrm{O}_{2} \) according to the following reaction:\[\mathrm{BaO}_{2} + \mathrm{CO}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{BaCO}_{3} + \mathrm{H}_{2} \mathrm{O}_{2}\]\( \mathrm{CO}_{2} \) helps in precipitating \( \mathrm{BaCO}_{3} \), leaving \( \mathrm{H}_{2} \mathrm{O}_{2} \) in solution.

Step 4: Analyze the Reaction Involving \( \mathrm{MnO}_{2} \) and Cold \( \mathrm{H}_{3} \mathrm{PO}_{4} \)

Adding \( \mathrm{MnO}_{2} \) to cold \( \mathrm{H}_{3} \mathrm{PO}_{4} \) does not produce \( \mathrm{H}_{2} \mathrm{O}_{2} \). This reaction might lead to other manganese transformations but not to the formation of hydrogen peroxide.

Key Concepts

Sodium Peroxide Reaction

Sodium peroxide (\( \mathrm{Na}_{2} \mathrm{O}_{2} \)) is a potent reagent that actively participates in the formation of hydrogen peroxide (\( \mathrm{H}_{2} \mathrm{O}_{2} \)) when introduced to water. When boiling water is added, sodium peroxide reacts energetically according to the equation: \[ \mathrm{Na}_{2} \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} + \mathrm{H}_{2} \mathrm{O}_{2}\]This process yields hydrogen peroxide and sodium hydroxide (\( \mathrm{NaOH} \)).
The reaction is advantageous because boiling water accelerates the reaction. This simplicity makes it a preferred method for laboratory preparation of hydrogen peroxide.

As an additional point, it's important to handle sodium peroxide with care. It is highly reactive, especially in the presence of moisture. Proper laboratory safety should be observed when conducting this reaction.

Barium Peroxide Reaction

Barium peroxide (\( \mathrm{BaO}_{2} \)) plays a vital role in producing hydrogen peroxide when it interacts with carbon dioxide and water. The following chemical reaction takes place when barium peroxide is added to water with carbon dioxide bubbling through it:\[ \mathrm{BaO}_{2} + \mathrm{CO}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{BaCO}_{3} + \mathrm{H}_{2} \mathrm{O}_{2}\]Barium carbonate (\( \mathrm{BaCO}_{3} \)) is precipitated out, leaving hydrogen peroxide dissolved in the solution.
This method benefits from the fact that it operates under milder temperature conditions, specifically using cold water.

The production method with barium peroxide is particularly useful due to its efficiency and the ability to generate relatively high concentrations of hydrogen peroxide. Care should always be taken when dealing with barium compounds due to their toxicity, ensuring the chemicals are handled in a well-ventilated area.

Redox Reactions

Redox reactions encompass a broad category where oxidation and reduction take place simultaneously. Here, some substances gain electrons while others lose them. In the context of hydrogen peroxide chemistry, redox reactions are crucial, as seen in several preparation methods.
In the reaction involving \( \mathrm{PbO}_{2} \) and acidified \( \mathrm{KMnO}_{4} \), although not producing hydrogen peroxide, redox chemistry is prominent. Manganese changes oxidation states, demonstrating classic redox behavior.

Understanding redox reactions requires focusing on two main aspects:

• **Oxidation**: The process where a substance loses electrons, increasing its oxidation state.
• **Reduction**: The opposite process where a substance gains electrons, reducing its oxidation state.

Identifying the agents of oxidation (oxidizing agents) and reduction (reducing agents) helps in balancing redox equations and predicts the products formed. This concept is foundational in many chemical reactions and is essential for students to grasp fully to master higher-level chemistry topics.