Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following on reaction with \(\mathrm{H}_{2} \mathrm{~S}\) does not produce metallic sulphide? (a) \(\mathrm{CdCl}_{2}\) (b) \(\mathrm{ZnCl}_{2}\) (c) \(\mathrm{CoCl}_{2}\) (d) \(\mathrm{CuCl}_{2}\)

Step-by-Step Solution

Answer

ZnCl₂ does not produce a metallic sulphide precipitate with H₂S.

Step 1: Understanding the Reaction

The reaction we are examining is between a metal chloride, such as \( CdCl_{2}, ZnCl_{2}, CoCl_{2}, \) or \( CuCl_{2} \), and \( H_{2}S \). In general, when a metal chloride reacts with \( H_{2}S \), a metal sulphide is formed, which is insoluble and precipitates out of the solution.

Step 2: Examining Precipitation Conditions

Metal sulphides are often insoluble in water, but the conditions for precipitation vary. Given the options, we need to identify which metal sulphide does not form or is soluble under standard conditions, meaning it won’t precipitate as a solid.

Step 3: Considering Solubility of Metal Sulphides

Cadmium sulfide \((CdS)\), zinc sulfide \((ZnS)\), cobalt(II) sulfide \((CoS)\), and copper(II) sulfide \((CuS)\) are the possible products. Generally, \( CdS, ZnS, CoS \) and \( CuS \) are insoluble in water and will form precipitates. However, zinc sulfide \((ZnS)\) can dissolve in excess acidic or alkaline conditions, making it less likely to precipitate than the others.

Step 4: Confirming the Least Likely Precipitate

Given the provided options and typical solubility rules, \( ZnCl_{2} \) is the compound that does not produce a metallic sulphide as a precipitate in the presence of \( H_{2}S \) because zinc sulfide dissolves in acidic solutions.

Key Concepts

Metal Chlorides

Metal chlorides are compounds composed of metal ions bonded to chloride ions. In this context, we focus on common metal chlorides like \( CdCl_2 \), \( ZnCl_2 \), \( CoCl_2 \), and \( CuCl_2 \). These chlorides are ionic in nature, meaning that they dissolve in water to give separate metal cations and chloride anions.

When metal chlorides react with other compounds, such as \( H_2S \), they often undergo a chemical transformation to form new substances. This is because the metal ions can react with sulfide ions from hydrogen sulfide to form metallic sulfides.

Precipitation Reaction

A precipitation reaction occurs when dissolved ions in a solution combine to form an insoluble solid, known as a precipitate. In our case, when metal chlorides like \( CdCl_2 \), \( ZnCl_2 \), \( CoCl_2 \), and \( CuCl_2 \) react with \( H_2S \), metal sulfides precipitate out of the solution as they form.

Here's how it works:

• Hydrogen sulfide (\( H_2S \)) provides sulfide ions (\( S^{2-} \)) in the solution.
• These sulfide ions combine with metal cations to form metal sulfides.
• The insoluble metal sulfides then settle out of the solution as a solid.

Precipitation reactions are a key part of various chemical processes, and they can be used to isolate or purify specific compounds from a mixture by taking advantage of their insolubility.

Solubility of Metal Sulphides

The solubility of metal sulfides in water varies depending on the specific metal involved. Most metal sulfides are poorly soluble in water and tend to precipitate out of the solution as solids.

In this specific case, cadmium sulfide \((CdS)\), cobalt(II) sulfide \((CoS)\), and copper(II) sulfide \((CuS)\) are typically insoluble under standard conditions. This insolubility is what causes them to form precipitates when they react with \( H_2S \).

However, zinc sulfide \((ZnS)\) is a bit different. It is known to dissolve in acidic or alkaline solutions, making it more soluble under certain conditions. This is why it is considered less likely to form a solid precipitate in comparison to its counterparts when examined under the same circumstances.

Understanding the solubility of various compounds is crucial in predicting the outcome of chemical reactions, especially when dealing with precipitation reactions.