Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A gas that cannot be collected over water is : [1985-1 Mark] (a) \(\mathrm{N}_{2}\) (b) \(\mathrm{O}_{2}\) (c) \(\mathrm{SO}_{2}\) (d) \(\mathrm{PH}_{3}\)

Step-by-Step Solution

Answer

\(\mathrm{SO}_{2}\) is the gas that cannot be collected over water due to its reactivity.

Step 1: Understand Collection Over Water

Collection over water involves trapping a gas above water in such a way that it can be measured or analyzed, typically in a gas collection jar. Gases that dissolve significantly or react with water are not collected using this method as they would not give accurate readings.

Step 2: Identify Gases That Dissolve or React With Water

We need to identify which gases among the options dissolve or react with water. \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\) are non-polar and only slightly soluble in water, making them suitable for collection over water. \(\mathrm{SO}_{2}\) dissolves in water to form sulfurous acid and \(\mathrm{PH}_{3}\) can react with water depending on conditions, but less so than \(\mathrm{SO}_{2}\). However, it is slightly more soluble and can engage in weak interactions.

Step 3: Select the Gas Least Suitable for Water Collection

Among the options, \(\mathrm{SO}_{2}\) is the gas that reacts with water to form sulfurous acid, making it unsuitable for collection over water specifically due to its high solubility and reactivity. \(\mathrm{PH}_{3}\) is less reactive in this context compared to \(\mathrm{SO}_{2}\).

Key Concepts

Gas Solubility

Gas solubility refers to the ability of a gas to dissolve in a liquid, such as water. This concept is crucial when considering how gases behave in various environments. Different gases have different levels of solubility:

• Some gases, like nitrogen (\( \mathrm{N}_{2} \), have very low solubility in water.
• Others, like carbon dioxide (\( \mathrm{CO}_{2} \), have moderate solubility and can significantly dissolve in water forming carbonic acid.
• Highly soluble gases react almost entirely when exposed to water, like sulfur dioxide (\( \mathrm{SO}_{2} \).

This varying solubility impacts how gases can be collected, stored, and analyzed. When collecting gases over water, solubility needs to be low. Otherwise, the gas will dissolve, leading to inaccurate measurements.

Reactions with Water

Many gases not only dissolve but also chemically react with water, leading to further considerations when collecting or handling them. An illustrative example is sulfur dioxide (\( \mathrm{SO}_{2} \).When \( \mathrm{SO}_{2} \) is exposed to water, it reacts to form sulfurous acid (\( \mathrm{H}_{2}\mathrm{SO}_{3} \):\[ \mathrm{SO}_{2} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{2}\mathrm{SO}_{3} \]This reaction changes the chemical identity of the gas, making it unsuitable for collection over water. Water provides the means for this transformation, emphasizing the importance of understanding chemical reactivity alongside simple solubility.

Gas Properties

The inherent properties of gases determine their interactions with water and their behavior during collection methods. These properties include:

• Polarity: Non-polar gases such as nitrogen (\( \mathrm{N}_{2} \) have low solubility in water.
• Reactivity: Reactive gases can undergo chemical changes upon contact with water, as seen with sulfur dioxide.
• Molecular Weight: This affects how heavy or light a gas is, influencing how quickly it dissolves or reacts.

Proper understanding of these properties allows scientists to predict the behavior of gases in different conditions, particularly when using them in experimental settings or industrial applications.

Sulfur Dioxide

Sulfur dioxide (\( \mathrm{SO}_{2} \) is an important industrial gas with significant environmental and chemical implications.

• In water, \( \mathrm{SO}_{2} \) forms sulfurous acid, a process important in the formation of acid rain.
• \( \mathrm{SO}_{2} \) is a component of volcanic gases and is used in processes like the production of sulfuric acid and paper manufacturing.
• Its ability to dissolve and react with water makes it unsuitable for collection over water, as this will alter its concentration and characteristics.

Understanding its behavior in water helps us manage its environmental impact and optimize its industrial use effectively.