Chemical Education Today
Comment on “Spectator Ions ARE Important! A Kinetic Study of the Copper-Aluminum Displacement Reaction”
Literature Cited
In the June 2010 issue of the Journal of Chemical Education, Sobel and Cohen (1) reported some interesting results on the chemistry of displacement of copper by aluminum. Such papers are important because they deal with some relevant elementary chemistry problems that one would expect to have been solved many years ago. The results and explanations of Sobel and Cohen are also in line with our studies of the reactions of aluminum with acids. Thus, of the commonly used acids, only HCl(aq) reacts vigorously with aluminum. H2SO4(aq) reacts very slowly, and HNO3(aq) shows no action at all (at least not at room temperature). However, providing the solution contains chloride or bromide anions, all of the above acids react vigorously. Interestingly, the major product with HNO3(aq) is N2O(g) (2). The meaning of the phrase “spectator ions” seems to be ambiguous. For example, one definition of spectator ions states, “A spectator ion is an ion that exists as a reactant and a product in a chemical equation” (3). Another definition is that spectator ions are those that do not react with water (4)! The definition given by Silberberg (5) seems to make more sense from a chemical point of view: “spectator ions [are so called] because they are not involved in the actual chemical change”. However, what is meant by “actual chemical change” remains unclear. It is the title of the paper by Sobel and Cohen (but also the content) and its possible implications that might be considered as slightly controversial, if the latter definition is adopted. For instance, it might be possible that the chloride anions take part in a side reaction. If this were true, are the chloride anions really spectator ions? They can be considered as spectator ions regarding the main equation (displacement of copper by aluminum), but not if the entire chemistry in the system were taken into account. Consider, for example, the possibility of formation of basic aluminum chloride. This compound would, one could speculate, be more soluble than the protective Al2O3 film (on the metal surface): Al2 O3 þ 2HCl þ H2 O f 2AlðOHÞ2 Cl Thus, chloride anions (and bromide, as well) might easily catalyze the dissolution of aluminum in acids, as well as the displacement of copper by aluminum, via formation of (virtual, as it is soon dissolved) Al(OH)2Cl. However, in the processes of catalysis, they take an active role in the overall chemistry, although when writing the chemical equations, they seem not to take part in the reaction. While in the above reaction Al(OH)2Cl should be treated as virtual compound, in a similar reaction (fast dissolution of magnesium powder in aqueous solution of sodium chloride) the Mg(OH)Cl appears as a real, white voluminous precipitate (6). To conclude, if Sobel and Cohen placed the “spectator ions” in the title of their paper in quotation marks, that would better correspond to the chemical reality and leave less room for potential misconceptions. 16
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Vol. 88 No. 1 January 2011
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1. Sobel, S. G.; Cohen, S. J. Chem. Educ. 2010, 87, 616–618. 2. Petrusevski, V. M.; Bukleski, M.; Stojanovska, M. Khim. Bulg. J. Chem. Educ. 2010, 19, 233-238. 3. Web Site for the Wikipedia Entry on Spectator Ions. http://en. wikipedia.org/wiki/Spectator_ion (accessed Oct 2010). 4. Web Page of The Chemistry Hypermedia Project, Section I-7: Spectator Ions. http://www.files.chem.vt.edu/chem-ed/courses/ equil/backgrnd/spectatr.html (accessed Oct 2010). 5. Silberberg, M. J. Chemistry: The Molecular Nature of Matter and Change, 4th ed.; McGraw-Hill: New York, 2006; p 140. 6. Petrusevski, V. M.; Monkovic, M. Chem. NZ 2006, 104, 25–27. evski* Marina Stojanovska and Vladimir M. Petrus Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Saints Cyril and Methodius University, 1001 Skopje, Republic of Macedonia *
[email protected] DOI: 10.1021/ed100617h Published on Web 11/03/2010
Reply to Comment on “Spectator Ions ARE Important! A Kinetic Study of the Copper-Aluminum Displacement Reaction” Stojanovska and Petrusevski's letter (1) is an excellent discussion that gets to my intended pedagogical point (2). The level of understanding about spectator ions can be increased beyond the general chemistry presentation. To further the discussion of what a spectator ion is, I located more definitions. One online dictionary defines spectator ions as “an ion that serves to balance the electrical charges in a reaction environment without participating in product formation” (3). A second online resource defines spectator ions as “ions in a solution that do not participate in a chemical reaction” (4). Including the Wikipedia definition offered by Stojanovska and Petrusevski, “A spectator ion is an ion that exists as a reactant and a product in a chemical equation” (5), one can see that no real consensus has emerged on what a spectator ion is. This could be due to efforts to give more “meaning” to the definition of spectator ions beyond ions that get canceled out from the total ionic equation of reaction to yield the net ionic equation of reaction. I spoke to an emeritus senior colleague, and he indicated that he never used the term “spectator ions” in teaching general chemistry, which may point to this being a more recent pedagogical phenomenon. What gets canceled out from an equation of reaction seems to depend on context, and the absolute presentation in most general chemistry texts should be recognized as an oversimplification. In my example, the copper-aluminum displacement reaction, the mechanism of how halide ions accelerate rate of reaction remains unknown, and may be catalytic, as discussed by Stojanovska and Petrusevski. This points out an interesting parallel between the idea of spectator ions and catalysts.
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Chemical Education Today
Both do not appear in the balanced equation of reaction, and remain unchanged from reactants to products. However, ideally, spectator ions do not change how the reaction occurs; yet catalysts do, by lowering the activation energy of reaction. Therefore, because the Wikipedia definition (5) does not distinguish between spectator ions and catalysts, it adds to potential confusion. As to the authors' contention to place spectator ions in quotation marks in the title of my paper, I must confess that I deliberately created a catchy title to draw readers in, which evidently worked! I look forward to continued discussion of spectator ions in this Journal to help resolve the issues presented.
3.
Web Site for Answers.com Sci-Tech Dictionary Entry for Spectator Ion. http://www.answers.com/topic/spectator-ion (accessed Oct 2010). 4. Web Site of Chemicool Chemistry Dictionary Entry for Spectator Ions. http://www.chemicool.com/definition/spectator_ions.html (accessed Oct 2010). 5. Web Site for the Wikipedia Entry on Spectator Ions. http:// en.wikipedia.org/wiki/Spectator_ion (accessed Oct 2010). Sabrina Godfrey Sobel Department of Chemistry, Hofstra University, Hempstead, New York 11549-1510, United States
[email protected] Literature Cited 1. Stojanovska, M.; Petrusevski, V. J. Chem. Educ. 2011, 88, DOI: 10.1021/ed100617h. 2. Sobel, S. G.; Cohen, S. J. Chem. Educ. 2010, 87, 616–618.
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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DOI: 10.1021/ed100959w Published on Web 11/01/2010
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Vol. 88 No. 1 January 2011
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