p-Block Elements - II — Important Questions

White (Yellow) phosphorus

Step 1: White phosphorus (also called yellow phosphorus) is the most reactive allotrope of phosphorus. Step 2: It glows in the dark due to slow oxidation in air — this property is called phosphorescence. Step 3: It also has a characteristic garlic smell and is highly poisonous. Step 4: Red phosphorus does NOT show phosphorescence and is much less reactive. Step 5: The word 'phosphorescence' itself comes from phosphorus, which highlights that white phosphorus is the allotrope associated with this glow-in-the-dark property.

Phosphonium iodide (PH₄I)

Step 1: Phosphine (PH₃) is a weakly basic molecule because phosphorus has a lone pair of electrons. Step 2: When it reacts with halogen acids like HI, the lone pair on phosphorus accepts a proton (H⁺) from HI. Step 3: The reaction is: PH₃ + HI → PH₄I (Phosphonium iodide). Step 4: This is similar to ammonia forming ammonium salts (NH₃ + HCl → NH₄Cl). Step 5: PH₄I is analogous to NH₄Cl but much less stable. Phosphonium salts decompose easily on adding water.

Contact process

Step 1: Sulphuric acid is industrially manufactured by the Contact process. Step 2: First, sulphur is burnt to give SO₂: S + O₂ → SO₂. Step 3: SO₂ is then oxidised to SO₃ using V₂O₅ catalyst at 720 K and 2 bar: 2SO₂ + O₂ → 2SO₃. Step 4: SO₃ is absorbed in concentrated H₂SO₄ to form oleum (H₂S₂O₇), which on dilution with water gives H₂SO₄. Step 5: Haber's process → NH₃; Ostwald's process → HNO₃; Electrolytic process → Cl₂/NaOH. Each process makes a specific product.

Barium chloride solution

Step 1: The standard test for sulphate (SO₄²⁻) ions is using barium chloride (BaCl₂) solution. Step 2: When BaCl₂ is added to a sulphate solution, a white precipitate of barium sulphate (BaSO₄) is formed: BaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl. Step 3: Barium sulphate is insoluble in dilute HCl, which confirms the test. Step 4: Silver nitrate (AgNO₃) is used to test for chloride ions (gives white precipitate of AgCl). Step 5: This is an important qualitative analysis test used in the laboratory.