Problem 48 - A Complete Resource Book in Chemistry for JEE Main

Question

A gas that cannot be collected over water is (a) \(\mathrm{PH}_{3}\) (b) \(\mathrm{O}_{2}\) (c) \(\mathrm{SO}_{2}\) (d) \(\mathrm{N}_{2}^{2}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{SO}_{2}\) cannot be collected over water.

Step 1: Understand the Concept

To solve this problem, recognize which gases are soluble or reactive with water. A gas that cannot be collected over water will either be very soluble in water or reactive with water.

Step 2: Analyze the Gases

(a) \(\mathrm{PH}_3\): Phosphine is only slightly soluble in water. However, it is a highly reactive and toxic gas.(b) \(\mathrm{O}_2\): Oxygen is not highly reactive with water and is only slightly soluble, so it can be collected over water.(c) \(\mathrm{SO}_2\): Sulfur dioxide is quite soluble in water, forming sulfurous acid (\(\mathrm{H_2SO_3}\)).(d) \(\mathrm{N}_2^2\): An error seems present in this option as \(\mathrm{N}_2^2\) is not a recognized stable molecule and likely a typo; assuming \(\mathrm{N}_2\), it is inert and slightly soluble.

Step 3: Identify the Correct Answer Based on Analysis

A gas that forms acids upon dissolving in water or is very soluble should not be collected over water. From the analyses:- \(\mathrm{SO}_2\) will form sulfurous acid in water, indicating it should not be collected over water.

Step 4: Confirm the Answer

After reviewing the solubility and reaction with water, \(\mathrm{SO}_2\) stands out as the most problematic gas to collect over water due to its high solubility and acid-forming nature.

Key Concepts

Gas Solubility

Gas solubility refers to the ability of a gas to dissolve in a liquid, such as water. This property can significantly affect how gases are collected and stored. Solubility is influenced by:

• Temperature: Typically, gas solubility decreases with an increase in temperature.
• Pressure: Higher pressure often increases gas solubility as gases are compressed into the liquid.

High solubility in water means a gas will dissolve readily, making it unsuitable for collection over water.
For example, sulfur dioxide ( SO_2 ) dissolves in water to form sulfurous acid ( H_2SO_3 ), indicating it has high solubility and should not be collected over water. In contrast, gases like oxygen ( O_2 ) that are only slightly soluble can be collected over water without significant loss.

Chemical Reactivity

Chemical reactivity describes how a gas interacts with other substances, like water, under set conditions. Gases with certain reactive properties are not ideal for collection over water because they may alter chemically upon contact. Such reactions might produce:

• New compounds, such as acids or bases.
• Changes in pH or other chemical properties of the water.

For instance, SO_2 demonstrates high chemical reactivity with water by forming sulfurous acid, meaning its presence dramatically changes the water's acidity. Alternatively, a gas like phosphine ( PH_3 ) has limited reactivity with water, making it less problematic for water-based collection despite being toxic. Understanding these reactivity traits is crucial for safely handling and collecting gases.

Gas Collection Methods

Different methods exist for collecting gases depending on their solubility and reactivity. Some primary methods include:

• Over Water: Suitable for gases that are neither highly soluble nor reactive, like O_2 or N_2 .
• Using Inverted Containers: This approach works well for gases slightly soluble in water, where bubbles can be trapped without the risk of dissolution.
• Dry Collection: Ideal for highly reactive or soluble gases, such as SO_2 , where they are collected in dry containers to prevent chemical reaction with water.

Choosing the correct method depends on a gas's characteristics, ensuring effective collection without losing or altering the gas. Selecting the appropriate technique safeguards the integrity of the gas and the security of the environment in which it is handled.