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Concept 08 / 09The p-Block Elements

Group 17: Halogens, Chlorine, Hydrogen Chloride, Oxoacids and Interhalogens

Trends in the halogens: electron gain enthalpy, bond dissociation enthalpy, oxidising power, colour, anomalous fluorine; preparation and properties of chlorine including bleaching and reactions with alkali; hydrogen chloride and aqua regia; oxoacids of chlorine; and interhalogen compounds.

Figure 1
Group 17 trends. Fluorine, chlorine, bromine, iodine, astatine; ns2np5ns^{2}np^{5}, one electron short of a noble gas. Electron gain enthalpy: very negative, but chlorine's (−349 kJ mol−1-349\,\mathrm{kJ\,mol^{-1}}) is more negative than fluorine's (−333-333), because the extra electron in the tiny 2p2p subshell of F suffers strong repulsion (see figure). Bond dissociation enthalpy: Cl2>Br2>F2>I2\mathrm{Cl_{2} > Br_{2} > F_{2} > I_{2}}; the F-F bond is anomalously weak because of repulsion between lone pairs on the small, closely spaced atoms. Colour: F2\mathrm{F_{2}} pale yellow, Cl2\mathrm{Cl_{2}} greenish yellow, Br2\mathrm{Br_{2}} red-brown (liquid), I2\mathrm{I_{2}} violet vapour (solid is black). Absorption of visible light excites electrons; larger molecules absorb lower-energy light. Oxidising power: F2>Cl2>Br2>I2\mathrm{F_{2} > Cl_{2} > Br_{2} > I_{2}}. Fluorine is the strongest because of its low bond enthalpy, high hydration enthalpy of F−\mathrm{F^{-}} and high electron affinity; each halogen oxidises the halide ions below it. Oxidation states: F only −1-1 (most electronegative, no d orbitals); Cl, Br and I also show +1+1, +3+3, +5+5, +7+7. Hydrogen halides: acid strength HF<HCl<HBr<HI\mathrm{HF < HCl < HBr < HI} (bond strength falls); boiling points HCl<HBr<HI<HF\mathrm{HCl < HBr < HI < HF} (HF is hydrogen-bonded). Chlorine. Preparation: MnO2+4HCl→MnCl2+Cl2+2H2O\mathrm{MnO_{2} + 4HCl \rightarrow MnCl_{2} + Cl_{2} + 2H_{2}O}; 2KMnO4+16HCl→2KCl+2MnCl2+8H2O+5Cl2\mathrm{2KMnO_{4} + 16HCl \rightarrow 2KCl + 2MnCl_{2} + 8H_{2}O + 5Cl_{2}}; Deacon's process 4HCl+O2→2Cl2+2H2O\mathrm{4HCl + O_{2} \rightarrow 2Cl_{2} + 2H_{2}O} (CuCl2\mathrm{CuCl_{2}}, 723 K723\,\mathrm{K}); electrolysis of brine. Reactions: with cold dilute NaOH, Cl2+2NaOH→NaCl+NaOCl+H2O\mathrm{Cl_{2} + 2NaOH \rightarrow NaCl + NaOCl + H_{2}O}; with hot concentrated NaOH, 3Cl2+6NaOH→5NaCl+NaClO3+3H2O\mathrm{3Cl_{2} + 6NaOH \rightarrow 5NaCl + NaClO_{3} + 3H_{2}O}; with dry slaked lime, bleaching powder. With excess NH3\mathrm{NH_{3}}: 8NH3+3Cl2→6NH4Cl+N2\mathrm{8NH_{3} + 3Cl_{2} \rightarrow 6NH_{4}Cl + N_{2}}; with excess Cl2\mathrm{Cl_{2}}: NH3+3Cl2→NCl3+3HCl\mathrm{NH_{3} + 3Cl_{2} \rightarrow NCl_{3} + 3HCl} (explosive). Bleaching: moist chlorine forms nascent oxygen, Cl2+H2O→2HCl+[O]\mathrm{Cl_{2} + H_{2}O \rightarrow 2HCl + [O]}, which oxidises coloured matter permanently. (SO2\mathrm{SO_{2}} bleaches by reduction, which is temporary.) Hydrogen chloride. Prepared from NaCl and concentrated H2SO4\mathrm{H_{2}SO_{4}}; very soluble, strong acid in water. Aqua regia (33 parts concentrated HCl, 11 part HNO3\mathrm{HNO_{3}}) dissolves gold and platinum: Au+4H++NO3−+4Cl−→AuCl4−+NO+2H2O\mathrm{Au + 4H^{+} + NO_{3}^{-} + 4Cl^{-} \rightarrow AuCl_{4}^{-} + NO + 2H_{2}O}; nitric acid oxidises, chloride stabilises the metal ion as a complex. Oxoacids of chlorine. Hypochlorous HOCl\mathrm{HOCl} (+1+1), chlorous HClO2\mathrm{HClO_{2}} (+3+3), chloric HClO3\mathrm{HClO_{3}} (+5+5), perchloric HClO4\mathrm{HClO_{4}} (+7+7). Acid strength increases with the number of oxygens (HOCl<HClO2<HClO3<HClO4\mathrm{HOCl < HClO_{2} < HClO_{3} < HClO_{4}}), because the negative charge of the conjugate base spreads over more oxygens. Oxidising power follows the reverse order. Fluorine forms only HOF. Interhalogens. Compounds between two halogens: XX′\mathrm{XX'} (ClF, BrF, ICl, IBr), XX3′\mathrm{XX'_{3}} (ClF3\mathrm{ClF_{3}}, BrF3\mathrm{BrF_{3}}, ICl3\mathrm{ICl_{3}}), XX5′\mathrm{XX'_{5}} (BrF5\mathrm{BrF_{5}}, IF5\mathrm{IF_{5}}), XX7′\mathrm{XX'_{7}} (IF7\mathrm{IF_{7}} only). X is the larger halogen. Shapes follow VSEPR: XX3′\mathrm{XX'_{3}} bent T-shaped, XX5′\mathrm{XX'_{5}} square pyramidal, IF7\mathrm{IF_{7}} pentagonal bipyramidal. They are more reactive than the parent halogens (except F2\mathrm{F_{2}}) because the X-X' bond is weaker than X-X, and they hydrolyse: ICl+H2O→HCl+HOI\mathrm{ICl + H_{2}O \rightarrow HCl + HOI}. ClF3\mathrm{ClF_{3}} and BrF3\mathrm{BrF_{3}} are used to make UF6\mathrm{UF_{6}} for uranium enrichment. Key Takeaways: - Electron gain enthalpy: Cl > F > Br > I (magnitude); bond enthalpy: Cl2>Br2>F2>I2\mathrm{Cl_{2} > Br_{2} > F_{2} > I_{2}}. - Oxidising power: F2>Cl2>Br2>I2\mathrm{F_{2} > Cl_{2} > Br_{2} > I_{2}}; F shows only −1-1. - Acid strength: HF<HCl<HBr<HI\mathrm{HF < HCl < HBr < HI} and HOCl<HClO2<HClO3<HClO4\mathrm{HOCl < HClO_{2} < HClO_{3} < HClO_{4}}. - Chlorine bleaches by oxidation (permanent); aqua regia dissolves gold. - Interhalogens are more reactive than halogens (except F2\mathrm{F_{2}}). - Pitfall: HF is the weakest hydrohalic acid, even though F is the most electronegative. Brain-Teaser 1: Predict the shape and hybridisation of ClF3\mathrm{ClF_{3}} and IF7\mathrm{IF_{7}}. Solution: ClF3\mathrm{ClF_{3}}: 77 valence electrons, 33 bonds, 22 lone pairs; SN 55, sp3dsp^{3}d; bent T-shape. IF7\mathrm{IF_{7}}: 77 bonds, no lone pairs; SN 77, sp3d3sp^{3}d^{3}; pentagonal bipyramid. Brain-Teaser 2: How many moles of chlorine are liberated when 0.2 mol0.2\,\mathrm{mol} of KMnO4\mathrm{KMnO_{4}} reacts with excess concentrated HCl? Solution: 2KMnO4+16HCl→2KCl+2MnCl2+8H2O+5Cl2\mathrm{2KMnO_{4} + 16HCl \rightarrow 2KCl + 2MnCl_{2} + 8H_{2}O + 5Cl_{2}}: 2 mol2\,\mathrm{mol} KMnO4\mathrm{KMnO_{4}} gives 5 mol5\,\mathrm{mol} Cl2\mathrm{Cl_{2}}, so 0.2 mol0.2\,\mathrm{mol} gives 0.5 mol0.5\,\mathrm{mol}. (Each Mn accepts 55 electrons; each Cl2\mathrm{Cl_{2}} needs 22.) Brain-Teaser 3: Why is the bond dissociation enthalpy of F2\mathrm{F_{2}} smaller than that of Cl2\mathrm{Cl_{2}}, and why does this make fluorine the strongest oxidant? Solution: The F-F bond is very short, so the three lone pairs on each atom repel strongly, weakening the bond (158.8158.8 versus 242.6 kJ mol−1242.6\,\mathrm{kJ\,mol^{-1}}). In the cycle 12X2→X→X−(g)→X−(aq)\frac{1}{2}\mathrm{X_{2}} \rightarrow \mathrm{X} \rightarrow \mathrm{X^{-}(g)} \rightarrow \mathrm{X^{-}(aq)}, fluorine's low dissociation energy and very high hydration enthalpy outweigh its slightly smaller electron gain enthalpy, giving the most positive reduction potential (E∘=+2.87 VE^{\circ} = +2.87\,\mathrm{V}).

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Group 17: Halogens, Chlorine, Hydrogen Chloride, Oxoacids and Interhalogens — DPP

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