Hydrogen Sulphide: Preparation and Properties & Test for S2− | Chemistry for JAMB PDF Download

Preparation

Hydrogen sulfide can be prepared by the following methods:

Reaction of a metal sulfide with a strong acid

• Metal sulfide + Strong acid → Hydrogen sulfide + Metal salt
  Example: FeS + 2HCl → H₂S + FeCl₂

Reaction of a metal with hydrogen sulfide gas

• Metal + Hydrogen sulfide → Metal sulfide + Hydrogen gas
  Example: Zn + H₂S → ZnS + H₂

Properties as a Weak Acid, Reducing Agent, and Precipitating Agent

Weak Acid

• Hydrogen sulfide is a weak acid that can undergo partial ionization in water, producing hydrosulfide ions (HS^-) and bisulfide ions (HSO^3-).
• It reacts with bases to form salts called sulfides.
  Example: H₂S + NaOH → NaHS + H₂O

Reducing Agent

• Hydrogen sulfide is a good reducing agent. It can reduce certain metal ions to their elemental forms.
  Example: H₂S + Cu²⁺ → CuS + 2H⁺ (Cu²⁺ is reduced to CuS)

Precipitating Agent

• Hydrogen sulfide can form insoluble metal sulfides when reacted with metal ions in an acidic medium.
• These metal sulfides appear as colored precipitates.
  Example: H₂S + Pb²⁺ → PbS↓ + 2H⁺ (Pb²⁺ forms a black precipitate of PbS)

Test for S^2- (Sulfide ions)

• To test for the presence of sulfide ions, we can use a group reagent called lead(II) acetate (Pb(CH₃COO)₂) or lead(II) nitrate (Pb(NO₃)₂).
• When hydrogen sulfide is passed through an acidic solution containing sulfide ions, it forms a black precipitate of lead(II) sulfide (PbS).
  Example: H₂S + Pb²⁺ → PbS↓ + 2H⁺
• The formation of a black precipitate confirms the presence of sulfide ions.

Conclusion

• In conclusion, hydrogen sulfide (H₂S) is a compound that can be prepared through reactions involving metal sulfides with strong acids or metals with hydrogen sulfide gas. It possesses several properties, including being a weak acid, a reducing agent, and a precipitating agent.
• As a weak acid, hydrogen sulfide partially ionizes in water, producing hydrosulfide and bisulfide ions. It reacts with bases to form sulfide salts. As a reducing agent, it can reduce certain metal ions to their elemental forms. When reacted with metal ions in an acidic medium, hydrogen sulfide forms insoluble metal sulfides, which appear as colored precipitates.
• To test for the presence of sulfide ions, a group reagent such as lead(II) acetate or lead(II) nitrate can be used. The addition of hydrogen sulfide gas to an acidic solution containing sulfide ions results in the formation of a black precipitate of lead(II) sulfide. The formation of this precipitate confirms the presence of sulfide ions.