⚗️ Visual Chemistry
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Oxygen, Sulfur, and Their Compounds

20min Part 3 / Ch5 / Lesson 3
Prerequisites: 3-5-1

Objectives

  • Know the difference between allotropes of oxygen (O₂ and O₃)
  • Understand sulfur's allotropes and the properties of sulfuric acid
  • Describe the Contact Process for industrial H₂SO₄ production

Oxygen and Ozone

Allotropes of oxygen:

  • Oxygen O₂: Colorless, odorless; essential for respiration and combustion
  • Ozone O₃: Pale blue, distinctive odor; powerful oxidizer

3O2→UV2O3\mathrm{3O_2 \xrightarrow{UV} 2O_3}

Ozone in the stratosphere absorbs UV radiation, protecting life on Earth.

Detecting ozone: KI-starch paper turns blue-purple

O3+2KI+H2O→I2+2KOH+O2\mathrm{O_3 + 2KI + H_2O \rightarrow I_2 + 2KOH + O_2}

Allotropes of Sulfur

NameFormula/StructureCrystal systemNotes
Rhombic sulfurS₈ (ring)OrthorhombicMost stable at room temperature
Monoclinic sulfurS₈ (ring)MonoclinicStable above 95.5°C
Plastic sulfurS (chains)AmorphousElastic; insoluble in CS₂

Sulfur allotropes

Sulfuric Acid H₂SO₄

Concentrated H₂SO₄ has three key properties:

  1. Hygroscopic: used as a drying agent
  2. Dehydrating: removes H₂O from compounds (sugar → carbon)
  3. Non-volatile: displaces volatile acids from their salts on heating

NaCl+H2SO4→heatNaHSO4+HCl↑\mathrm{NaCl + H_2SO_4 \xrightarrow{heat} NaHSO_4 + HCl \uparrow}

Dilute H₂SO₄ behaves as a normal strong acid (neutralization, reactions with metals, etc.).

Hot concentrated H₂SO₄ acts as an oxidizer, dissolving metals like Cu that dilute acids cannot:

Cu+2H2SO4(hot, conc.)→CuSO4+SO2+2H2O\mathrm{Cu + 2H_2SO_4(hot,\,conc.) \rightarrow CuSO_4 + SO_2 + 2H_2O}

Contact Process (Industrial H₂SO₄)

S + O₂Combustion→SO₂→V₂O₅ catalystSO₃→+ H₂OH₂SO₄
Contact Process flow
  1. S+O2→SO2\mathrm{S + O_2 \rightarrow SO_2}
  2. 2SO2+O2→V2O52SO3\mathrm{2SO_2 + O_2 \xrightarrow{V_2O_5} 2SO_3} (the “contact” reaction — named for the catalyst contact)
  3. SO3+H2O→H2SO4\mathrm{SO_3 + H_2O \rightarrow H_2SO_4}

Check Your Understanding

Q1 Which sulfur allotrope is most stable at room temperature?

Q2 The hygroscopic nature of conc. H₂SO₄ is used for...

Q3 What catalyst is used in the Contact Process (SO₂ → SO₃)?

Exercises

Q1. When concentrated sulfuric acid is poured on sugar, it turns black. Explain this reaction and name the property of H₂SO₄ responsible.

Solution

Sugar (sucrose C12H22O11\mathrm{C_{12}H_{22}O_{11}}) has hydrogen and oxygen removed as water, leaving elemental carbon:

C12H22O11→conc. H2SO412C+11H2O\mathrm{C_{12}H_{22}O_{11} \xrightarrow{conc.\,H_2SO_4} 12C + 11H_2O}

This is due to the dehydrating property of concentrated sulfuric acid. The black residue is carbon.