What are "heavy" sulfides
Heavy or black sulfides refer to four insoluble heavy metal sulfides: CuS (copper(II) sulfide), Ag₂S (silver sulfide), HgS (mercury sulfide), and PbS (lead sulfide). All of them are black, which is reflected in the group name.
The main feature of these compounds is their resistance to non-oxidizing acids. Metathesis reactions do not occur with dilute hydrochloric, hydrobromic, and dilute sulfuric acids: the sulfide ion is held too firmly in the crystal lattice, and hydrogen sulfide is not released.
At the same time, black sulfides actively participate in redox reactions with strong oxidizing agents. On the Chemistry Unified State Exam, this almost always refers to the reaction with concentrated nitric acid. Concentrated sulfuric and dilute nitric acids are generally not included in exam tasks on this topic.
Two models of the redox reaction
For each black sulfide, two oxidation schemes are formally possible:
- First model. Sulfur remains with "its" metal: the sulfide is oxidized to the sulfate of the same metal. Sulfur changes its oxidation state from −2 to +6, and nitrogen is reduced from +5 to +4 (NO₂ is released).
- Second model. Sulfur is released as free sulfuric acid, and the metal binds to the nitrate ion, forming a nitrate. The oxidation states of sulfur and nitrogen change in the same way.
Both schemes are chemically correct, but when drafting an answer for the Unified State Exam, it is important to choose the one that reflects the actual direction of the process.
Thermodynamic criterion for product selection
Any reaction tends to proceed irreversibly, i.e., toward the formation of the least soluble product — a precipitate or a gas. This is a consequence of thermodynamics: a decrease in product concentration in solution shifts the equilibrium to the right. Therefore, when choosing between two models, the one containing an insoluble compound among the products is written down.
- PbS + HNO₃(conc.). PbSO₄ is formed as an insoluble precipitate, whereas Pb(NO₃)₂ is soluble. The first model is written: lead sulfate, NO₂, water.
- HgS + HNO₃(conc.). HgSO₄ is sparingly soluble, while Hg(NO₃)₂ is soluble. The first model with the formation of mercury sulfate is preferred.
- CuS + HNO₃(conc.). Both CuSO₄ and Cu(NO₃)₂ are soluble, so both notations are acceptable. It is more convenient to use the first model.
Special case: silver sulfide
Ag₂SO₄ is formally a precipitate, so the first model seems logical. However, in an excess of concentrated HNO₃, normal silver sulfate is not formed: it converts into a soluble acid salt — silver hydrogensulfate. On the Unified State Exam, acid salts are not expected in such equations, and answers with Ag₂SO₄ have been rejected by experts and led to appeals.
The only safe writing option is according to the second model: the reaction products are AgNO₃, H₂SO₄, NO₂, and H₂O. Sulfur completely converts into sulfuric acid, and silver forms a soluble nitrate.
Guidelines for Unified State Exam formatting
To successfully complete tasks 29 and 31, it is recommended to follow a single scheme:
- For CuS, HgS, and PbS with concentrated HNO₃, write the first model: the products are the corresponding metal sulfate, NO₂, and H₂O.
- For Ag₂S with concentrated HNO₃, write the second model: the products are AgNO₃, H₂SO₄, NO₂, and H₂O.
- With dilute non-oxidizing acids (HCl, HBr, dilute H₂SO₄), black sulfides do not react — this is a typical distractor in the tasks.
- Do not use concentrated H₂SO₄ and dilute HNO₃ as reagents with heavy sulfides in standard Unified State Exam equations.
When balancing coefficients, the ion-electron (or oxidation number) balance method should be used: sulfur gives up 8 electrons (−2 → +6), and nitrogen accepts 1 electron (+5 → +4 in NO₂).
Key points
- Black sulfides are CuS, Ag₂S, HgS, PbS; all are insoluble in water and non-oxidizing acids.
- A redox reaction occurs with concentrated HNO₃: sulfur −2 → +6, nitrogen +5 → +4.
- The reaction direction is determined by the formation of the least soluble product.
- PbS is characterized by the formation of a PbSO₄ precipitate.
- For CuS, both models are equivalent due to the solubility of both copper salts.
- For Ag₂S on the Unified State Exam, only the variant with AgNO₃ and H₂SO₄ is written — Ag₂SO₄ is not credited.
- HgS yields sparingly soluble HgSO₄ as the preferred product.
- The topic regularly appears in tasks 29 and 31 and requires an understanding of the mechanism rather than rote memorization.