In the Classroom Tested Demonstrations
Lead Globules submitted by:
Francisco J. Arnáiz and María R. Pedrosa Laboratorio de Química Inorgánica, Universidad de Burgos, 09001 Burgos, Spain
checked by:
Gordon A. Parker Department of Natural Sciences, University of Michigan–Dearborn, Dearnborn, MI 48128-1491
The facile deposition of lead from its salts is well known. A number of experiments illustrating the growth of lead crystals and the formation of lead trees have been described in this Journal (1–5). Typical demonstrations of lead deposition are conducted by reacting aqueous solutions of lead(II) acetate with zinc rods. Here we refer to the less known, also attractive, reaction of plumbate solutions with aluminum. A variety of lead closo-films can be formed depending on the conditions— mainly, lead concentration, basicity of the solution, size and shape of the aluminum pieces, and temperature. The method described below permits obtaining a large lead globule in less than 1 minute. The globule is 1–3 cm in diameter and usually remains floating for a long time. Procedure CAUTION : Lead compounds are toxic and concentrated sodium hydroxide solutions are corrosive. Gloves and safety goggles should be worn to protect against possible spattering during hydrogen evolution. Twenty-five milliliters of 0.1 M Pb(NO3)2 solution in a 100-mL beaker is treated with 25 mL of 4 M NaOH so that the Pb(OH)2 initially precipitated is completely dissolved and a large excess of NaOH with respect to the trihydroxoplumbate is present. The resulting solution is heated to 60–80 °C and then a granule of aluminum (3–4 mm, 40–60 mg) is added. A spheroidal floating gray globule, which eventually sinks, forms immediately. Successive addition of aluminum pieces leads to the formation of new globules until most of the lead in the solution is consumed (Fig. 1). It is worth noting that, in case aluminum granules are not available, similar results are obtained with a piece of aluminum foil or wire rolled into a ball. The formation of floating “serpents” is also attractive and quite easy to achieve by using a piece of aluminum wire approximately 50 mm long and 1 mm in diameter (Fig. 2). Discussion A plausible explanation for this behavior is as follows. First, aluminum is a very strong reducing agent in basic medium (E ° = ᎑2.33 V). Addition of aluminum to the trihydroxoplumbate solution provokes simultaneous liberation of Pb and H2, as represented in a simplified form in eqs 1 and 2.
and remains in solution as tetrahydroxoaluminate because of the large excess of NaOH. In contrast, lead reacts very slowly with NaOH so that the lead film formed is not significantly redissolved in the excess of base. Third, aluminum and lead do not form solid solutions, so the lead film that begins to form on the surface of the aluminum is readily released. Finally, the hydrogen liberated under the surface of the lead film initially formed, the continuous deposition of lead while the reaction is in progress, and the malleability of the metal work together to grow a thin film in the form of a large globule that goes to the surface of the liquid. The continuous nature of most of the film is evidenced by the fact that, despite the high density of lead and the tendency of hydrogen to escape, the globule usually remains floating for hours. The main discontinuity in the film is produced in the lower part of the globule while the piece of aluminum is reacting. Because of the oscillations of the globule, occasionally the hydrogen escapes and the globule sinks. No similar results have been obtained by using zinc instead of aluminum, but lead nitrate can be replaced by lead acetate. However, use of the nitrate permits simultaneous testing of the ability of aluminum to reduce nitrate to ammonia without significant additional requirements. To this end the reaction is more conveniently conducted in a test tube. A typical run is as follows: In a 16 × 160-mm test tube is placed 5 mL of each of the above solutions. To the warm solution a granule of aluminum is added and, immediately, a glass-wool plug is placed above the surface of the liquid to allow only gases to escape from the solution. A strip of wet indicator paper—or a drop of the Nessler reagent on a glass rod— placed on the mouth of the tube evidences the liberation of ammonia at the time that the lead globule is formed. Literature Cited 1. 2. 3. 4. 5.
Stone, C. H. J. Chem. Educ. 1929, 6, 355. Brewington, C. P. J. Chem. Educ. 1929, 6, 2228. Taft, R.; Stareck, J. J. Chem. Educ. 1930, 7, 1520. Fillinger, H. H. J. Chem. Educ. 1935, 12, 92. Hurd, C. B.; Lamareaux, H. F. J. Chem. Educ. 1959, 36, 472.
2Al + 3[Pb(OH)3]᎑ → 3Pb + 2[Al(OH) 4] ᎑ + OH᎑ (1) Al + OH᎑ + 3H2O → [Al(OH)4]᎑ + 3/2H2
(2)
Second, aluminum reacts rapidly with aqueous NaOH
JChemEd.chem.wisc.edu • Vol. 75 No. 11 November 1998 • Journal of Chemical Education
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In the Classroom
Figure 1. Floating lead globules still forming (left), show tendency to self-weld (right).
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Figure 2. Top and side views of the floating lead “snake”.
Journal of Chemical Education • Vol. 75 No. 11 November 1998 • JChemEd.chem.wisc.edu
