Group 18 - The Elements

Group 18 - The Elements ! All found in minute quantities in air. • Argon is most abundant (and cheapest), comprising 0.934% of air by volume. ! Although rare on earth, helium is the second most abundant element in the universe (H, 76%; He, 23%), being a major component of stars. ! All radon isotopes are short-lived and are continuously being produced by natural decay processes. • Longest lived isotope is 222Rn (á, t½ = 3.825 days). ! Condensed phases are held together by van der Waals forces, which increase smoothly down the group. Element b.p. (K) I (kJ/mol) He 4.18 2372 Ne 27.13 2080 Ar 87.29 1520 Kr 120.26 1351 Xe 166.06 1169 Rn 208.16 1037 Helium ! Helium is found in certain natural gas deposits (e.g., in Kansas), where it probably forms as a result of radioactive decay in the rocks. ! Because of its low boiling point, it is used widely in cryogenic applications. • Also used in airships (e.g., Goodyear blimp) and as a balloon filler. • Used as a fill gas for deep diving, because it is less soluble in blood than nitrogen, thus avoiding the "bends." ! The 4He isotope has the lowest know boiling point, 4.12 K, at which point it is called He-I. • On cooling to 2.178 K a phase transition to He-II occurs. • He-II has an expanded volume, almost no viscosity, and is superconducting. • He-II can readily flow uphill to equalize volumes in adjacent vessels. • It cannot be stored in glass Dewars, because it leaks through glass into the evacuated space, posing an explosion risk. ! At 0.9 K, 3He floats on 4He, permitting separation of the isotopes. Chemistry of the Noble Gases ! Before 1962 it was assumed that no compounds could be formed, owing to the "stability of the octet." ! In 1962 Neil Bartlett reacted O2 with PtF6, forming [O2]+[PtF6]–. • O2 and Xe have similar ionization energies, so Bartlett tried the same reacion with Xe, producing a compound reported to be [Xe]+[PtF6]–. • Today the reacion is believed to be the following: Xe + 2PtF6 ÷ [XeF]+ [PtF6]– + PtF5 ÷ [XeF]+[Pt2F11]– • Shortly after this discovery, the xenon fluorides were prepared by direct combination at high temperature and pressure. Xe + xsF2 XeF2, XeF4, XeF6 • All are solids. ! Today many compounds and ions of Xe with F and O are known, and even some with Kr and F are known, although Kr compounds are stable only at very low temperatures. Structures ! Most noble gas compounds have structures predictable by VSEPR considerations. D4h D4h C3v C4v ! In XeF6 and XeF82–, the lone pairs do not seem to occupy a fixed position, and the structures are fluxional, on average distorted octahedral (C3v) and distorted square antiprism (~D4d), respectively. Xenon Fluorides ! XeF2 forms with alacrity when a stoichiometric mixture of Xe and F2 in a glass flask is irradiated with sunlight: Xe + F2 XeFÄ2Hfo = –105 kJ • XeF2 is a powerful oxidant: XeF2 + 2H+ + 2e– ÷ Xe + 2HF Eo = +2.64 V ! Nonetheless, both XeF2 and XeF4 are only mildly reactive, whereas XeF6 attacks glass: 2XeF6 + SiO2 ÷ 2XeOF4 + SiF4 ! The lone pair on XeF6 allows it to act as a Lewis base, forming adducts such as XeF6:AsF3, XeF6:BF3, XeF6:SbF5. • XeF6 can also act as a Lewis acid in forming XeF7– and XeF82–. ! Xenon fluorides hydrolyze to give characteristic mixtures of products: 2XeF2 + 2H2O ÷ 2Xe + O2 + 4HF (slow) 4XeF4 + 8H2O ÷ 2Xe + O2 +16HF + 2XeO3 (violent!) XeF6 + 3H2O ÷ 6HF + XeO3 (violent!) ! Carefully controlled hydrolysis of XeF6 with the stoichiometric amount of H2O gives XeOF4, a colorless liquid. XeF6 + H2O ÷ XeOF4 + 2HF ! Xenon fluorides have been used as fluorinating agents. 4SF5 + XeF4 ÷ 4SF6 + Xe 2C6H6 + XeF2 ÷ 2C6H5F + Xe + H2 Krypton Fluoride ! Kr has one molecular fluoride, KrF2, which is formed by direct combination at low temperature, and compounds with the ions KrF+ and Kr2F3+ and [MF6]– anions (M = As, Sb, Bi). • KrF2 is unstable: Kr(g) + F2(g) ÷ KrF2(g) ÄHfo = +63 kJ • Solid has two structures, both tetragonal: low temperature á (<< –80 oC; isomorphous with RT XeF2) and higher temperature â.1,2 á-KrF2 at –125 oC â-KrF2 at –80 oC XeO3 and Its Anions ! Solutions of XeO3, formed by hydrolysis of XeF4 or XeF6, are stable up to 11 M. • With careful evaporation XeO3 is obtained as a white, deliquescent solid. K XeO3 is dangerously explosive! ! XeO3 dissolves in water as molecules, but in base an equilibrium with OH– occurs, forming HXeO4–, which slowly disproportionates to the perxenate ion, XeO64–: XeO3 + OH– º HXeO4– K = 1.5 x 10–3 2HXeO4– + 2OH– ÷ XeO64– + Xe + O2 + 2H2O • XeO64– is isoelectronic and isostructural with paraperiodate. ! Both XeO3 and XeO64– are powerful oxidants: XeO3 + 6H+ + 6e– º Xe + 3H2O XeO64– + 6H+ + 2e– º XeO3 + 3H2O Eo = +2.12 V Eo = +2.36 V