edited by GEORGEL. GILBERT Denison University Granville. Ohio 43023
tefted demon/trcltion/ Collapsing Can S u s ~ l m oBY
Equipment One 12-cm crystallizing dish; 10 cm of copper (or platinum) wire; forceps; overhead projector.
Richard D. Sands Alfred University Alfred. NY 14802 CMECKEO BY
David Blackman University of the District of Columbia Washington, DC The considerable solubility of ammonia gas in water can he demonstrated hv the introduction of a small amount of water into a closed c& filled with the gas. The water dissolves the ammonia. creatina such a vacuum that the can collaoses. A gallon ran (a plastic CloroxQhottle dfers the a(ivantage that it can he reused) with a small screw-cap is dried and filled with ammonia. Enough of the gas is introduced in 30-60 see by a fairly slow stream from a cylinder of ammonia. -~ltern&vely,the gas may hegenerated hy the addition of concentrated ammonium hydroxide solution to sodium hydroxide pellets. A side-arm flask is fitted with a separatory funnel and a tube leading to the can. A teaspoonful of sodium hydroxide pellets (about 50) is placed in the flask and covered by the slow addition of the ammonium hydroxide solution (50-100 ml) from the separatory funnel. When the can has been filled with ammonia, it is sealed with a one-hole rubber stopper fitted with astopcock to which an carsyringeor largedropping bulhcan beattarhed. From the ear svrinee. throueh t h itnowck. ~ a suuirt cahout 30 mlf nf wate; is i%roducA into theean and t i e stopcock is quickly closed. Sometimes a t once. other times after a little shakine. -. the can collapses. When a can secretlv filled with ammonia ahead of time collapses upon the introduction of water, the listeners' interest will certainlv he held lone " enouah to exolain and to fill and collapse another can.
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Reduction Potentials and Hydrogen Overvoltage: An Overhead Projector Demonstration SUBMITTEDBY:
Richard W. Ramette Carleton College Northfieid. MN 55057 CHECKED 0 ~ :
Bob Olsen University of Wisconsin. Madison Madison, WI 53706 This demonstration relates the scale of standard reduction potentials to the nbserv~dbehavior of me& in their renrtions w ~ t hhydrogen ion to producr hydrogen gas. It illustrates the kinetic inhibition of hvdroeen eas evolution on a mercurv surface and provides dramatic evidence for the validity of an electron-transfer mechanism. I t provides a setting for the discussion of the difference between thermodynamic reaction tendency and kinetic tendency. ~
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886
Experimental
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Journal of Chemical Education
Reagents 100 mL 6M HC1; 0.5 g HgClz dissolved in 25 mL 1M HCI; mossy zinc Procedure Step I: Place the crystallizing dish, containing the 6M HCI and the copper wire, on the overhead projector and focus sharply on the wire. There is no visible reaction to produce hydrogen by the reaction Cu(s) + 2H+ = Cu2+ Hp(g). On the blackboard draw a vertical line representing the scale of reduction potentials from -1 to +1V, and mark it to show the SHE reference point, the Eofor Cu2+/Cu at +0.34 V, and the E o for Zn2+/Zn a t -0.76 V. Discuss the use of potentials to predict thermodynamic tendencies, pointing out the very different positions of copper and zinc relative to hydrogen. Step 2: Remove the wire and place a piece of mossy zinc in the acid. The evolution of hydrogen is in accord with the thermodynamic prediction, and the fact that i t is rapid shows that the reaction also has a strong kinetic tendency as well, that is, a mechanism with low activation energy. Remove the zinc with forceps. Step 3: At the lecture bench place another piece of zinc metal in the mercuric chloride solution and let i t stand while discussing the reduction potential of HgZ+/Hg (EO = +0.85 V). Thermodynamics predicts that mercury metal will not reduce hydrogen ion, hut that zinc metal has a very strong tendency to reduce mercuric ion to mercury metal. The surface of the immersed zinc will appear shinier, but only at close range, due to the coatine of mercurv. Remove the amaleamated zinc with forceps and rinse very thoroughly to remove traces of mercuric chloride solution. C a ~ o e dvials containine mossy zinc, and freshly-amalgamatedzinc, may he passe& around the class for inspection. The amaleamated zinc should l prevent zincoxide furmntim on be covered wirh 1 M ~ i ' to the surface hut rihndd not he stored with a tight cao because there may be a pressure buildup due to veryslow hydrogen evolution. Step 4: Place the piece of amalgamated zinc in the acid and observe that there is no rapid evolution of hydrogen. After a few minutes there will he a few huhhles clinging to the surface, but the rection is extremely slow in comparison to that of the untreated zinc. A tentative explanation (elicited from the class if possihle) is that the coatine of mercurv on the zinc surface acts as a shield, preventingihe co~~isiodof hydrogen ions with the zinc. However, zinc metal is fairly soluble in mercury. The coating is a saturated solution (amalgam) with a substantial supply of zinc atoms throughout and a t the surface itself. Therefore one would still predict, on the hasis of thermodynamics, that there should be evolution of hydrogen gas. The extremely slow evolution must he explained in terms of kinetics, not thermodynamics. Althoueh hvdroeen atoms may he formed on the surface, H30+ e - 2 ~:(honhedto the
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