JEE Chemistry The P Block Elements Notes Set 09

Group 17: Properties and Trends

1. The halogens are F, Cl, Br, I and At with the configuration ns²np⁵. They are diatomic non-metals: F₂ and Cl₂ are gases, Br₂ is a red-brown liquid, and I₂ is a violet solid. The weak van der Waals forces make them volatile.
2. Chlorine has the most negative electron gain enthalpy (not fluorine, because of its small size and strong electron repulsion). Electronegativity and oxidising power fall from F to I: F₂ > Cl₂ > Br₂ > I₂.
3. Fluorine shows only −1 (and 0) because it has no d orbitals and is the most electronegative element. The others also show +1, +3, +5 and +7 in oxyacids and interhalogens.
4. Bond dissociation enthalpy: Cl₂ > Br₂ > F₂ > I₂ (F₂ is low because of repulsion between lone pairs on the small F atoms).
5. Hydrogen halides HX: acidity and reducing power increase from HF to HI while thermal stability falls. HF is a weak acid and liquid at room temperature because of hydrogen bonding.
6. Silver halides: AgF is soluble, while AgCl (white), AgBr (pale yellow) and AgI (yellow) are precipitates. Iodine turns starch blue.

Chlorine and Hydrogen Chloride

1. Chlorine is made in the laboratory by oxidising HCl with MnO₂ (MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O), by the Deacon process (4HCl + O₂ → 2Cl₂ + 2H₂O over a CuCl₂ catalyst), and industrially by electrolysis of brine.
2. Cl₂ is a greenish yellow, toxic gas that bleaches moist coloured materials by oxidation: Cl₂ + H₂O → HCl + HOCl, HOCl → HCl + [O]. Cl₂ reacts with cold dilute NaOH to give NaCl + NaOCl and with hot concentrated NaOH to give NaCl + NaClO₃.
3. Bleaching powder is Ca(OCl)₂·CaCl₂·Ca(OH)₂·2H₂O, made from slaked lime and chlorine.
4. HCl is made from NaCl and conc. H₂SO₄ on heating: NaCl + H₂SO₄ → NaHSO₄ + HCl. Aqua regia dissolves gold and platinum.
5. Chromyl chloride test for chloride: NaCl + K₂Cr₂O₇ + conc. H₂SO₄ gives deep red vapours of CrO₂Cl₂, which in NaOH give yellow Na₂CrO₄ and with acetic acid and lead acetate give a yellow precipitate of PbCrO₄. The test is not given by bromides and iodides.

Interhalogen Compounds and Oxoacids

1. Interhalogens are of the types AB (ClF, BrF, ICl), AB₃ (ClF₃), AB₅ (BrF₅, IF₅) and AB₇ (IF₇), where the larger halogen is the central atom. They are more reactive than the halogens (except F₂) and are diamagnetic.
2. Shapes: AB₃ is T-shaped, AB₅ is square pyramidal and AB₇ is pentagonal bipyramidal. BrF₅ is an interhalogen compound, whereas CN⁻, SCN⁻ are pseudohalides.
3. AB-type interhalogens hydrolyse to the halide of the smaller halogen and the hypohalite of the larger one: ICl + H₂O → HOI + HCl.
4. Oxoacids of chlorine: HOCl (hypochlorous), HClO₂, HClO₃ and HClO₄. The acid strength rises HOCl < HClO₂ < HClO₃ < HClO₄ and the oxidising power falls. Cl₂O₇ is the anhydride of HClO₄, and Cl₂O₆ is a mixed anhydride giving HClO₃ and HClO₄.
5. Fluorine forms only HOF. HIO₃ is made by oxidising I₂ with conc. HNO₃.
6. Iodine is soluble in KI because of the formation of I₃⁻. It does not dissolve in water but dissolves in organic solvents like CCl₄ and benzene to form violet solutions.

Noble Gases

1. Group 18 consists of He, Ne, Ar, Kr, Xe and Rn with complete octets (He has 1s²). They are monoatomic, colourless and odourless, and their boiling points rise from He to Rn because of stronger dispersion forces as polarisability increases.
2. Argon is the most abundant noble gas in air (about 0.93 percent). Helium is mostly obtained from natural gas, and radon is radioactive.
3. The first noble gas compound was Xe[PtF₆] (Bartlett, 1962), made by reaction of Xe with PtF₆. Xenon forms compounds only with F and O.
4. Xenon fluorides: XeF₂ (linear, sp³d), XeF₄ (square planar, sp³d²) and XeF₆ (distorted octahedral, sp³d³), made by direct reaction of Xe and F₂ at different ratios and conditions. XeO₃ (pyramidal) and XeOF₄ (square pyramidal) are formed on hydrolysis; XeO₃ is explosive. XeCl₄ is not known.
5. XeF₂ + SbF₅ → [XeF]⁺[SbF₆]⁻, XeF₄ + SbF₅ → [XeF₃]⁺[SbF₆]⁻ and XeF₆ + HF gives [XeF₅]⁺ and HF₂⁻.
6. Clathrates form when Ar, Kr or Xe is trapped in cavities of ice or quinol; He and Ne are too small and do not form them.
7. Uses: helium in balloons, in cryogenics (liquid helium) and mixed with oxygen for deep-sea diving (it is less soluble in blood than nitrogen), neon in discharge tubes and advertising signs, argon to provide an inert atmosphere in welding and bulbs, and radon in radiotherapy.