Concept Explanation: Formation of Colloidal Sulphur with H₂S

Hydrogen sulphide (H₂S) can react with oxidizing agents to produce colloidal sulphur (S). Colloidal sulphur forms when sulphur is generated in a finely divided state that remains suspended in the solution. The reaction generally involves the oxidation of H₂S to sulphur, and colloidal sulphur is often observed when the reaction is mild or controlled.

The reaction for the formation of sulphur from H₂S is:

Now, let's evaluate each species to see if it produces colloidal sulphur with H₂S:

Step 1: Identify the oxidizing agents

All given species are oxidizing agents except Pb(CH₃COO)₂, which is not an oxidizer but reacts with H₂S to form black PbS precipitate.

Step 2: Check for each species

• FeCl₃: Oxidizes H₂S to sulphur. Reaction: $2{\mathrm{FeCl}}_3+H_{2}S\to 2{\mathrm{FeCl}}_2+2\mathrm{HCl}+S\downarrow$ Produces colloidal sulphur.
• KMnO₄ (acidified): Strong oxidizer, oxidizes H₂S to sulphur or sulphate depending on conditions. In dilute or mild conditions, it can give colloidal sulphur.
• K₂Cr₂O₇ (acidified): Similar to KMnO₄, it can oxidize H₂S to colloidal sulphur under controlled conditions.
• Cl₂ (moist): Reacts with H₂S: ${\mathrm{Cl}}_2+H_{2}S\to 2\mathrm{HCl}+S\downarrow$ Produces colloidal sulphur.
• O₂ (excess): With excess oxygen, H₂S is oxidized to SO₂ or sulphate, not sulphur. No colloidal sulphur.
• Pb(CH₃COO)₂: Not an oxidizing agent. Forms black PbS precipitate: ${\mathrm{Pb}}_{\left({\mathrm{CH}}_3{\mathrm{COO}}_2\right)}+H_{2}S\to {\mathrm{PbS}}_{\left(\text{black}\right)}+2{\mathrm{CH}}_3\mathrm{COOH}$ Does not produce colloidal sulphur.

Step 3: Count the species that produce colloidal sulphur

From the above: FeCl₃, KMnO₄ (acidified), K₂Cr₂O₇ (acidified), Cl₂ (moist) can produce colloidal sulphur. O₂ (excess) and Pb(CH₃COO)₂ do not.

Final Answer

The number of species that produce colloidal sulphur with H₂S is 4.

Related Topics

• Oxidation-Reduction Reactions
• Colloidal Chemistry
• Properties of Hydrogen Sulphide
• Qualitative Analysis of Anions

Important Formulae and Theory

General Reaction for Sulphur Formation:

Key Point: Colloidal sulphur forms when the oxidation is not too vigorous, leading to fine particles that do not settle easily.

Note: Strong oxidizers like concentrated HNO₃ or excess O₂ may oxidize H₂S to SO₂ or sulphate, bypassing sulphur formation.