9 The Volume Expansion of Alkali Metals in Liquid Ammonia WILLIAM H. BRENDLEY, JR. and E. CHARLES EVERS
Downloaded by UNIV OF NEW ENGLAND on February 11, 2017 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch009
Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pa. 19104
The volume expansions of alkali metals in liquid ammonia are discussed in the light of the current available data. Special emphasis is made of the anomalous volume minimum found with sodium-ammonia and potassium— ammonia solutions. Recent studies of potas sium in ammonia at — 3 4 ° C. were found to exhibit a large minimum in the volume expan sion, ΔV, vs. concentration curve. The re sults of these findings were compared with the previous results of potassium in ammonia at — 4 5 ° C. The volume minimum was found to be temperature dependent in that the depth of the minimum increased and shifted to higher concentrations with increasing temperature. No temperature effect was observed on either side of the minimum. These findings are dis cussed in light of the Arnold and Patterson and Symons models for metal-ammonia solutions.
Jolutions of alkali metals i n liquid ammonia at all concentrations, with the exception of cesium, are less dense than either of the constituents. T h i s behavior for metal ammonia solutions is unique i n that the expan sion i n volume is much larger than that shown on forming solutions of normal electrolytes or non-electrolytes. In very dilute metal solutions where dissociation is considered com plete, the alkali metal ion is considered to be a normal solvated ion, as i n electrolytic solutions, and it is generally conceded that the large volume change is to be ascribed chiefly to the solvated electron. A s the con centration is increased it is quite obvious from conductance and magnetic data that metal ions interact with electrons to form some sort of a n ion pair and also that electrons couple to form spin-paired species. T h e manner i n which these species form is not entirely clear, nor is their 111
Hart; Solvated Electron Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
SOLVATED ELECTRON
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structure, but there is still a n expansion i n volume which exceeds the volume of solvent plus metal. T h e effect also persists at high metal concentrations where the solutions exhibit unquestionably metallic properties. However, the volume expansion is not uniform on going from extremely dilute solutions to saturation. I n the case of sodium and potassium solutions, a pronounced minimum is found i n the region of concentration where the conductance passes through a minimum, and a maximum is observed i n the volume change at high metal concentration. A large volume expansion for solutions of sodium i n ammonia was first reported b y K r a u s and Lucasse (17). Since this initial report, many investigations have been made of the volume expansion for a number of alkali metal-ammonia solutions. T h e techniques employed i n these investigations have varied from density measurements for concentrated solutions using the Westphal Balance or Pycnometer to dilatometric studies for dilute solutions, which measure the volume expansion directly. T h e volume expansion per gram atom of metal, A V , as used i n this report, will be defined as: V.oln. — [ V N H , + Vmetal]
A A
V
=
gram-atom metal
where V H , is the volume of the liquid ammonia i n the solution at the temperature studied, Vmetai is the volume of the metal, and Veoin. is the volume of the solution. One might also express the data i n terms of the apparent molar volume of metal. I n this case the volume change would be computed as the volume of solution, Veoin., minus the volume of a m monia, V N H „ per gram atom of metal. While either method may be used, we feel the former is more appropriate since we do not know the partial molar volumes of the constituents. Owing to the metastability of the metal-ammonia solutions, meticulous and detailed experimental procedures are discussed i n great detail i n the doctoral dissertation of Brendley (3) and i n the book, " M e t a l - A m m o n i a Solutions" (7). A brief summary of the data from various sources will be presented below and some of these data will be discussed i n more detail i n a following section. N
Lithium L i t h i u m , the smallest atom i n Group I of the periodic table, exhibits the largest volume expansion of the alkali metals i n liquid ammonia. Johnson, Meyer, and Martens (14) studied the density of lithium solutions at —33.2° C . T h e y reported a maximum i n the volume expansion vs. concentration curve at 1.2622V. T h e AV value at the maximum is 46.59 cc. /gram-atom. G u n n and Green (10) have reported volume changes for lithium i n ammonia at 0 ° C . T h e volume change results of G u n n at 0 ° C . are much lower than the volume change data of Johnson at —33° C . T h e results of these investigations are plotted i n Figure 1. It should be noted that a line has been drawn through the points obtained
Hart; Solvated Electron Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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BRENDLEY AND EVERS
Volume Expansion
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Downloaded by UNIV OF NEW ENGLAND on February 11, 2017 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch009
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