Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal gives two chlorides \(\mathrm{A}\) and \(\mathrm{B}\). A gives black precipitate with \(\mathrm{NH}_{3}\) and \(\mathrm{B}\) gives white. With \(\mathrm{KI}, \mathrm{B}\) gives a yellow precipitate, which is soluble in excess of \(\mathrm{KI}\). A and \(\mathrm{B}\) are respectively (a) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}, \mathrm{HgCl}_{2}\) (b) \(\mathrm{HgCl}, \mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{ZnCl}_{2}, \mathrm{HgCl}_{2}^{2}\) (d) \(\mathrm{HgCl}_{2}, \mathrm{ZnCl}_{2}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{Hg_2Cl_2}, \mathrm{HgCl_2} \)

Step 1: Identify Chloride A

Chloride A gives a black precipitate with \( \mathrm{NH}_3 \) (ammonia), which is characteristic of \( \mathrm{Hg_2Cl_2} \), also known as calomel. \( \mathrm{Hg_2Cl_2} \) reacts with \( \mathrm{NH}_3 \) to form a black mercury-ammonia complex. Therefore, Chloride A is \( \mathrm{Hg_2Cl_2} \).

Step 2: Identify Chloride B

Chloride B gives a white precipitate with \( \mathrm{NH}_3 \) and a yellow precipitate with \( \mathrm{KI} \) which is soluble in excess \( \mathrm{KI} \). This behavior is characteristic of \( \mathrm{HgCl_2} \). The yellow precipitate indicates the formation of \( \mathrm{HgI_2} \), which can dissolve in excess \( \mathrm{KI} \) to form a soluble \( \mathrm{KI_3} \). Therefore, Chloride B must be \( \mathrm{HgCl_2} \).

Step 3: Compare Results with Options

We have determined that Chloride A is \( \mathrm{Hg_2Cl_2} \) and Chloride B is \( \mathrm{HgCl_2} \). Reviewing the provided options, (a) \( \mathrm{Hg_2Cl_2} \), \( \mathrm{HgCl_2} \) correctly matches our findings. The other options do not match the properties we identified in Steps 1 and 2.

Key Concepts

Mercury Chlorides

Mercury is a fascinating element, and it forms several distinct compounds with chlorine, notably mercury(I) chloride (\(\mathrm{Hg}_2\mathrm{Cl}_2\)) and mercury(II) chloride (\(\mathrm{HgCl}_2\)). These two compounds exhibit different physical appearance, solubility, and chemical behavior.

• **Mercury(I) chloride**, also known as calomel, appears as a white, insoluble powder. Being less reactive, it is commonly used in medicinal applications.
• **Mercury(II) chloride**, on the other hand, is a white crystalline substance that is soluble in water but highly toxic. It is often used as a disinfectant or catalyst in industrial processes.

Mercury chlorides are known for their unique reactivity based on their oxidation states. Understanding the fundamental differences between \(\mathrm{Hg}_2\mathrm{Cl}_2\) and \(\mathrm{HgCl}_2\) is essential for grasping their application in chemical reactions, such as precipitation reactions.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine and form an insoluble solid called a precipitate. This type of reaction is common when dealing with metal chlorides like mercury chlorides. Precipitation can be visualized as the process of ions in solution finding each other to form a solid compound that falls out of solution. For instance:

• When \(\mathrm{Hg}_2\mathrm{Cl}_2\) is mixed with \(\mathrm{NH}_3\) (ammonia), a noticeable black precipitate forms. This tells us that a chemical change has occurred, forming a complex such as the ammonia-mercury complex.
• Similarly, \(\mathrm{HgCl}_2\) forms a yellow precipitate with \(\mathrm{KI}\), which is \(\mathrm{HgI}_2\). This precipitate is strikingly pigmented and dissolves in excess \(\mathrm{KI}\), resulting in a soluble \(\mathrm{KI_3}\) complex.

Precipitation reactions are significant in analytical chemistry for detecting the presence of certain ions in a solution. The distinct color changes provide a visual cue, helping chemists identify various metal ions.

Chemical Behavior of Metal Chlorides

Metal chlorides each exhibit a variety of chemical behaviors based on their specific properties, such as solubility, ionic character, and reactivity. Understanding this behavior is crucial, especially for mercury chlorides.

• **Solubility**: Metal chlorides vary widely in terms of solubility in water. For instance, \(\mathrm{HgCl}_2\) is soluble in water, making it more active in reactions compared to \(\mathrm{Hg}_2\mathrm{Cl}_2\), which is not soluble.
• **Ionic Character and Reactivity**: The degree of ionic character affects reactivity. More ionic chlorides like \(\mathrm{HgCl}_2\) may participate more readily in reactions involving ions. They interact differently with reagents like \(\mathrm{NH}_3\) and \(\mathrm{KI}\).

The reactive nature of mercury chloride compounds is evident when looking at their interaction with common precipitators. The ability to form complex ions and precipitates allows them to be utilized in extraction, purification, and even in the medical field. Understanding these dynamic interactions ensures we use metal chlorides safely and effectively.