liquid oxygen to b u m much more rapidly than expanded polystyrene bums under ordinary conditions. Liquid nitrogen is very cold (B.P. -196 'C) and can rapidly freeze tissue, and liquid oxygen is not only very cold (B.P. -183 "C), but also it is a n oxidizine aeent. Liquid oxveen can be vem dangerous if allowed to eontact ~educingsubstancesand must be kept awav from oil,. erease, powdered carbon, or any other reducing agents. Caution: Filter-tipped cigarettes should not be used; the filtertipmay ignite and take offlikearocket. (Once, one of the t i ~flew s across the room and struck a balloon held by awaitingstudent. This mightbeagreattrickifrepeatahle, but the ballistics of burning cigarette filters are unpredictable.) There is slight exposure to combustion products, but these are typically particulate matter representing actual ash and probably innocuous compared to actual cigarette smoke. Those who wish to modify this experiment may make chanees to emphasize a n antismoking message, but I believe the message is more effective if understated. In the United States this year, the number of deaths due to exposure to radon, asbestos or cocaine, although substantial, will seem low when compared to the estimated 320,000 to 390,000 projected deaths due to cigarette ~ m o k i n g . ~ Principles Demonstrated Charles's Law Production and properties of liquid oxygen Effect of concentration of reactants on rate of reaction Reaction between substances in two differentstates of matter Liquefaction and boiling of oxygen Cam~arisonof aartial and com~letecombustion ~~
~~
A Reversible Blue-and-Gold Reaction Submitted by Marie C. Sherman and Deborah Weil Ursuline Academy 341 S. Sappington Rd. St. Louis. MO 63122 Checked by
Allan R. Burkett Dillard University New Orleans. LA 70122 Are blue and gold your school colors? If so, here is the reaction for you! (It is also perfect for a Cub Scout Blue-andGold dinner or award ceremony.) If not, your students will still be fascinated bv the reversible colors and sudden precipitate formation. This is also a n eye-catching way to introduce students to redox reactions. The students can easily see the colors of the copper ion a s it changes back and forth between the +1 and +2 oxidation states. Solutions and Equipment 1M KNaC4H406.4H20 (Rwhelle Salts) 1M CuS04. 5Hz0 3%H202(from grocery or drug store) l-L beaker with stir bar heater-stirrer hat late (or heat with burner and stir with a stirring rod) thermometer 3 A n exce en! acco~nto f o ~incons~stenlapproacn r todng Lse and abJse is conla ned in an editor al en1 I ea 'M nor Vices, Mortal Slns' written by David Morris of the Saint Paul, Minnesota Pioneer Press Disuatch. This editorial appeared on p 18 of the Omaha, NE. World ~eialdonFebruary 15, 1990
Procedure 1.Measure 60 mL of the potassium sodium tartrate solution into the beaker. Add 40 mL of 3%hydrogen peroxide. Stir continuously. 2. Heat until the temperature reaches 50 "C.Turn offthe heat. (Littlereaction will be apparent at this point.) 3. Add 1mL (20 drops) of the copper sulfate solution. 4. Observe the following: the blue color of the Cu-tartrate complex ion, the reaction starting, the temperature rising to abaut 80 'C, the vigorous bubbling as gas is formed,and the color suddenly changing to opaque orange-gold as the precipitate of copper(1)oxide, CuzO,is formed. 5.Add another 40 mL of hydrogen peroxide and watch as the orange precipitate dissolves, the blue color returns, the reaction repeats itself and the sudden orange precipitate occurs. 6. This cycle of orange-blue-orangecolors can be repeated 5 or 6 times before the solution becomes too dilute and the tartrateion is depleted. It is necessaryto keep the solution h o h t least 70 "C. Discussion One of the authors (DW) investigated this reaction a t great length and made the following observation: 1.The gas bubbles are mostly oxygen, with a small amount of carbon dioxide. 2. with - Onlv abaut 2-3 %of the tartrate is oxidized into COQ each cycle uf the rroetinn; thus, the reaction can hc rrpeated numerous rimes by smply adding mow pcrnxlde and keeping the mixture hot. 3. The blue color is due to the copper-tartrate complex ion. This is a much more intense color than that of hydrated Cu2+ ion. 4. The pH of the mixture changes from 5 initially to about 9 at the end of several cycles. 5. The orange precipitate is CuzO,the same compound that is formed in a positive Benedict's test. ~
This reaction is analogous to the well-known demonstration that uses potassium sodium tartrate and hydrogen peroxide, with Co2+a s the catalyst.' In that reaction the cobalt ion is regenerated and does not precipitate. Espenson proposed a theory that the cobalt cycles between the +2 and +3 state.' Other theories have been proposed by ~ o t h ?The reaction described here was discovered in the process of a student investigation of the cobalt-catalyzed process? There are a number of unusual features connected with this copper catalyzed cycle of reactions, as well as several unanswered questions. The copper ion a t first is a catalyst that speeds the decomposition of the hydrogen peroxide. (This can be demonstrated bv usine onlv the comer sulfate and the peroxide.) Then the cu2+;on Lecomes reactant and is reduced to the +1 state a s shown bv the precipitate of CuzO. When more peroxide is added, th; CU+;S oxidized back to the +2 state. where it aeain a d s as acatalvst. Thus, the same ion acts first a s a catalyst and then as a reactant. One of the mvsteries connected with this reaction is the identity ofthe-species that reacts with theCu2'and reduces it LO Cu'. Also. what oreanic com~oundis formed during this reduction? The au&ors woufd appreciate correspon~ dence with any chemists who can shed light on these questions.
a
'Klatz, D. Colotfui Catalysis: The Oxidation of Tartaric Acid by Hydrogen Peroxide with a Cobalt Chloride Catalyst. Presented at CHEMED 81. Universitv of Waterloo. Waterloo, Ontario, August, 2Espenson,J. Iowa State University, Ames, IA. Personal commu.nication. ... -. ...
3Toth,Z. J. Chem. Educ. 1980, 57, 464. Brewer, R. Northeast Missouri State University, Kirksville, MO, Unpublished data
Volume 68 Number 12 December 1991
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