Oxidizing acids: general characteristics
Oxidizing acids include nitric acid HNO₃ of any concentration and concentrated sulfuric acid H₂SO₄. Their characteristic feature is that their oxidizing properties are determined not by the hydrogen cation, but by the acid-forming element in its highest oxidation state (N⁺⁵, S⁺⁶).
Principal differences from standard acids:
- they react with metals regardless of their position in the electrochemical series of metals, including metals to the right of hydrogen (Cu, Ag, Hg);
- in reactions with metals, no hydrogen gas is evolved; instead, a reduction product of the acid's central element is formed;
- they are capable of oxidizing non-metals (S, P, C, I₂), hydracids (HBr, HI), and a number of salts (sulfides, bromides, iodides).
Reactions of concentrated H₂SO₄ with metals
The sulfur reduction product depends on metal reactivity. The more active the metal, the more electrons the sulfur atom (S⁺⁶) accepts:
- Active metals (alkali, alkaline earth, Mg, excess Zn) → H₂S (S⁻²), transfer of 8 electrons;
- Moderately active metals (Zn, Fe, Cr, etc.) → S (S⁰), transfer of 6 electrons;
- Noble and inactive metals (Cu, Ag, Hg) → SO₂ (S⁺⁴), transfer of 2 electrons.
Important: dilute H₂SO₄ behaves like a regular acid — it reacts only with metals to the left of hydrogen, evolving H₂. In the cold, concentrated H₂SO₄ passivates Fe, Al, Cr.
Reactions of HNO₃ with metals
The nitrogen reduction product is determined by two factors: metal reactivity and acid concentration. The more active the metal and the more dilute the acid, the deeper the reduction of nitrogen (logic: upon dilution, there are fewer "competing" HNO₃ molecules, so each receives more electrons).
- Dilute HNO₃ + active metal → NH₄NO₃ (N⁻³), transfer of 8 electrons;
- Dilute HNO₃ + moderately active metal → NO (N⁺²);
- Dilute HNO₃ + inactive metal → NO (N⁺²), transfer of 3 electrons;
- Concentrated HNO₃ + active metal → N₂O (N⁺¹);
- Concentrated HNO₃ + inactive metal → NO₂ (N⁺⁴), transfer of 1 electron.
Concentrated HNO₃ in the cold passivates Fe, Al, Cr, Ni; does not react with Au and Pt (dissolved by aqua regia).
Interaction with non-metals
Oxidizing acids are capable of oxidizing S, P, C, I₂. The non-metal generally converts to its highest oxidation state:
- S → H₂SO₄ (S⁺⁶);
- P → H₃PO₄ (P⁺⁵);
- C → CO₂ (C⁺⁴);
- I₂ → HIO₃ (I⁺⁵, not reaching the maximum +7).
Standard reduction products: for concentrated HNO₃ — NO₂, for dilute — NO, for concentrated H₂SO₄ — SO₂.
Exception: the reaction of concentrated H₂SO₄ with sulfur is comproportionation, with SO₂ as the sole product (H₂SO₄ is not formed, otherwise the reaction would shift back to the left): S + 2H₂SO₄(conc) → 3SO₂ + 2H₂O.
Reactions with hydracids and salts
Oxidizing acids oxidize HI and HBr (but not HCl and HF). This clearly demonstrates the difference in oxidizing strength:
- with HI, concentrated H₂SO₄ yields I₂, H₂S, and H₂O — transfer of 8 electrons to sulfur;
- with HI, concentrated HNO₃ yields I₂ and NO₂ — transfer of 1 electron to nitrogen;
- with HBr, reactions proceed via a similar scheme, but sulfur reduction stops at SO₂ (bromine is a weaker reducing agent than iodine).
With hydrogen halide salts (KBr, KI), the products are a free element (Br₂, I₂), an acid salt (KHSO₄ or KNO₃), and the acid reduction product along with water.
Reactions with "black sulfides"
There are four insoluble black sulfides: CuS, PbS, HgS, Ag₂S. They react exclusively with concentrated HNO₃ via a specific pathway: the metal converts to a nitrate, sulfur is oxidized directly to S⁺⁶ (as H₂SO₄), and nitrogen is reduced only to NO₂ (gaining 1 electron):
MeS + HNO₃(conc) → Me(NO₃)ₙ + H₂SO₄ + NO₂ + H₂O.
Reaction feature: the sulfide is insoluble and the acid is concentrated, so deep reduction of nitrogen does not occur, yet sulfur undergoes maximum oxidation — by 8 electrons.
Key points
- Oxidizing acids: HNO₃ at any concentration and H₂SO₄ only when concentrated.
- In reactions with metals, no hydrogen gas is evolved — a reduction product of N or S is formed.
- For H₂SO₄(conc): active metals → H₂S, moderate → S, inactive → SO₂.
- For HNO₃: the more active the metal and the more dilute the acid, the deeper the reduction of nitrogen (from NO₂ to NH₄NO₃).
- Concentrated HNO₃ and H₂SO₄ in the cold passivate Fe, Al, Cr.
- Non-metals (S, P, C, I₂) are oxidized to their highest oxidation state; exception — the reaction of H₂SO₄(conc) with sulfur yields only SO₂.
- Black sulfides CuS, PbS, HgS, Ag₂S are oxidized by concentrated HNO₃, converting sulfur to H₂SO₄ and evolving NO₂.
- HCl and HF are not oxidized by oxidizing acids; HBr and HI are oxidized, with iodine being a stronger reducing agent.