Sulfur (S)

Sulfur exists primarily as cyclic S₈ rings in the rhombic (stable at room temperature) and monoclinic (stable at ~95–119 °C) forms. When molten sulfur is poured into cold water it can form plastic sulfur consisting of long S_n chains, which slowly revert to S₈.

On heating, sulfur vaporizes and reacts with oxygen to give sulfur dioxide with an exothermic reaction that emits blue light:

\(\mathrm{S(s) + O_2(g) \rightarrow SO_2(g)}\)

Under more oxidative conditions, SO₂ can be further oxidized to SO₃.

Sulfur spans from −2 to +6:

• −2: H₂S, metal sulfides (e.g., FeS)
• +4: SO₂, sulfites (SO₃²⁻)
• +6: SO₃, sulfates (SO₄²⁻), H₂SO₄

Industrial sulfuric acid production involves:

1. Burn sulfur to SO₂: \(\mathrm{S + O_2 \rightarrow SO_2}\)
2. Catalytic oxidation (V₂O₅): \(\mathrm{2\,SO_2 + O_2 \rightleftharpoons 2\,SO_3}\)
3. Absorb SO₃ in H₂SO₄ to make oleum, then dilute: \(\mathrm{SO_3 + H_2SO_4 \rightarrow H_2S_2O_7 \;;\; H_2S_2O_7 + H_2O \rightarrow 2\,H_2SO_4}\)

H₂S is a toxic, corrosive gas where sulfur is in the −2 state. Its characteristic odor is due to strong receptor binding at very low concentrations. It is a reducing agent and blackens lead acetate paper by forming PbS.

SO₂ is a reducing gas that decolorizes acidified potassium permanganate and can reduce dichromate from orange to green. Typical test:

\(\mathrm{2\,MnO_4^- + 5\,SO_2 + 2\,H_2O \rightarrow 2\,Mn^{2+} + 5\,SO_4^{2-} + 4\,H^+}\)

Vulcanization is the process of forming cross-links between polyisoprene chains in rubber using sulfur, improving elasticity, strength, and thermal stability. Sulfur forms mono-/di-/polysulfide bridges that limit chain slippage.

Thiosulfate (S₂O₃²⁻) is a mild reducing agent used to titrate iodine:

\(\mathrm{2\,S_2O_3^{2-} + I_2 \rightarrow S_4O_6^{2-} + 2\,I^-}\)

This reaction underlies iodometry for determining oxidizing agents (e.g., chlorine, copper(II)).

SO₂ from combustion oxidizes in the atmosphere to H₂SO₄, contributing to acid deposition:

\(\mathrm{SO_2 + \tfrac{1}{2}O_2 + H_2O \rightarrow H_2SO_4}\)

Acid rain acidifies soils and waters, damaging ecosystems and infrastructure. Flue-gas desulfurization (FGD) reduces emissions.

Yes. In hot, concentrated alkali, sulfur can yield a mixture of sulfide (S²⁻) and thiosulfate (S₂O₃²⁻) ions. This reflects sulfur’s ability to occupy intermediate oxidation states in redox processes.