JEE Chemistry The P Block Elements Notes Set 08

Group 16: Properties and Trends

1. Group 16 elements are O, S, Se, Te and Po, with the configuration ns²np⁴. Oxygen is a gas of discrete O₂ molecules, sulphur is a solid made of S₈ rings, and the other elements show increasing metallic character down the group.
2. Oxidation states are −2, +2, +4 and +6. Oxygen shows mainly −2 because of its high electronegativity and has no d orbitals. Sulphur to tellurium show +4 and +6, and the stability of +4 rises down the group because of the inert pair effect.
3. Oxygen's electron gain enthalpy is less negative than that of sulphur because of the small size and strong inter-electronic repulsion in the compact 2p subshell.
4. Hydrides H₂E: bond angle and thermal stability fall from H₂O to H₂Te and the acidity and reducing power rise. Water is a liquid because of hydrogen bonding, while the other hydrides are gases.
5. Oxides of sulphur, selenium and tellurium in the +4 state (SO₂, SeO₂) are acidic and the oxide acidity falls down the group; PoO₂ is amphoteric.
6. SeO₂ has the same bent shape as SO₂ in the vapour state.

Dioxygen and Ozone

1. Dioxygen is paramagnetic in the gaseous, liquid and solid states because of two unpaired electrons in antibonding orbitals, with a bond length of 121 pm.
2. Ozone is made by passing a silent electric discharge through dry oxygen: 3O₂ → 2O₃, ΔH = +142 kJ per mole (endothermic, entropy decreases). It is a bent diamagnetic molecule with an O–O bond length of 128 pm and an angle of about 117 degrees; the bonds have partial double-bond character through delocalisation.
3. Ozone is a powerful oxidising agent because it readily gives nascent oxygen: O₃ → O₂ + [O]. It oxidises PbS to PbSO₄, KI to I₂ (turns starch-iodide paper blue), Hg (mercury loses its meniscus, the tailing of mercury), and bleaches by oxidation even when dry.
4. Ozone also acts as a reductant towards H₂O₂ and Ag₂O. Ozone in the upper atmosphere absorbs harmful ultraviolet rays.

Sulphur Dioxide and Hydrogen Sulphide

1. SO₂ is made by burning sulphur or roasting sulphide ores (4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂) and in the laboratory by Cu + 2H₂SO₄(conc.) → CuSO₄ + SO₂ + 2H₂O or by treating sulphite with dilute acid.
2. SO₂ is a colourless, pungent, bent molecule; its aqueous solution H₂SO₃ is acidic and has never been isolated as the pure acid. It is a reducing agent: it turns acidified K₂Cr₂O₇ from orange to green and decolourises KMnO₄ and iodine solution.
3. SO₂ is also an oxidising agent towards H₂S: 2H₂S + SO₂ → 3S + 2H₂O. It bleaches in the presence of moisture by reduction, and the colour slowly returns, unlike chlorine, which bleaches permanently by oxidation.
4. H₂S is made by FeS + 2HCl → FeCl₂ + H₂S. It has a rotten egg smell, burns with a blue flame, is a strong reducing agent, and turns lead acetate paper black due to PbS.

Sulphuric Acid

1. The contact process: burn S or FeS₂ to give SO₂, oxidise it to SO₃ over V₂O₅ at about 720 K and 2 bar, absorb SO₃ in conc. H₂SO₄ to form oleum (H₂S₂O₇) and dilute the oleum to get the 98 percent acid. SO₃ is not absorbed directly in water because a fine mist forms.
2. Conc. H₂SO₄ is an oily, dense liquid with strong affinity for water. It is a dehydrating agent (sugar to carbon, HCOOH to CO), an oxidising agent (C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O; Cu → CuSO₄ with SO₂) and a non-volatile acid for preparing HCl, HF and HNO₃.
3. Conc. H₂SO₄ cannot dry H₂S, HBr or HI because it oxidises them, and it cannot dry NH₃ because it reacts with it.
4. H₂SO₄ is a dibasic acid giving two series of salts: NaHSO₄ (hydrogen sulphates) and Na₂SO₄ (sulphates). It is the most widely used industrial chemical, for fertilisers, petroleum refining, detergents and batteries.

Other Oxygen Family Compounds

1. Thiosulphate ions have an S–S bond, and iodine oxidises thiosulphate to tetrathionate: 2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻.
2. Polythionic acids contain a chain of sulphur atoms between two SO₃H groups. SO₃ forms a trimer (S₃O₉) with a ring of alternating S and O atoms and no S–S bond.
3. Peroxydisulphuric acid H₂S₂O₈ contains an O–O bridge and sulphur is +6, as it is in oleum H₂S₂O₇, which has an S–O–S bridge.
4. Fluorine reacts with water to give O₂ and HF, but chlorine only disproportionates. O₂F₂ converts plutonium in spent nuclear fuel into volatile PuF₆ for separation, and OF₂ is oxygen difluoride (a fluoride of oxygen).