Chapter 9 The sp-block elements (II) CHEM 462 Wednesday, Nov. 10 T. Hughbanks Alkali Metals - Group 1 Chemistry is dominated by the low ionization energies, extreme electropositive character Atomic configurations ns1 Bonding in the elements is weak, bonding in most compounds is ionic Li is the only element that shows any slight covalent chemistry. The symptoms of having only one loosely bound valence electron . . . I.E. (kJ•mol-1) m.p. b.p. Density Hatomization (˚C) (˚C) (g•cm3) (kJ•mol-1) Li 520 180 1347 0.53 162 Na 496 98 881 0.97 108 K 420 64 766 0.86 90 Rb 403 39 688 1.53 82 Cs 376 29 705 1.87 78 More Physical Properties . . . Hhyd E˚ ionic rad (pm) kJ/mol Shyd J/mol•K Metallic radius (pm) M+/M (V) Li 76 -519 -140 152 -3.04 Na 102 -404 -110 186 -2.71 K 138 -321 -70 230 -2.93 Rb 149 -296 -70 247 -2.98 Cs 170 -271 -60 267 -3.03 Alkaline-Earth Metals - Group 2 Electropositive character of elements still very important Atomic configurations ns2 Bonding in the elements is stronger than group 1, but weaker than other metals. Bonding in most compounds is ionic, but some covalent character is seen for Be, slightly so for Mg. 2 valence electrons are better than one . . . I.E.1 I.E.2 (kJ•mol-1) (kJ•mol-1) melting Density pts. ˚C (g•cm3) Hvap (kJ•mol-1) Be 899 1758 1287 1.848 309 Mg 738 1450 649 1.738 127 Ca 590 1146 839 1.55 155 Sr 549 1064 768 2.63 158 Ba 503 964 727 3.62 136 More Physical Properties . . . metallic ionic radius, pm radius, pm Hhyd Shyd (kJ/mol) (J/mol•K) Be 112 27 -2500 -300 Mg 160 72 -1931 -320 Ca 197 100 -1586 -230 Sr 215 126 -1456 -220 Ba 224 142 -1316 -200 Sources/uses of alkali metals Li and Na from Down’s cell electrolysis K, Rb, Cs from reaction of MCl with Na. e.g,: Na(l) + KCl(l) K(g) + NaCl(l) ~ 850 ˚C (What drives this reaction? For the reaction, Na(s) + KCl(s) K(s) + NaCl(s), G˚ = +25 kJ) Na is used heavily in industrial processes, including its use to make other metals. Na2CO3 is used heavily in glass manufacture. Li2CO3 is used to treat manic-depressives. Sources/uses of alkaline-earth metals Be comes from the mineral beryl, Be3Al2Si6O18. (with 2% AlIII/CrIII ‘doping’ this is emerald!) MgO (from CaMg(CO3 )2 , dolomite) is a main source. Mg metal is obtained by electrolysis of MgCl2. Over half of the worldwide production (4 105 tons) goes to lightweight Al/Mg alloys. Thermite reaction, Al reduction Ca, Sr, Ba are produced by Al reduction at high T: 3 MO(s) + Al(s) Al2O3(s) + M(s) The reaction shown here is for Al reduction of iron. Chemistry of alkali metals All alkali metals react with water: 2 M(s) + 2 H2O 2MOH + H2(g) with hydrogen: M(s) + H2(g) 2MH With halogens: M(s) + X2(g) 2MX with oxygen… 2Li(s) + O2(g) 2 Li2O(s) Chemistry of alkali metals For the other alkali metals the reaction with O2 doesn’t go all the way to M2O: 2 Na(s) + O2(g) 2 Na2O2(s) M + O2(g) 2 MO2(s), M = K, Rb, Cs (What is the nature of oxygen in these products?) Li is one of the few metals that reacts directly with nitrogen at room temperature: 3 Li(s) + 1/2 N2(g) Li3N(s) Other MI metals make no nitrides, MII all do. Crown Ethers and Cryptands These are macrocyclic ligands that can be designed to selectively bind different alkali metal cations (‘87 Nobel prize: Cram, Lehn, Pederson). Dye used these to prepare alkalides and electrides. Stability Constants for Alkali Metals Formed with Cryptands of Various Cavity sizes in H2O O O z O O Oy O N x N Li x y z Na K Rb Cs (86 pm) (112 pm) (144 pm) (158 pm) (184 pm) 0 0 1 4.30 2.80 2.0 2.0 2.0 1 1 0 2.50 5.40 3.95 2.55 2.0 1 1 1 2.0 3.90 5.40 4.35 2.0 1 1 2 2.0 2.0 2.2 2.05 2.20 2 2 1 2.0 2.0 2.0 0.7 2.0 3 3 3 2.0 2.0 2.0 0.5 2.0 Liquid Ammonia solutions! All the alkali metals and the three last three alkaline earths form amazing solutions in liquid ammonia. E.g., Na(s) Na (soln) Na+ + e– (solvated) Electrons in dilute solutions behave like “particles-in-boxes” formed by NH3 molecules. All these solutions have the same deep blue color. Concentrated solutions have a golden color and are electronic conductors. Reaction Summary (C&W: p. 301) Na Reaction Summary (C&W: p. 301) K, Rb, Cs - Reaction Summary (C&W: p. 302)