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Concept 07 / 09The p-Block Elements

Group 16: Oxygen, Ozone, Sulfur Dioxide and Sulfuric Acid

Trends in group 16 elements and their hydrides, anomalous oxygen, classification of oxides, preparation, structure and oxidising action of ozone, sulfur allotropes, sulfur dioxide as a reducing agent, manufacture of sulfuric acid by the contact process, its properties, and the oxoacids of sulfur.

Figure 1
Group 16 trends. Oxygen, sulfur, selenium, tellurium, polonium (chalcogens); ns2np4ns^{2}np^{4}. Oxygen shows mainly −2-2 (and +2+2 in OF2\mathrm{OF_{2}}); the others show −2-2, +2+2, +4+4 and +6+6, with +4+4 becoming more stable than +6+6 down the group (inert pair effect). Oxygen's electron gain enthalpy (−141 kJ mol−1-141\,\mathrm{kJ\,mol^{-1}}) is less negative than sulfur's (−200-200), because the incoming electron is crowded in the small 2p2p shell. Hydrides H2E\mathrm{H_{2}E}: acidic character increases H2O<H2S<H2Se<H2Te\mathrm{H_{2}O < H_{2}S < H_{2}Se < H_{2}Te} (weaker H-E bonds), thermal stability decreases, reducing character increases. Water boils anomalously high because of hydrogen bonding. Anomalous oxygen. Small, highly electronegative, no d orbitals: forms pπp\pi-pπp\pi double bonds, so it exists as O2\mathrm{O_{2}} while sulfur forms S8\mathrm{S_{8}} rings; maximum covalency 44; hydrogen bonding in water. Oxides. Acidic (non-metal oxides: SO2\mathrm{SO_{2}}, Cl2O7\mathrm{Cl_{2}O_{7}}, Mn2O7\mathrm{Mn_{2}O_{7}}), basic (metal oxides: Na2O\mathrm{Na_{2}O}, CaO), amphoteric (Al2O3\mathrm{Al_{2}O_{3}}, ZnO, PbO), neutral (CO, NO, N2O\mathrm{N_{2}O}). Ozone, O3\mathrm{O_{3}}. Made by a silent electric discharge through dry oxygen: 3O2→2O3\mathrm{3O_{2} \rightarrow 2O_{3}}, ΔH=+142 kJ mol−1\Delta H = +142\,\mathrm{kJ\,mol^{-1}} (endothermic, so ozone is thermodynamically unstable and a strong oxidant). Structure: bent (about 117∘117^{\circ}), two equivalent O-O bonds of 128 pm128\,\mathrm{pm} (resonance hybrid), diamagnetic, pale blue gas. Oxidations: PbS+4O3→PbSO4+4O2\mathrm{PbS + 4O_{3} \rightarrow PbSO_{4} + 4O_{2}}; 2I−+H2O+O3→2OH−+I2+O2\mathrm{2I^{-} + H_{2}O + O_{3} \rightarrow 2OH^{-} + I_{2} + O_{2}} (used to estimate ozone, the liberated iodine being titrated with thiosulfate). The stratospheric ozone layer absorbs UV; it is depleted by NO from supersonic jets (NO+O3→NO2+O2\mathrm{NO + O_{3} \rightarrow NO_{2} + O_{2}}) and by chlorine atoms from CFCs. Sulfur. Rhombic (α\alpha) sulfur, stable below 369 K369\,\mathrm{K}, and monoclinic (β\beta) sulfur, stable above it; both contain puckered crown-shaped S8\mathrm{S_{8}} rings. S2\mathrm{S_{2}} in the vapour at high temperature is paramagnetic, like O2\mathrm{O_{2}}. Sulfur dioxide. From burning sulfur, or SO32−+2H+→H2O+SO2\mathrm{SO_{3}^{2-} + 2H^{+} \rightarrow H_{2}O + SO_{2}}. Bent (119.5∘119.5^{\circ}), resonance-stabilised, acidic (SO2+H2O→H2SO3\mathrm{SO_{2} + H_{2}O \rightarrow H_{2}SO_{3}}). Mainly a reducing agent in moist conditions: it decolourises acidified KMnO4\mathrm{KMnO_{4}} (5SO2+2MnO4−+2H2O→5SO42−+4H++2Mn2+\mathrm{5SO_{2} + 2MnO_{4}^{-} + 2H_{2}O \rightarrow 5SO_{4}^{2-} + 4H^{+} + 2Mn^{2+}}), turns acidified dichromate green, and reduces Fe3+\mathrm{Fe^{3+}} to Fe2+\mathrm{Fe^{2+}}. It bleaches by reduction (temporary). It also acts as an oxidant towards H2S\mathrm{H_{2}S}: 2H2S+SO2→3S+2H2O\mathrm{2H_{2}S + SO_{2} \rightarrow 3S + 2H_{2}O}. Sulfuric acid. Contact process (see figure). Properties: strong dibasic acid; low volatility (displaces volatile acids from their salts, NaCl+H2SO4→NaHSO4+HCl\mathrm{NaCl + H_{2}SO_{4} \rightarrow NaHSO_{4} + HCl}); powerful dehydrating agent (C12H22O11→12C+11H2O\mathrm{C_{12}H_{22}O_{11} \rightarrow 12C + 11H_{2}O}, charring sugar); hot concentrated acid is an oxidant: Cu+2H2SO4→CuSO4+SO2+2H2O\mathrm{Cu + 2H_{2}SO_{4} \rightarrow CuSO_{4} + SO_{2} + 2H_{2}O}; C+2H2SO4→CO2+2SO2+2H2O\mathrm{C + 2H_{2}SO_{4} \rightarrow CO_{2} + 2SO_{2} + 2H_{2}O}. Dilution releases much heat, so acid is always added to water. Oxoacids of sulfur. Sulfurous H2SO3\mathrm{H_{2}SO_{3}} (S +4+4), sulfuric H2SO4\mathrm{H_{2}SO_{4}} (+6+6), pyrosulfuric (oleum) H2S2O7\mathrm{H_{2}S_{2}O_{7}} (S-O-S link), peroxomonosulfuric (Caro's) H2SO5\mathrm{H_{2}SO_{5}} and peroxodisulfuric (Marshall's) H2S2O8\mathrm{H_{2}S_{2}O_{8}} (O-O links, S still +6+6). Thiosulfuric acid H2S2O3\mathrm{H_{2}S_{2}O_{3}} is known through its salts. Key Takeaways: - Acidity of hydrides: H2O<H2S<H2Se<H2Te\mathrm{H_{2}O < H_{2}S < H_{2}Se < H_{2}Te}. - Ozone: bent, resonance, strong oxidant; 3O2→2O3\mathrm{3O_{2} \rightarrow 2O_{3}} is endothermic. - SO2\mathrm{SO_{2}} is mainly reducing (turns dichromate green); bleaches by reduction. - Contact process: SO2→SO3\mathrm{SO_{2} \rightarrow SO_{3}} over V2O5\mathrm{V_{2}O_{5}}; SO3\mathrm{SO_{3}} absorbed in H2SO4\mathrm{H_{2}SO_{4}} as oleum. - Pitfall: peroxo acids (H2SO5\mathrm{H_{2}SO_{5}}, H2S2O8\mathrm{H_{2}S_{2}O_{8}}) have S at +6+6, not +8+8 or +7+7. Brain-Teaser 1: Why is H2S\mathrm{H_{2}S} more acidic than H2O\mathrm{H_{2}O}, though oxygen is more electronegative than sulfur? Solution: Acid strength of binary hydrides down a group is governed by H-E bond strength, not electronegativity. The H-S bond is longer and weaker (larger, more diffuse S orbitals), so it breaks more easily in water, releasing H+\mathrm{H^{+}}. Brain-Teaser 2: Why is SO3\mathrm{SO_{3}} absorbed in concentrated sulfuric acid rather than in water? Solution: SO3\mathrm{SO_{3}} reacts with water so exothermically that the heat vaporises the water, producing a fog of tiny sulfuric acid droplets that is very hard to condense. Absorbing it in 98%98\% acid gives oleum (H2S2O7\mathrm{H_{2}S_{2}O_{7}}) smoothly; oleum is then diluted with the correct amount of water. Brain-Teaser 3: 100 mL100\,\mathrm{mL} of ozonised oxygen is passed through KI solution, and the iodine liberated needs 10 mL10\,\mathrm{mL} of 0.01 M0.01\,\mathrm{M} Na2S2O3\mathrm{Na_{2}S_{2}O_{3}}. How many moles of ozone were present? Solution: Thiosulfate: 0.1 mmol0.1\,\mathrm{mmol}, reacting with 0.05 mmol0.05\,\mathrm{mmol} I2\mathrm{I_{2}} (I2+2S2O32−\mathrm{I_{2} + 2S_{2}O_{3}^{2-}}). Each O3\mathrm{O_{3}} liberates one I2\mathrm{I_{2}}, so 5×10−5 mol5 \times 10^{-5}\,\mathrm{mol} O3\mathrm{O_{3}}.

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Group 16: Oxygen, Ozone, Sulfur Dioxide and Sulfuric Acid — DPP

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