Problem 54 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A white solid is either \(\mathrm{Na}_{2} \mathrm{O}\) or \(\mathrm{Na}_{2} \mathrm{O}_{2}\). A piece of red litmus paper turns white when it is dipped into a freshly made aqueous solution of the white solid. (i) Identify the substance and explain with balanced equation. (ii) Explain what would happen to the red litmus if the white solid were the other compound.

Step-by-Step Solution

Answer

The substance is \(\mathrm{Na}_{2} \mathrm{O}_{2}\). If it were \(\mathrm{Na}_{2} \mathrm{O}\), red litmus would turn blue.

Step 1: Analyze the Reaction

When red litmus paper turns white, it indicates bleaching due to the release of oxygen which typically occurs in the presence of peroxides. Here, \(\mathrm{Na}_{2} \mathrm{O}_{2} \) is known to produce hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)) in an aqueous solution. According to the chemical reaction: \[ \mathrm{Na}_{2} \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} + \mathrm{H}_{2} \mathrm{O}_{2} \]The \(\mathrm{H}_{2} \mathrm{O}_{2}\) can decompose to release oxygen, which bleaches the red litmus paper and turns it white.Thus, the substance is \(\mathrm{Na}_{2} \mathrm{O}_{2}\) due to this bleaching action.

Step 2: Consider the Alternative

If the white solid were \(\mathrm{Na}_{2} \mathrm{O}\), when dissolved in water, it would form sodium hydroxide, a basic solution. The reaction is:\[ \mathrm{Na}_{2} \mathrm{O} + \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} \]A basic solution turns red litmus paper blue due to its alkaline nature rather than bleaching it. Therefore, if the red litmus turned blue instead of white, it would indicate \(\mathrm{Na}_{2} \mathrm{O}\).

Key Concepts

Redox Reactions

Redox Reactions, short for reduction-oxidation reactions, play a fundamental role in many chemical processes.
They involve the transfer of electrons between substances, often highlighting a change in oxidation states.
In the case of the provided exercise, we have a redox reaction occurring with sodium peroxide \(\mathrm{Na}_{2} \mathrm{O}_{2}\) in water.

When \(\mathrm{Na}_{2} \mathrm{O}_{2}\) is dissolved in water, a redox reaction takes place:

• \(\mathrm{Na}_{2} \mathrm{O}_{2}\) reacts with water to form sodium hydroxide (\(\mathrm{NaOH}\)) and hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)).
• Further decomposition of \(\mathrm{H}_{2} \mathrm{O}_{2}\) can release oxygen, demonstrating oxidizing behavior.

This release of oxygen illustrates the oxidizing property of \(\mathrm{Na}_{2} \mathrm{O}_{2}\), leading to the bleaching effect on the litmus paper by oxidizing organic dyes.
Understanding such redox interactions helps explain the observed chemical behavior.

Acid-Base Indicators

Acid-base indicators are substances that change color according to the pH of a solution.
Litmus paper is a common indicator used to identify whether a solution is acidic or basic.

In an alkaline environment, red litmus paper turns blue, but an interesting exception is observed in this exercise:

• Red litmus paper turns white when exposed to the solution of \(\mathrm{Na}_{2} \mathrm{O}_{2}\).
• This is due to the bleaching action of oxygen released from hydrogen peroxide \((\mathrm{H}_{2} \mathrm{O}_{2})\).

Had the compound been \(\mathrm{Na}_{2} \mathrm{O}\), it would dissolve in water to form a basic solution \((2\mathrm{NaOH})\), andred litmus paper would change to blue instead.
This demonstrates the unique interaction of peroxide compounds with litmus paper, where bleaching occurs rather than a color change due to pH variations.

Chemical Equations

Chemical equations represent the substances involved in a chemical reaction and their transformation.
They are crucial for depicting the changes accurately, including balancing both reactants and products.

Let's delve into the equations relevant to this exercise:

• For \(\mathrm{Na}_{2} \mathrm{O}_{2}\):
  The chemical reaction when it reacts with water is balanced as:
  \[ \mathrm{Na}_{2} \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} + \mathrm{H}_{2} \mathrm{O}_{2} \]
• This equation shows the formation of sodium hydroxide and hydrogen peroxide, highlighting the generation of bleaching oxygen.
• For \(\mathrm{Na}_{2} \mathrm{O}\):
  Its interaction with water can be shown as:
  \[ \mathrm{Na}_{2} \mathrm{O} + \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH} \]
• Here, the result is a basic sodium hydroxide solution without any bleaching capability.

Having the correct balanced chemical equation is vital for predicting the outcome of reactions and explaining experimental observations.
This precision allows chemists to understand exactly how substances interact and transform.