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Published byNeil Blair Modified over 8 years ago
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Halogens AS
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F Cl Br I (At) Generally: Oxidising agents Germicides Note: Atoms are halogens Ions are halides Ions have 8 electrons by borrowing one, so single negative charge (F¯ etc.) Not very soluble in water, more soluble in organic solvent, usually cyclohexane
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F Cl Br I (At) Electronic configurations 2s 2 2p 5 3s 2 3p 5 4s 2 (3d 10 )4p 5 5s 2 (4d 10 )5p 5
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F Cl Br I (At) Atomic radius increases down group because of: More shielding / repulsion More shells (Even though) Bigger nucleus Ionic radius matches atomic radius
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F Cl Br I (At) Boiling point (X 2 molecules) Increases down G7 Because of increased Van der Waal’s forces F & Cl gases Br liquid I & At solid
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F Cl Br I (At) Reactivities (oxidising ability) ½X 2 + e ˉ X ˉ Reactivity of atoms decreases down group because of: Stronger X-X bond Electron affinity not changing Hydration / Lattice energy decreasing – making, say, NaI releases less energy than making, say, NaF
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F Cl Br I (At) Displacement Higher-reactivity halogens will displace lower-reactivity halogens F > Cl > Br > I > (At) E.g. Cl 2 + 2NaBr Br 2 + 2NaCl Or Cl 2 + 2Br - 2Cl - + Br 2 Displaced bromine – yellow/brown colour Displaced iodine – brown and/or black precipitate Fluorine too dangerous
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F Cl Br I (At) Electronegativity and polarisation Electronegativity decreases down group More shells so more shielding Radius increases so less nuclear attraction for shared electrons Bigger nucleus should attract electrons but it has less effect than other two Therefore hydrogen halides are decreasingly polar
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F Cl Br I (At) Hydrogen halides (HCl etc.) All colourless gases, very soluble in water, dissociate well, strong acids E.g. HCl (aq) H + (aq) + Cl - (aq) Bond enthalpy decreases down group, easier to separate H + and X - So strength of acid increases HI > HBr > HCl These acids are oxidising agents – the hydrogen ion (H + ) can take an electron Note: conc. HF not a strong acid – bond dissociation enthalpy too high, diluted is stronger
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F Cl Br I (At) Tests for ions: Test solutions with acidified silver nitrate to make a silver halide: (acidified to avoid formation of carbonates) F – no change - AgF is soluble Cl – white AgCl precipitate Br – cream AgBr precipitate I – yellow AgI precipitate
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F Cl Br I (At) General reaction: AgNO 3 (aq) + Xˉ (aq) AgX (s) + NO 3 ˉ (aq) orAg + (aq) + Xˉ (aq) AgX (s) Check with dilute and conc. ammonia solution (NH 4 OH): Fˉ no reaction anyway AgCl precipitate soluble in dilute or concentrated ammonia AgBr precipitate soluble in concentrated ammonia only AgI precipitate insoluble, even in concentrated ammonia
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F Cl Br I (At) A redox reaction: Cl 2 + H 2 O HCl + HClO Note oxidation states: One chlorine goes from 0 to -1 in HCl Other goes 0 to +1 in HClO This is a disproportionation – one element is simultaneously oxidised and reduced.
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F Cl Br I (At) Reduction by halide ions Iodide ion I - is very reducing – “happy” to lose electron didn’t really want it bromide less so, etc. Decreasing reducing power up the group Electrons lost particularly to strong oxidising agents (H 2 SO 4, F 2 etc.)
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F Cl Br I (At) Homework Find out the reactions of H 2 SO 4 with F - to I - and memorise
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F Cl Br I (At) Halide ion reactions with concentrated H 2 SO 4 Sulphur has an oxidation number of +6 in SO 4 2 ˉ Fˉ and Clˉ cannot reduce sulphur – only white fumes of HF or HCl seen when NaF or NaCl added to conc. sulphuric acid Fˉ + H 2 SO 4 HF + HSO 4 ˉ Clˉ + H 2 SO 4 HCl + HSO 4 ˉ MEMORISE!!
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F Cl Br I (At) (Brˉ) NaBr + H 2 SO 4 Brˉ can reduce sulphur from +6 to +4, producing HBr, SO 2, Br 2 Observe white fumes of HBr, invisible SO 2 (bubbles) and brown fumes of Br 2 (or possibly liquid Br 2 ) Brˉ + H 2 SO 4 HBr + HSO 4 ˉ Then 2HBr + H 2 SO 4 Br 2 + SO 2 + 2H 2 O
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F Cl Br I (At) (Iˉ) NaI + H 2 SO 4 Iˉ can reduce sulphur from +6 to +4, then 0, then -2, producing HI, SO 2, S, H 2 S, I 2 White fumes of HI, invisible SO 2 and H 2 S (bubbles, distinctive smell), yellow sulphur, and purple fumes of I 2 Iˉ + H 2 SO 4 HI + HSO 4 ˉ Then 2HI + H 2 SO 4 I 2 + SO 2 + 2H 2 O then
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General reaction: NaX (s) + H 2 SO 4 (aq) NaHSO 4 (aq) + HX (g) fumes in moist air Or Xˉ + H 2 SO 4 HSO 4 ˉ + HX In these, the halide is acting as a base (proton acceptor), accepting H + E.g. NaF (s) + H 2 SO 4 (aq) NaHSO 4 (aq) + HF (g)
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F Cl Br I (At) Uses of chlorine and chlorate(I) compounds Under most conditions: H 2 O + Cl 2 Ý HCl + HClO This is a disproportionation, one chlorine atom is reduced, the other is oxidised. This makes “chlorine water” which can decompose photolytically (in sunlight): 2Cl 2 + 2H 2 O O 2 + 4HCl
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F Cl Br I (At) Chlorination Water treatment Drinking water – 0.7mg dm -3 Swimming pools more concentrated Kills bacteria, especially E.coli from bottoms.
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F Cl Br I (At) Reaction with NaOH Cl 2 + 2NaOH Ý NaCl + NaClO + H 2 O This mixture of sodium chloride and sodium chlorate(I) is used as a bleach.
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