Chemical Education Today
Letters Gelatinous Aluminum Hydroxide A recent demonstration in this Journal (1) clearly shows the amphoteric behavior of aluminum ions in aqueous solution. Nevertheless, it is stated that the gelatinous white precipitate, initially formed upon addition of 1.0 M NaOH to a solution of Al2(SO 4)3, is [Al(OH)3(H2O) 3]?xH2O. A similar formulation is also found in some textbooks (2, 3), and seems to indicate a structure consisting of discrete neutral octahedral molecules. Indeed, the formulation as isolated molecules is clearly shown in Rodgers’ textbook (2). That structure is not very likely in the light of the known aluminum chemistry (4–7 ). The well-characterized crystalline aluminum hydroxides, Al(OH)3 (gibbsite, bayerite, and nordstrandite), all have polymeric layered structures with octahedrally coordinated Al atoms and bridging OH groups. The crystalline aluminum oxyhydroxides, AlO(OH) (boehmite and diaspore), also have polymeric structures with bridging O and OH groups. Furthermore, although the hydrated ion [Al(H 2O) 6]3+ exists in acidic solutions, when aqueous solutions of aluminum salts are treated with alkali, complex condensation reactions occur via a sequence of basic polynuclear aluminum cations such as [AlO 4Al12(OH) 24(H2O)12]7+ with Al–O–Al or Al–OH–Al bonding. Precipitation of Al(OH)3 occurs when the polymers reach the appropriate size. Therefore, it is unlikely that gelatinous aluminum hydroxide contains monomeric [Al(OH)3(H2O)3] molecules. The structure of the gelatinous precipitate formed through base hydrolysis of aqueous Al3+ solutions or by acid hydrolysis of sodium aluminate solutions is not yet clearly understood. Nevertheless, both processes have been studied (8, 9). In the case of base hydrolysis, it has been proposed that fresh gels are formed through the coalescence of [AlO4Al12(OH)24(H2O)12]7+ units, containing aluminum atoms both 4- and 6-coordinate, and any other species present in solution, leading to a “pseudo-spinel” structure (8). Aging of the gels forms a pseudo-boehmite phase (γ -AlO(OH)?H2O), which subsequently rearranges to form a hydroxide phase (bayerite and, finally, gibbsite). The so-called “pseudo-spinel” phase possesses both 4- and 6-coordinate aluminum, whereas pseudo-boehmite, bayerite, and gibbsite have only 6-coordinate aluminum. The transformation sequence 1 has also been proposed for gels precipitated by acid hydrolysis of sodium aluminate solutions (9). pseudo-spinel → pseudo-boehmite → bayerite → gibbsite (1) No evidence was found of the proposed ion [Al(OH)4(H2O)2]{ (1–3), but rather tetrahedral [Al(OH)4]{, in dilute alkaline solutions. In conclusion, base hydrolysis of aqueous Al3+ solutions and acid hydrolysis of sodium aluminate solutions are complex reactions that form many polynuclear species. Although the exact nature of the precipitated amorphous gels is not yet completely understood, they are not isolated [Al(OH)3(H2O)3] molecules, and they finally transform to crystalline Al(OH) 3. Therefore, if for simplicity condensed polynuclear species are excluded, the sequence of transformations 2 is more correct than sequence 3 as the pH of the solution is increased. 756
[Al(H2O) 6] 3+ → Al(OH)3 → [Al(OH) 4] {
(2)
[Al(H2O)6]3+ → [Al(OH) 3(H2O)3] → [Al(OH)4(H2O)2]{ (3) The wrong sequence 3 (1–3) could indicate that the amphoterism of aluminum hydroxide consists simply in the protonation of coordinated hydroxide ions or the deprotonation of a coordinated water molecule in neutral [Al(OH)3(H2O)3]. Literature Cited 1. Koubek, E. J. Chem. Educ. 1998, 75, 60. 2. Rodgers, G. E. Introduction to Coordination, Solid State, and Descriptive Inorganic Chemistry; McGraw-Hill: New York, 1994; p 362. 3. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997; p 225. 4. Wade, K.; Banister, A. J. The Chemistry of Aluminium, Gallium, Indium and Thallium; Pergamon: Oxford, 1973. 5. Taylor, M. J.; Brothers, P. J. In Chemistry of Aluminium, Gallium, Indium and Thallium; Downs, A. J., Ed.; Blackie: London, 1993; Chapter 3. 6. Evans, K. A. In Chemistry of Aluminium, Gallium, Indium and Thallium; Downs, A. J., Ed.; Blackie: London, 1993; Chapter 4. 7. Tuck, D. G. In Chemistry of Aluminium, Gallium, Indium and Thallium; Downs, A. J., Ed.; Blackie: London, 1993; Chapter 8. 8. Bradley, S. M.; Kydd, R. A.; Howe, R. F. J. Colloid Interface Sci. 1993, 159, 405. 9. Bradley, S. M.; Hanna, J. V. J. Am. Chem. Soc. 1994, 116, 7771. David Tudela Departamento de Química Inorgánica Universidad Autónoma de Madrid 28049-Madrid, Spain
The author replies: I agree completely with the statement that “The structure of the gelatinous precipitate formed…is not yet clearly understood.” My demonstration (J. Chem. Educ. 1998, 75, 60) does not deal with “freezed dried” material or “crystalline” or “aged” materials. I see no reason to state that my demonstration gives a “wrong sequence” based upon a “proposed transformation sequence” for the gel upon “aging” (eq 1). Also the letter states that the amorphous gel is not composed of “isolated Al(OH)3(H2O)3 molecules”. (Please note that eq 2 as proposed by Tudela implies a simple molecular form and does not explain the gel nature of the precipitate.) I did not say this was the case, but suggested the material may be described as Al(OH)3(H2O)3?xH2O, with the ?xH2O being hydrogen bonded and responsible for the gel nature of the precipitate. I do not object to Tudela’s suggestion that my explanation is perhaps an oversimplification of a rather complex process; however, I do not feel his cited evidence regarding “crystalline and aged” materials explains the nature of the initially produced gel any better. I feel the sequence I have proposed is entirely consistent with that given in two well-regarded referenced texts (Rogers and Greenwood) and I believe it is well suited to explain the demo as presented. Edward Koubek Department of Chemistry U. S. Naval Academy Annapolis, MD 21402-5026
Journal of Chemical Education • Vol. 76 No. 6 June 1999 • JChemEd.chem.wisc.edu
Chemical Education Today
Acid Rain Demonstration
The Bobbing Bird
In late December 1997, I spent an hour playing with the demonstration published in J. Chem. Educ. 1997, 74, 1424 because I found the idea that high temperature combustion generates acid rain to be important. Even though I had done many H2 + O 2 explosions (filled soap bubbles are a favorite!), the idea that I might mix some unburned H2 in the O2 flask did not occur to me until a very minor “flashback” occurred. The demo is probably safe enough provided that anyone who does it recognizes that under no circumstances should the flame be allowed to “burn out” while H2 is flowing out of the delivery tube and into the O2 flask. As soon as the H2 flame begins to be hard to see, the safe approach is to shut off the H2 flow and conclude the demonstration. I had removed the H2 delivery tube without stopping the H2 flow, relit the H2, and reinserted the delivery tube into the O 2 flask when the flashback occurred near the mouth of the 6-L Erlenmeyer flask. The danger is that the explosive limits for H2 and O2 are very wide—roughly 5% H 2 in the O2 flask is an explosive mixture. I was excited to try this demo and to use it in class, but I now think that the risk is too great. If done competently according to your instructions, the demo is probably “safe”. But lack of attention or an attempt to salvage a less than successful demo could put an entire class at risk. I wish that I could think of a fail-safe approach to the demo because the idea of seeing NO2 form is appealing—although it was my fooling around in order to produce enough NO2 to be visible that nearly got us in trouble.
My note on the Bobbing Bird (J. Chem. Educ. 1996, 73, 355) neglected to point out that the demonstration would fail in a room with high relative humidity. A cure for this situation is to add a small amount of alcohol (methyl or ethyl) to the water reservoir. H. D. Gesser Department of Chemistry The University of Manitoba Winnipeg, Manitoba R3T 2N2 Canada
About Letters to the Editor Letters to the Editor may be submitted to the editorial office by regular mail (JCE, University of Wisconsin– Madison, Department of Chemistry, 209 N. Brooks, Madison, WI 53715-1116), by fax (608/262-7145), or by email (
[email protected]) . Be sure to include your complete address including email, your daytime phone number, and your signature. Your letter should be brief (400 words or less) and to the point; it may be edited for style, consistency, clarity, or for space considerations.
Charles L. Braun Department of Chemistry Dartmouth College Hanover, NH 03789
[email protected] The author replies: We have tried several modifications of the demonstration; however, they compromise its effectiveness. The only procedure that showed any promise was dilution of the oxygen with 50% nitrogen. This would reduce the power of any explosion significantly. However, the intensity of the brown nitrogen dioxide was reduced about 50% making it difficult to observe; yet, the test for the nitrous and nitric acids works fine. This is the best recommendation that I can make at this time. Charles L. Braun’s letter helps create an awareness that one needs to exercise extreme caution when dealing with hydrogen–oxygen mixtures. Jerry A. Driscoll Department of Chemistry University of Utah Salt Lake City, UT 84112
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JChemEd.chem.wisc.edu • Vol. 76 No. 6 June 1999 • Journal of Chemical Education
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