Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

When dilute \(\mathrm{HCl}\) is added to a solution of substance (X) which is water soluble, a white precipitate (Y) is formed. Treatment of \((\mathrm{Y})\) with \(\mathrm{NH}_{2} \mathrm{OH}\) turns it black (Z). Treatment of \((\mathrm{Y})\) with \(\mathrm{H}_{2} \mathrm{~S}\) also turns it black. Identify ( \(\mathrm{X}\) ) here? (a) \(\mathrm{HgO}\) (b) \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{HgSO}_{4}\) (d) \(\mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

(X) is \( \mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2}\).

Step 1: Analyze Precipitate Formation

When dilute \( \text{HCl} \) is added to the solution containing \( \text{X} \), a white precipitate \( \text{Y} \) is formed. This indicates the formation of a chloride, typically an insoluble chloride. Mercury chlorides such as \( \text{Hg}_2\text{Cl}_2 \) are commonly white precipitates.

Step 2: Reaction with Hydroxylamine

When the precipitate \( \text{Y} \) is treated with \( \text{NH}_2\text{OH} \), it turns black. The black compound suggests the formation of metallic mercury \( \text{Hg} \). This transformation is indicative of \( \text{Hg}_2\text{Cl}_2 \), which can break down to form \( \text{Hg} \).

Step 3: Reaction with Hydrogen Sulfide

Treatment of \( \text{Y} \) with \( \text{H}_2\text{S} \) also turns it black, indicating the formation of \( \text{HgS} \) (mercury sulfide). This is a characteristic behavior of \( \text{Hg}_2\text{Cl}_2 \) reacting with \( \text{H}_2\text{S} \).

Step 4: Identify Suitable Compound

Given the transformations and reactions observed, you are tasked with identifying \( \text{X} \). The white precipitate that turns black in both reactions aligns with \( \text{Hg}_2\text{Cl}_2 \), which would originate from \( \text{Hg}_2\left(\text{NO}_3\right)_2 \). This compound is known to form \( \text{Hg}_2\text{Cl}_2 \) when treated with \( \text{HCl} \). Therefore, \( \text{X} \) must be \( \text{Hg}_2\left(\text{NO}_3\right)_2 \).

Key Concepts

Precipitation Reactions

Precipitation reactions are intriguing chemical processes where two soluble substances, when mixed, create an insoluble product. This insoluble material is called a precipitate. Imagine combining two clear solutions and suddenly seeing a solid appear. Neat, right? This solid can appear as a cloudiness or a separate layer, which is what happens when certain ions meet each other. For instance, if you add dilute hydrochloric acid (HCl) to a solution with a specific water-soluble compound, you might see a white solid forming. In our example, the reaction between HCl and mercury nitrate (Hg_2(NO_3)_2) forms a white precipitate known as mercury chloride (Hg_2Cl_2). This solid isn't soluble in water and separates from the liquid. What's fascinating is how these reactions can help us identify unknown compounds. Each precipitate's unique color and formation conditions can serve as clues in the detective work of chemistry. It's a primary technique chemists use to understand what's happening in a solution.

Transition Metal Chemistry

When it comes to transition metals, chemistry can get quite colorful and exciting. These metals are found in the d-block of the periodic table and exhibit a range of interesting properties. Unlike regular metals, transition metals can form variously charged ions due to their intricate electron structures.In our example, mercury is a transition metal that behaves uniquely compared to others in its group. Mercury forms a diatomic ion, \({Hg}_2^{2+}\), which under certain reactions, like with HCl, will lead to the formation of solid white mercury chloride \(Hg_2Cl_2\). Even more interesting, when this solid is treated with hydroxylamine \(NH_2OH\) or hydrogen sulfide \(H_2S\), it turns black, indicating a change to elemental mercury \(Hg\) and mercury sulfide \(HgS\) respectively.This ability to switch from one color to another demonstrates a key aspect of transition metal chemistry: variable oxidation states and complex formation. Such transitions add layers to the chemical reactions we observe, making them quite dynamic and sometimes even surprising. Transition metals are key players in many reactions and essential in industries ranging from manufacturing to medical applications.

Analyzing Chemical Reactions

Chemical reactions are the heart of chemistry. To decode these, especially with transition metals, you need to look at what substances you start with and what you end up with. This analysis helps define the steps of a reaction and reveals the mysteries of compounds.Let's take a closer look at how we use chemical reaction analysis in our mercury example. When a white precipitate \(Hg_2Cl_2\) forms after adding \(HCl\) to \(Hg_2(NO_3)_2\), something exciting is happening. By introducing other reactants, such as \(NH_2OH\) or \(H_2S\), the precipitate changes color — turning black. These color changes are vital signs of new compounds forming, telling us about complex ion exchanges or reductions occurring.Breaking it down, each step in the reaction provides a clue:

• The initial white precipitate suggests the formation of \(Hg_2Cl_2\).
• Turning black with \(NH_2OH\) hints at producing elemental mercury \(Hg\).
• The blackening with \(H_2S\) indicates the presence of mercury sulfide \(HgS\).

Chemists use these transformations, alongside knowledge about solubility and color changes, to understand reaction pathways and identify unknown substances. So each reaction isn't just a change; it's a page of a broader story in the book of chemistry.