Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not precipitated as sulphide by passing \(\mathrm{H}_{2} \mathrm{~S}\) in presence of conc. \(\mathrm{HCl}\) (a) arsenic (b) lead (c) copper (d) cadmium

Step-by-Step Solution

Answer

Arsenic

Step 1: Understand the Context

Passing hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) gas into a solution may cause some metal ions to precipitate as insoluble sulfides. However, when concentrated hydrochloric acid (\(\mathrm{HCl}\)) is present, it affects the solubility of the sulphides, and not all metal ions will precipitate.

Step 2: Analyze the Options

Consider each metal: arsenic, lead, copper, and cadmium. We need to determine which of these does not form an insoluble sulphide in the presence of \(\mathrm{H}_2 \mathrm{S}\) and \(\mathrm{HCl}\).

Step 3: Recall Solubility Principles

Sulfides of arsenic (\(\mathrm{As}_2\mathrm{S}_3\)), lead (\(\mathrm{PbS}\)), copper (\(\mathrm{CuS}\)), and cadmium (\(\mathrm{CdS}\)) are generally insoluble in water. However, in the presence of \(\mathrm{HCl}\), arsenic sulfide remains soluble due to complex formation, while sulfides of lead, copper, and cadmium are still insoluble and precipitate.

Step 4: Conclusion Based on Solubility

Thus, arsenic sulphide does not precipitate in the presence of \(\mathrm{HCl}\), unlike the other metal sulfides which do. Therefore, arsenic is the metal that does not precipitate out as its sulfide in this reaction.

Key Concepts

Hydrogen Sulfide Reactions

Hydrogen sulfide reactions play a significant role in precipitating metal ions as metal sulfides. When \(\mathrm{H}_{2} \mathrm{S}\) gas is bubbled into a solution containing metal ions, it reacts with those ions to form sulfide precipitates. This process occurs because sulfides of many metals, such as lead, copper, and cadmium, have low solubility in water. As \(\mathrm{H}_{2} \mathrm{S}\) is introduced, these sulfides can form solid precipitates that settle out of the solution.

• Reactions depend heavily on the specific metal ion and its affinity for sulfide ions.
• The acidic environment can influence the extent of precipitation.

Understanding these reactions is essential to ascertain which metal ions will form precipitates and which will remain dissolved under specific conditions.

Solubility Principles

Solubility principles determine whether a metal sulfide formed by a hydrogen sulfide reaction will precipitate. These principles revolve around the concept that some substances can dissolve in certain solvents, while others cannot. For metal sulfides:

• Generally insoluble sulfides will precipitate when hydrogen sulfide gas is present.
• Solubility is affected by the presence of concentrated acids, which alter the ionic equilibrium in the solution.

The role of concentrated hydrochloric acid (\(\mathrm{HCl}\)) is crucial. It increases the solubility of certain metal sulfides, such as arsenic sulfide, by shifting the chemical equilibria, whereas sulfides of metals like lead, copper, and cadmium remain insoluble and precipitate.

Insoluble Sulfides

Insoluble sulfides are metal sulfides that cannot dissolve in water, forming solid precipitates instead. For example, copper (\(\mathrm{CuS}\)), lead (\(\mathrm{PbS}\)), and cadmium (\(\mathrm{CdS}\)) sulfides are known for their low water solubility:

• This characteristic of forming solid precipitates is leveraged in separating or identifying metals.
• They remain insoluble even in the presence of strong acids like concentrated \(\mathrm{HCl}\).

However, it's noteworthy that these properties can be manipulated under different chemical environments, altering their behavior in complex mixtures.

Complex Formation

Complex formation can drastically change the solubility properties of metal ions in a solution. When certain metal ions, such as arsenic, come in contact with concentrated \(\mathrm{HCl}\), complex ions can form. Here’s how it generally happens:

• The acidic conditions provided by \(\mathrm{HCl}\) facilitate the formation of complex ions, which are often more soluble than their corresponding simple metal sulfides.
• This outcome ensures that no precipitate forms even in the presence of hydrogen sulfide.

For arsenic, this complex formation makes its sulfide remain in solution under acidic conditions, explaining why arsenic sulfide does not precipitate when \(\mathrm{H}_{2} \mathrm{S}\) is passed in the presence of \(\mathrm{HCl}\). Understanding this concept is key to predicting and controlling reactions in analytical chemistry.