Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

A gas ' \(X\) ' is passed through water to form a saturated solution. The aqueous solution on treatment with the \(\mathrm{AgNO}_{3}\) gives a white precipitate. The saturated aqueous solution also dissolves magnesium ribbon with evolution of a colourless gas ' \(\mathrm{Y}\) '. Identify ' \(\mathrm{X}\) ' and ' \(\mathrm{Y}\) '. (a) \(\mathrm{X}=\mathrm{Cl}_{2}, \mathrm{Y}=\mathrm{H}_{2}\) (b) \(\mathrm{X}=\mathrm{H}_{2}, \mathrm{Y}=\mathrm{Cl}_{2}\) (c) \(\mathrm{X}=\mathrm{Cl}_{2}, \mathrm{Y}=\mathrm{CO}_{2}\) (d) \(\mathrm{X}=\mathrm{CO}_{2}, \mathrm{Y}=\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The gases are \(\mathrm{X} = \mathrm{Cl}_2\) and \(\mathrm{Y} = \mathrm{H}_2\).

Step 1: Identify White Precipitate

When gas 'X' dissolved in water forms a saturated solution that gives a white precipitate with \(\mathrm{AgNO}_3\), it suggests the presence of chloride ions \(\mathrm{Cl}^-\). This is due to the formation of silver chloride \(\mathrm{AgCl}\), which is a white precipitate.

Step 2: Evaluate Gas Dissolving Metal

The saturated solution also dissolves magnesium ribbon with the evolution of a gas 'Y'. Magnesium reacting with an acid typically releases hydrogen gas \(\mathrm{H}_2\). This suggests the solution formed by 'X' is acidic.

Step 3: Identify Possible Gases for 'X'

Based on the information, gas 'X' should be able to produce chloride ions in solution and make the solution acidic enough to release hydrogen gas when it reacts with magnesium. Among the given options, \(\mathrm{Cl}_2\) gas, when dissolved in water, forms hydrochloric acid \(\mathrm{HCl}\), which can fulfill both conditions.

Step 4: Confirm Solution Gas 'Y'

The colorless gas evolved when magnesium reacts with the solution of 'X' is \(\mathrm{H}_2\) gas. In acidic solutions, such as one formed by dissolving chlorine gas in water, magnesium reacts to form magnesium chloride and releases hydrogen gas \(\mathrm{H}_2\).

Step 5: Conclude Identification

Thus, the gas 'X' is \(\mathrm{Cl}_2\) and the gas 'Y' is \(\mathrm{H}_2\), which matches the information provided. So the correct answer is option (a).

Key Concepts

Chlorine gas

Chlorine gas (\(\mathrm{Cl}_2\)) is a pale yellow-green gas at room temperature and is known for its strong, pungent smell. Chlorine gas is highly reactive and easily dissolves in water to form a solution that contains hydrochloric acid (HCl) and hypochlorous acid (HOCl). This reaction is significant because it produces aqueous chloride ions (\(\mathrm{Cl}^-\)), which are important for various chemical reactions. Chlorine is commonly used as a disinfectant and for water purification due to its ability to kill bacteria and other pathogens. When studying chemical reactions involving chlorine gas, it’s important to understand its role in forming acids and precipitates, which makes it useful in tests for the presence of chloride ions.

Silver nitrate reaction

The silver nitrate reaction is a well-known chemical test used to detect the presence of chloride ions in a solution. In this reaction, silver nitrate (\(\mathrm{AgNO}_3\)) reacts with chloride ions to form silver chloride (\(\mathrm{AgCl}\)), which appears as a white precipitate. The formation of this precipitate is an important indicator in qualitative analysis. This method is not limited to chloride ions but can also identify other halides (such as bromides and iodides), though the precipitate may vary slightly in color. Yet, silver chloride's distinctive white color helps specifically confirm the presence of chloride ions, making it an invaluable tool in chemical testing and analysis.

Magnesium reaction

Magnesium is a silvery-white metal that is quite reactive, especially with acids. When magnesium comes in contact with an acidic solution, such as hydrochloric acid (\(\mathrm{HCl}\)), it reacts vigorously to form magnesium chloride (\(\mathrm{MgCl}_2\)) and releases hydrogen gas (\(\mathrm{H}_2\)). This reaction is exothermic, which means it releases heat as it proceeds. The reaction of magnesium and an acid can be represented by the equation:

• \(\mathrm{Mg}(s) + 2\mathrm{HCl}(aq) \rightarrow \mathrm{MgCl}_2(aq) + \mathrm{H}_2(g)\)

Understanding this reaction helps in predicting how magnesium will behave when exposed to acid solutions and is fundamental in many chemical processes involving reactive metals.

Hydrochloric acid

Hydrochloric acid (\(\mathrm{HCl}\)) is a strong acid formed when chlorine gas dissolves in water. It is a vital component in both laboratory and industrial chemical processes. \(\mathrm{HCl}\) is widely used in the production of fertilizers, dyes, and as a cleaning agent. As a strong acid, it fully dissociates in water, releasing hydrogen ions (\(\mathrm{H}^+\)) and chloride ions (\(\mathrm{Cl}^-\)). This dissociation is what makes hydrochloric acid capable of participating in reactions that release gases such as hydrogen or form precipitates like silver chloride. Its ability to react with metals, releasing hydrogen gas, is particularly important in the context of understanding corrosion and reactivity in various materials.

Hydrogen gas evolution

Hydrogen gas (\(\mathrm{H}_2\)) is a colorless, odorless gas that is much lighter than air and highly flammable. The evolution of hydrogen gas occurs in many chemical reactions, especially those involving acids and reactive metals. In the context of magnesium reacting with hydrochloric acid, the hydrogen gas that is released is a sign of a typical single-replacement reaction, where magnesium displaces hydrogen from hydrochloric acid. This reaction produces hydrogen gas and magnesium chloride as products. One can observe tiny bubbles forming, which is the hydrogen gas being liberated. Understanding hydrogen gas evolution is crucial for safety and with applications ranging from fuel cells to refining metals. Its role in chemical reactions signifies energy release and is fundamental in energy conversion processes.