Exocharmic Reactions Up Close

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Exocharmic Reactions Up Close

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by R. W. Ramette

My greatest enjoyment as a teacher was performing classroom demonstrations that were visually delightful. Coining the term “Exocharmic Reactions,” I proposed the “Laws of Charmodynamics” and mentioned two reactions worth observing with a microscope (1). The present article elaborates on these and adds another that has elicited awed responses from my students, including high school teachers in summer programs. Because chemistry departments may not have their own stereo (dissecting) microscopes, 20⫻ or 30⫻, the biology department must be sweet-talked into making theirs available. They may express shock, and they will rightly demand a blood oath that no chemicals, including water, come into contact with the objective lenses. While these reactions may be observed with a monocular scope, the effects are especially striking in stereo. If the teacher has access to a scope adapter that can route the images to a color monitor, these experiments will make good demonstrations but, unfortunately, not in stereo. The four exercises below1 are written in the form of handouts to students. I am grateful to Jim Maynard (2) for his careful work in producing the accompanying photomicrographs.

interesting student projects. For example, discover the optimal concentration of an acidic lead nitrate solution, so that a crystal of potassium iodide, nudged to the edge of a drop, results in glinting golden hexagons of lead iodide. WSupplemental

Material

Both still images and video of the reactions shown in all four Exercises are available in this issue of JCE Online. Note 1. CLIPs containing safetly information are available for most of the substances mentioned in this article. Two are in this issue (pp 33, 34). Others may be found using the JCE Index online to search for the name of the substance in the title.

Literature Cited

Hazards Although the quantities of toxic reagents are small, standard measures for disposal are required. Residues may be rinsed into a common waste jar for temporary storage.1

1. Ramette, R. W. Exocharmic Reactions, J. Chem. Educ. 1980, 57, 68. Also in: Shakhashiri, B. Z. Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 1; University of Wisconsin Press: Madison, WI, 1983. 2. Jim Maynard, Chemistry Demoist, University of Wisconsin– Madison, private communication.

Conclusion There must be dozens of exocharmic reactions that can be observed microscopically. Searches for them would make

R. W. Ramette is an emeritus member of the Department of Chemistry, Carleton College, Northfield, MN 55057; [email protected]

1: Getting to Know the Microscope Exercise Put a tiny scoop of table salt (NaCl) onto a dry watch glass and, following the instructor’s suggestions, learn how to adjust and use the microscope. Compare illumination from the top with that from the bottom. Try different magnifications, and play around with the focus to explore the depth of field. In all these exercises be prepared to readjust the focus and watch glass position, as needed. Try pushing single crystals around with a dissecting needle. Note the crystal shapes. Add a drop of water and watch the crystals

dissolve. Set the watch glass aside until later, and then observe the recrystallized salt after the water has evaporated. At another time, this dissolving and recrystallization might be tried with various soluble salts. Figure 1. (A) The NaCl crystals under study are illuminated from the top, and shot at 30⫻ magnification. (B) A close-up of the larger of the two crystals in (A), shot at 40⫻, and illuminated from below (40⫻). (C) An NaCl crystal has been guided into the water droplet with a stylus. photos: J. Maynard and J. Harris

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Journal of Chemical Education

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Vol. 84 No. 1 January 2007

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photos: J. Maynard and J. Harris

Figure 2. The sequence above reveals the dissolution of NaCl crystals in water and their subsequent recrystallization (A–H). The final two photos in the sequence (G and H) are close-ups of the reformed NaCl crystals (49⫻).

2. Growing Silver Chromate Crystals Exercise

The Chemistry

Use a spatula to place 3–6 small crystals of potassium dichromate at one side on a dry watch glass. Observe their shapes with top illumination. Keep them away from the middle. Add a drop of 0.05 M silver nitrate to the middle of the watch glass, and get it in focus. Use a needle to push a single crystal of potassium dichromate barely into the edge of the drop. Keep watching and enjoy, using bottom illumination. This exercise sometimes works better than others. If you get really nice results, say “Wow!” and let your less fortunate friends peek through your scope.

The solution contains silver ions. When K2Cr2O7 dissolves, the dichromate ion comes to equilibrium with HCrO4⫺ and CrO42⫺ ions, which diffuse into the drop. The solubility of silver chromate is low, and slow diffusion makes crystals form as large, thin, redorange spears, while countless microcrystals also swim around. Cr2O72⫺ + H2O HCrO4⫺

2HCrO4⫺

H⫹ + CrO42⫺

2Ag⫹ + CrO42⫺

Ag2CrO4(s)

log K = ⫺1.55 log K = ⫺6.47 log K = 12


Figure 3. (A) A top-lit crystal of (NH4)2Cr2O7 is introduced into a drop of 0.05M AgNO3 (20⫻). (B) The crystal dissolves into the surrounding solution, reacts with the water in the droplet, and then reacts further with the resulting chromate ions in a concentric ring around the solid crystal, forming silver chromate (30⫻). (C) As the reaction continues, a small ring of red silver chromate crystals forms near the outer edge of the crystal (40⫻). (D) Near the conclusion of the reaction, the outline of the now-dissolved crystal is clearly seen as a concentrated region of silver chromate crystals (20⫻).

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Report 3: Growing Aluminum Oxide Whiskers Exercise Put a little scoop of granular aluminum onto a dry watch glass, off to the side to make sure no spattering contaminates the microscope. Add a few drops of a solution that contains both 6 M hydrochloric acid and 0.1 M mercuric chloride. Let it bubble for about one minute. Then use an absorbent tissue to “blot” the granules, soaking away nearly all of the solution. Use a needle to quickly brush the granules off the tissue and back onto the watch glass. Immediately begin continuous microscope observations (top illumination). If the lab humidity is very low, it may help to “mouth breathe” on the sample. Move the watch glass around so that you can check on various granules, and readjust focus as needed. The whiskers are very fragile, so you must not sneeze. Later, write a haiku about your observations. When no further change seems to occur, add 1 drop of 3 M HCl. Can you see tiny mercury droplets? Do you think a 1-cm square of aluminum foil would also work?

The Chemistry Aluminum is a very active metal (Eo = ⫺1.66V) and reacts quickly with the oxygen in air: Al(s) + 3/2O2(g)

Al2O3(s)

log K = 227

Aluminum buildings, however, do not crumble into piles of white dust because the oxide forms a tightly-bonded protective

monolayer on the aluminum surface. This layer is so thin that it doesn’t prevent highly polished aluminum from being an excellent mirror. In this exercise the solution performs two functions. The hydrogen ions dissolve the protective oxide film, and then attack the aluminum: Al2O3(s) + 6H⫹ Al(s) + 3H⫹

2Al3⫹ + 3H2O log K = 17

Al3⫹ + 3/2H2(g)

log K = 85

The clean aluminum surface is then able to reduce the mercury(II) to form an adhering film of liquid mercury: 2Al(s) + 3HgCl2(aq)

2Al3⫹ + 6Cl⫺ + 3Hg(l) log K = 217

The bubbling slows down because the mercury-coated surface displays the phenomenon of hydrogen overvoltage. When the granules are blotted nearly dry and exposed to air, the underlying aluminum atoms dissolve in the mercury film and diffuse to the surface, reacting as usual with oxygen. But on the liquid surface the aluminum oxide can no longer form the impervious protective monolayer, so the oxidation merrily proceeds, and spectacular white Al2O3 “whiskers” grow out in all directions. Finally, the addition of HCl dissolves the aluminum oxide, forming a solution of aluminum chloride.


Figure 4. (A) A 30 mesh spherical granule of aluminum metal is secured to a slide, treated with acidified 0.2 M HgCl2, and then lightly blotted with lens paper. The slide upon which the crystal is attached was placed at a 45 degree angle with respect to the objective lens for photography purposes (30⫻). (B) Once exposed to air, small “whiskers” of aluminum oxide begin to grow out from the surface of the granule. (C) As the reaction proceeds, the “whiskers” continue to form larger structures (30⫻).

Editor’s Note The Photographs The photos illustrating this article were taken in the Department of Chemistry at the University of Wisconsin– Madison. The photographer was James Maynard, the department’s lecture demonstrator, who was assisted by Julie Harris, a student intern. Many photographs were taken be-

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fore capturing the superb results shown here and on the cover. Computer hard drives were quickly filled to capacity only to be replaced by larger ones, which were subsequently filled. Copious notes were recorded in a laboratory notebook. Crystals and reactions were always fascinating but usually undisciplined—sometimes the best action was off camera!


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4: Growing Crystals of Silver Metal Exercise

2Hg(l) + 2Ag⫹

The Chemistry Mercury and silver have nearly the same reduction potentials (E⬚ = 0.80 V). Silver ions are reduced to silver metal, which is only slightly soluble in mercury, and it forms eerily interesting silver crystals.

K~1

Ask the instructor whether (s)he has determined the mass of a typical mercury droplet. If so, calculate its amount in millimoles. How does it compare with the 0.7 mmol of silver ion added? photo: J. Maynard and J. Harris

The instructor will use a fine-tipped dropper to put a droplet of mercury onto a watch glass. Get it in focus using top illumination and enjoy its reflective surface. Add about 1 mL of a solution that contains both 0.7 M silver nitrate and 0.1 M nitric acid. Simply watch what happens until no further change occurs. Try to keep your “Oohs!” at a modest decibel level. A skilled demoist has reported fantastic results with a higher (1 M) concentration of silver nitrate (3).

Hg22⫹ + 2Ag(s)

Figure 5. A droplet of mercury is placed on a 2-cm watch glass. The photograph shows the reflected image of the objective lens of the microscope and the laboratory as seen in the droplet (20⫻).


Figure 6. (A) After the drop is treated with a solution of nitric acid and silver nitrate, crystals of silver growing on the drop distort the spherical shape of the mercury (30⫻). (B) The distortion of the droplet becomes more pronounced as the silver crystals grow larger. (C) At this point in the reaction, the mercury drop is clearly distorted by the large silver crystals growing through it in all directions. (D) The drop is now dominated by the silver crystal structure. (E) This photograph shows that there are more silver crystals now than mercury. Note the fine structure of the silver crystals in the lower right of the photograph. (F) The final photograph represents the final state of the reaction. The silver crystals form a fine dendrite structure, while the mercury dissolves into solution as the mercury(II) ion.

The Equipment The reactions were carried out on glass slides, with the exception of the Hg + AgNO3/HNO3 reaction, which was performed in a 2-cm watch glass. The microscope used was the Nikon SMZ-10A from the department’s crystallography laboratory. A Nikon digital sight DS-2Mv microscope-

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compatible camera was mounted to the microscope’s photo lens adapter. The Nikon ACT-2U camera operation software was used to capture the images.


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Exocharmic Reactions Up Close

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