In the Classroom Tested Demonstrations
Acid–Base Chemistry of the Aluminum Ion in Aqueous Solution submitted by:
Edward Koubek Department of Chemistry, U.S. Naval Academy, Annapolis, MD 21402-5026
checked by:
Cole McWherter and George L. Gilbert Department of Chemistry, Denison University, Granville, OH 43023
Abstract This demonstration deals with the amphoteric behavior of Al3+ ions in aqueous solution. Slow addition of OH- to an Al3+ containing solution results in the formation of a precipitate which redissolves upon further addition of OH -. Keywords Demonstrations Introductory/High School Chemistry Inorganic Chemistry Coordination Chemistry Equilibrium Acid-Base Chemistry Supplementary Materials No supplementary material available.
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Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
In the Classroom Tested Demonstrations
Acid–Base Chemistry of the Aluminum Ion in Aqueous Solution submitted by:
Edward Koubek Department of Chemistry, U.S. Naval Academy, Annapolis, MD 21402-5026
checked by:
Cole McWherter and George L. Gilbert Department of Chemistry, Denison University, Granville, OH 43023
A demonstration dealing with the amphoteric behavior of aluminum was reported many years ago (1). However, we feel that this demo can be improved upon by presenting it somewhat differently. The method outlined below seems to generate more student interest. To 500 mL of distilled water, we add 25 mL of a stock solution of Al2(SO4)3. A large stirring bar is added and the solution is placed on a magnetic stirrer. Then 1.0 M NaOH is added by means of a 50-mL buret, and students are told to observe the result. As the solution continues to flow, a white gelatinous precipitate begins to form. At first the students are unimpressed, realizing that perhaps aluminum hydroxide is insoluble. But as more NaOH is added the precipitate begins to redissolve until the last few milliliters turn the solution clear. This usually does impress the students (what about the common ion effect?)! After this demo they are eager to learn about amphoteric behavior and the properties of coordinated water molecules. The coordinated molecules are formed as the Al3+ ions in solution become surrounded and bonded to water molecules, forming the hydrated ion [Al(H 2O)6]3+. This ion can serve as an acid (pKa ≈ 5.0, roughly the same as acetic acid). This is a consequence of the high charge and small size of the Al3+ ion, which polarizes the attached water molecules. This polarization is sufficient to make the hydrogen atoms attached to the bonded water molecules more proton-like
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or acidic than normal water molecules. The hydrated Al3+ ion can also serve as a polyprotic acid, losing additional protons as more base is added, until the compound [Al(OH)3(H2O)3] ? xH2 O is formed. This uncharged compound is insoluble and settles out of solution as a gelatinous white precipitate. The gelatinous nature of the precipitate results from additional (? xH2 O) water hydrogen bonding to the coordinated water molecules. As more base is added, additional protons are removed, and the newly formed negatively charged ion [Al(OH)4(H 2O)2]{ reenters solution (2). This entire process could be reversed by starting with a basic solution of Al3+ and adding a strong acid. Thus, the ability of the hydrated Al3+ ion to serve as either proton donor or proton acceptor clearly demonstrates the meaning of the term amphoteric behavior. The stock solution contains 25 g of Al2(SO4)3 ? 18 H2 O, 30 mL of 6 N H2 SO4, and 220 mL of distilled water. This solution sometimes appears slightly cloudy and may need to be filtered before being used and stored. Literature Cited 1. Alyea, H. N. Tested Demonstrations in General Chemistry, 6th ed.; Journal of Chemical Education: Easton, PA, 1965; p 31. 2. For a more detailed discussion, including the inductive effect on coordinated amines (e.g., The pKa of Pt(NH3)64+ is 7.2) see: Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions, 2nd ed; Wiley: New York, 1967; pp 33–34.
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
