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

Question

A gas ' \(\mathrm{X}\) ' 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 ' \(\mathrm{Y}^{\prime}\). 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

Option (a) \( \mathrm{X} = \mathrm{Cl}_{2}, \mathrm{Y} = \mathrm{H}_{2} \).

Step 1: Analyzing the Reaction with Silver Nitrate

When the saturated solution formed by gas 'X' is treated with \( \mathrm{AgNO}_{3} \), it gives a white precipitate. Silver nitrate reacts with chloride ions to form silver chloride, which is a white precipitate. This indicates that the gas \( \mathrm{X} \) must contain chloride ions.

Step 2: Identifying Gas 'X'

Based on Step 1, gas \( \mathrm{X} \) should contain chloride ions to give a white precipitate with \( \mathrm{AgNO}_{3} \). \( \mathrm{Cl}_{2} \) (chlorine gas) dissolves in water to form hydrochloric acid (\( \mathrm{HCl} \)), containing chloride ions. Therefore, \( \mathrm{X} = \mathrm{Cl}_{2} \).

Step 3: Analyzing Reactions that Produce a Colorless Gas

The saturated solution (likely containing \( \mathrm{HCl} \)) dissolves magnesium ribbon and evolves a colorless gas. When \( \mathrm{HCl} \) reacts with \( \mathrm{Mg} \), hydrogen gas \( (\mathrm{H}_{2}) \), which is colorless, is evolved.

Step 4: Identifying Gas 'Y'

Since the reaction of \( \mathrm{HCl} \) with \( \mathrm{Mg} \) releases hydrogen gas, \( \mathrm{Y}\) is identified as hydrogen gas, \( \mathrm{H}_{2} \).

Step 5: Final Identification

Based on the analysis, gas \( \mathrm{X} \) is chlorine \( (\mathrm{Cl}_{2}) \) and gas \( \mathrm{Y} \) is hydrogen \( (\mathrm{H}_{2}) \). Therefore, the correct option is (a) \( \mathrm{X} = \mathrm{Cl}_{2}, \mathrm{Y} = \mathrm{H}_{2} \).

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into new substances, called products. These changes occur due to the breaking and forming of chemical bonds. Reactions can be classified into several types, such as synthesis, decomposition, single replacement, double replacement, and combustion. In this specific exercise, we are dealing with a series of reactions that include gas dissolution, interaction with ions, and metal-acid reactions.
A notable reaction type in this scenario is the formation of a precipitate, a solid formed from the reaction of solutions. Here, chlorine gas (\(\mathrm{Cl}_{2}\)) dissolves in water to form hydrochloric acid (\(\mathrm{HCl}\)), which then reacts with ions from silver nitrate (\(\mathrm{AgNO_{3}}\)) to produce silver chloride (\(\mathrm{AgCl}\)), the white precipitate observed in the solution.

Gas Identification

Identifying gases involved in chemical reactions is crucial for understanding the overall reaction mechanism and the substances involved. Different gases have characteristic properties that can be detected through observation and by their chemical behavior with other substances.
In this case, gas X was identified as chlorine (\(\mathrm{Cl}_{2}\)), and this was deduced through its ability to dissolve in water to form an aqueous solution that precipitates silver ions as silver chloride (\(\mathrm{AgCl}\)). Subsequently, after the magnesium ribbon reacts with the aqueous chlorine solution, the observation of a colorless gas suggests the evolution of hydrogen gas (\(\mathrm{H}_{2}\)). These deductions help in accurately naming the gases involved: \(\mathrm{Cl}_{2}\) and \(\mathrm{H}_{2}\).

Precipitation Reactions

Precipitation reactions occur when two solutions combine to form an insoluble solid, known as a precipitate. These reactions are typically double replacement reactions where ions exchange partners, resulting in the formation of a solid.
The exercise provided exemplifies a classic precipitation reaction, where \(\mathrm{AgNO_{3}}\) reacts with a solution containing chloride ions, forming silver chloride (\(\mathrm{AgCl}\)), a white precipitate. The presence of this precipitate is key evidence that helped us identify gas X. Such reactions are not only visually noticeable but are also helpful in qualitative analysis to confirm the presence of specific ions.