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Inorganic Chemistry of the Main-group Elements Volume 1
A Review of the Literature Published Between July 1971 and September 1972
By C. C. Addison The Royal Society of Chemistry
Copyright © 1973 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-637-0
CHAPTER 1
Elements of Group I
BY R. J. PULHAM
In this chapter individual references which are inter-related are grouped together to make a section and, therefore, reference to several alkali metals may feature in a single section. Each reference, however, appears once only within this chapter so that if described in one section, it will not be duplicated in any other. Single references to topics are presented systematically in the section on the appropriate metal.
The elements of Groups I and II are so closely linked in some instances that a section describing them jointly is presented to avoid duplication in Chapter 2. Such a case is the section on 'Molten Salts' which covers the chemistry of the molten salts of both Groups I and II but is presented only in this chapter. A similar situation exists with the section on 'Solutions in Liquid Ammonia' and also with a few other isolated references.
1 The Alkali Metals
Isolation. — The preparation of lithium, (like calcium, strontium, and barium) may be achieved by reduction of its fluoride or oxide by hydrogen in the presence of noble metals. The method seems applicable, in principle, to all alkali metals. A finely ground mixture of metal oxide (Li2O, CaO, SrO, or BaO) and noble metal (Pt, Pd, or Ir) is heated above 1100 °C in a stream of pure hydrogen. The noble metal functions as a catalyst in this reduction which is normally not possible. The required metal forms an intermetallic compound of composition AB2, AB3, AB5, and/or AB7, where A = Li, Ca, Sr, or Ba and B = Pt, Pd, or Ir. At higher temperatures and lower pressures (10-5 — 10-6 Torr) part of the more volatile component A volatilizes and is isolated by condensation.
Vapours. — The vapour pressure of potassium up to the critical point (1925 ± 30 °C and 155 ± 15 bar) has been obtained by measuring the pressure needed to collapse a bubble of vapour in the liquid metal. The appearance and disappearance of the vapour bubble was followed by its effect on the resistance and thermo-power of the system. The tabulated data are largely summarized by the equation
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
The rate of ionization of alkali-metal atoms in flames of H2 + O2 + N2 has been measured using a quadrupole mass spectrometer to determine concentrations of each ion in the flame. The results confirm that the atoms, M, ionize by collision with flame gas molecules, X:
M + X -> M+ + e- + X
The measured activation energy is, within experimental error, the ionization potential of the alkali metal. The associated cross-sections πσ2 are anomalously large, being similar for each metal and in the range 2.5 ± 1.1 × 10-16. The transfer of excitation energy in caesium vapour induced by collision with a molecule of nitrogen or hydrogen at 450 K has also been studied. The effective cross-sections (Å2)of the transfer of excitation energy from the 82P1/2 to the 82P3/2, 72P3/2, and 72D5/2 states of caesium are 55 ± 11, 1.8 ± 0.5, and 3.3 ± 1 for nitrogen and 80 ± 16, 4.7 ± 1.3, and 6.6 ± 2 for hydrogen, respectively. An indication of the electron density in ionized caesium vapour has been deduced from the emission spectrum and an increase in the intensity of certain spectral lines during the post-luminescence of the ionized vapour was attributed to the process of recombination.
Theoretical Aspects. — An approximate equation of state for liquid metals, including the alkali metals, has been developed using a corrected entropy of melting (constant for all metals), the Lindemann law for melting, and a specific heat which has universal dependence on the ratio of the temperature to the melting temperature. A mean spherical model for the structure of liquid metals is also proposed, applicable to sodium, which is a perturbation of the Percus–Yevick hard-sphere model.
There are several reports on alkali-metal molecules. The bond energies (calculated by LCAO MO) in small clusters of lithium atoms are shown to change with both the number of atoms in the cluster and their configuration. A previously developed valence-electron model is used in theoretical calculations on states of the molecules Li2, Na2, K2, LiNa, LiK, and NaK, which dissociate either to give two ground-state (2S) atoms or to give one ground-state atom and one atom in the first excited state (2P). Many of the potential curves show stable minima. The results are in agreement with the meagre experimental evidence available. This model, which was originally developed to explain an anomalous feature in the bonding of diatomic alkali-metal molecules, is modified and used to calculate the dissociation energies and equilibrium geometries of the species Na3, Na+3, K3, and K+3. The neutral triatomic molecules are predicted to be metastable only and their calculated ionization potentials are in agreement with experiment. In recent years the ionization potentials of many small clusters of alkali-metal atoms have been measured mass spectroscopically. In the sodium series there are some odd–even irregularities but the ionization potentials generally show a reduction towards the metallic work function as the number of atoms in the cluster increases. The properties of clusters as they progress from atoms to bulk metals is further pursued in a quantitative attempt to explain bonding in small particles. Extended Hückel and modified CNDO methods are applied to calculations of the electronic properties of Na2, Ca2, and analogous molecules. The ordering of molecular energy levels calculated by the two methods is the same but their dependence on intemuclear distance is different.
Solutions in the Metals. — The use of alkali metals as liquid coolants has stimulated research into purity control of these chemically reactive liquids, into corrosion of materials used to contain them, and into the way in which this is influenced by dissolved impurities, particularly the non-metals hydrogen, carbon, and oxygen. In this connection, a solvation model for non-metals in liquid alkali metals has been described assuming that electronegative nonmetals dissolve as anions and are variously solvated according to charge density by metal atoms. The solvation energy is derived from the proposed electrostatic forces set up in the vicinity of the anion and, for the halogens, the model predicts solvation energies which agree well with those derived from experimental data. The model has also been used to examine the solutes hydrogen, carbon, and oxygen with the aim of suggesting the form in which they exist in liquid-alkali-metal solution. Pursuing the same theme is a second theoretical model for these solutions. Again, the non-metals dissolve as anions and separation of the quantities determining the enthalpy of solvation is achieved. Reasonable agreement between these and the experimentally determined entities is obtained.
The noble gases also have a small but finite solubility in liquid alkali metals. In sodium, the solubility of helium and also that of argon has been determined as functions of pressure and temperature and obeys Henry's law up to at least 9 atm. From 330 to 550 °C the solubility is represented by the linear equations log γ = 0.516 – 3078/T for helium (γ = Ostwald coefficient) and log γ = 1.08 – 4462/T for argon.
The vapour pressure of solutions of tin in liquid lithium at 1200 °C is reported. The composition range 10 — 90 atom% tin is covered in 10 atom% intervals. Measurements were made by the transpiration technique except for the solutions of 10 and 20 atom% Sn. For these the boiling-point method was used to measure the total vapour pressure. Activity coefficients for lithium were calculated directly from the data but those for tin were obtained by a graphical Gibbs–Duhem integration.
Ultrasonic absorption and velocity measurements made on liquid sodium–caesium and potassium–rubidium alloys from 25 °C, or the liquidus to 250 °C, indicate that potassium–rubidium behaves as an ideal liquid mixture but that substantial molecular association occurs in sodium–caesium. An absorption seen in the vicinity of 75 atom% Na is attributed to molecular association in the liquid of the form 3Na + Cs -> Na3Cs. Na3Cs has been identified as the critical composition of an assumed miscibility gap under the inflection point of the liquidus. Electrical resistivity proves to be a valuable technique in the study of reactions in liquid metallic solvents. In this context, the phase diagram for solutions containing up to 44 atom% Ba in sodium was determined from breaks in resistivity–temperature curves. The technique appears more sensitive than others employed previously for this system. The eutectic occurs at 4.5 atom% barium and 83 °C with a peritectoid reaction at 70 °C and a peritectic at 197 °C commencing at 24 atom% barium. Subsequently, the resistance method was used in conjunction with a thermal method to determine the sodium–lead phase diagram up to 7.8 atom% Pb. A eutectic occurs adjacent to the sodium axis at 0.10 atom% Pb at 97.32 °C. Using the data obtained on the solubility of lead in sodium, the partial molar enthalpy and entropy of solution of lead in liquid sodium were calculated as 29.6 kJ (g atom)-1 and 23.1 J K(g atom)-1, respectively. Finally, the electrical resistivity of liquid potassium and of solutions containing up to 30 atom% Na in liquid potassium are reported from the liquidus to 300 °C. Over the range T = 65 — 300 °C, the resistivity of potassium is represented by
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
and corresponds with the mean of previous determinations. Equations for the temperature dependence of resistivity of the alloys are of similar form. Those representing composition dependence are parabolic, as is usual for liquid binary metallic solutions in which there is no strong interaction between components. Resistivity–temperature curves at constant composition are extended into the solid phase to allow determination of part of the phase diagram. The eutectic occurs at 67.8 atom% potassium and the eutectic is confirmed at — 12.5 °C. Liquid metallic solutions are not normally amenable to study by the common spectroscopic techniques. By the ingenious use of F-centres, however, these solutions may be rendered tractable, albeit indirectly, to spectroscopic study. Thus the activity of potassium in potassium–lead alloys has been determined from changes in the characteristic absorption spectrum of F-centres in crystals of potassium chloride or bromide suspended above the liquid alloy and in equilibrium with the metallic vapour. At 600 °C potassium exhibits positive and negative deviations from ideality at high and low concentrations respectively of potassium. The free energy of mixing is derived for the binary liquid alloy over the entire composition range at 600 °C.
Solutions of the Metals. — Liquid Ammonia. A solvation model based on an analysis of the vapour pressure of alkali metal-ammonia solutions is suggested for these solutions in which the solvated entity, M(NH3)n, is the solute. The model permits the calculation of the critical temperature and concentration with precision and allows interpretation of the positive and negative deviation from Raoult's law in dilute and concentrated metal solutions respectively. The magnetic susceptibility of such a species, tetra-amminelithium, Li(NH3)4, prepared by mixing lithium and ammonia in the stoicheiometric proportions, has been measured by the Faraday method from 1.5 to 194 K. The molar susceptibility ranged from 68.5 × 10-6 to 97.9 × 10-6 e.m.u. at 194 K. In the liquid the susceptibility is paramagnetic and shows a small positive temperature coefficient that can be attributed to variation in bandwidth with temperature. At the freezing point, 88.8 K, the susceptibility undergoes a 6% decrease, also attributable to a change in bandwidth. At 82.2 K, where there is a change from cubic to hexagonal structure, the susceptibility shows an abrupt 25% drop and Curie–Weiss behaviour is then observed down to about 15 K. Below 15 K the susceptibility levels out. Two models can account for the behaviour. In one, Li(NH3)4 is likened to an antiferro-magnetic metal with a Néel temperature of about 10 K; in the other it is likened to a nearly degenerate electron gas. As a sequel to studies of ammonia tes of Group II metals, the equilibrium pressure of ammonia over strontium ammoniate, Sr(NH3), is presented from –18 to +7 °C. The standard enthalpy, free energy (at 0°C), and entropy of dissociation according to
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
are 9.74 ± 0.15 and 1.68 ± 0.02 kcal and 29.5 ± 0.06 cal K-1 respectively per mole of ammonia. A difficulty which must be overcome in these experiments is the decomposition M(NH3)6(s) -> M(NH2)2(s) + 4NH3(g) + H2(g). The gaseous formation energy of the strontium hexa-ammine complex according to Sr2+(g) + 6NH3(g) [??] [Sr(NH3)6]2+(g) is calculated as –256 kcal mol-1. This value is qualitatively consistent with the electrostatic theory of co-ordination complexes in which the ligand molecules are assumed to have progressively greater induced dipoles as the central ion diminishes in size. The Hall coefficient and electrical conductivity of Li–NH3, K–NH3, and Ca–NH3 solutions have been determined from 203 to 243 K and from metal saturation down to 4 mol % metal (MPM). The Hall coefficient is independent of temperature but varies with concentration, rising upon dilution to above the free-electron value for a specific electron concentration. The conductivity ranges from 100 to 5000 Ω-1 cm-1 and dσ/dT is positive. Using an ionized impurity scattering model and an adjustable effective mass, a qualitative fit for the conductivity and Hall coefficient can be made in the region 4 — 8 MPM. The loss of degeneracy on dilution seems responsible for anomalies in this region and hence the transition from metal to nonmetal can be narrowed down to the 1 — 4 MPM region. Significantly, a change in the velocity of ultrasound through caesium–ammonia solutions also occurs at 4 MPM. The effects of varying temperature and caesium concentration on the velocity of 10 MHz waves through these solutions have been examined. Analysis of the latter reveals a structural change in the liquid at about 4 MPM with no evidence of compound formation. The technique also reveals the liquidus for solutions of caesium in ammonia. The melting curve drops smoothly from 195 to 125 K as the caesium concentration is increased to 16 MPM. A comparison of the sound velocity in Cs–NH3 with the other alkali-metal solutions at 12 MPM and 240 K shows that the velocity decreases with increasing ionic radius of metal. For very dilute solutions of sodium in liquid ammonia at –67 °C, the Raman spectrum is almost indistinguishable from the spectrum of ammonia itself. However a weak band at 300 cm-1, present in ammonia, gradually vanishes as the concentration of sodium is raised to 2.0 × 10-3 mol l-1. This band may be due to unresolved rotational structure which is lost as the solvent becomes more ordered.
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Excerpted from Inorganic Chemistry of the Main-group Elements Volume 1 by C. C. Addison. Copyright © 1973 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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