tested Jemonstrcrtions A Spontaneous Exothermic Reaction between Two Solids A Safe Demonstration Submitted by Earle S. Scott Ripon College Ripon, WI 54971 Checked by Ronald Stmthkamp Hofstra Universitv Long Island, NY i1550 When equal masses (1-2 g) of hydroxylamine hydrochloride and sodium nitrite are ground, mixed and then ground together, a reaction occurs that generates enough heat to char the paper on which the chemicals were mixed and to vaporize the water produced in the reaction to form a visible cloud. The reaction occurring may be represented by eq 1.
AH for this reaction is -136.5 kJ per mole or -32.62 kcal per mole. This reaction does not start spontaneously if the solids have been thoroughly dried before grinding and mixing them. However, both reagents are mildly hygroscopic so the required amount of water is usually present in the solids as they are taken from the bottle. If a mixture of the dried solids is sprayed with a mist ofwater, reaction occurs immediately. If a solution of sodium nitrite is added to a solution of hydroxylamine hydrohloride, nitrous oxide is formed and bubbles from the~solution.Thus, this system may be used to demonstrate an exothermic reaction between two solids, the importance of providing a mechanistic pathway for even such an exothermic reaction, or the preparation of Ntrous oxide through the reverse disproportionation type reaction that often occurs when compounds of nitrogen in high and low oxidation states are mixed. The customary preparation of nitrous oxide by the thermal decomposition of ammonium nitrate is another example of this type of reaction but is inherently more dangerous than this preparation of nitrous oxide, because overheating frequently leads to an explosion. For average-sized classrooms, the reaction can be run in a n 8-in. test tube to good effect. The concentrations need not be rigidly controlled. Dissolve between 1 and 1.5 e of sodium nitrite in 10 mL water. Dissolve between 1and-1.5 g hydroxylamine hydrochloride in 10 mL water. (This r e p resents arange ofioncentration from approximately 1.5 2 M for each reagent before mixina.~Pourthe two solutions together in a n s i n . test tube andshake enough to assure mixing. Hold the test tube against a dark background so the bubbles that form can be observed. Shortly, a froth will form on the top of the solution, and it will increase in height to 2 or 3 in. Gas evolution continues for several minutes. By the end of the reaction, the solution is hot to the touch. 1028
Journal of Chemical Education
GEORGE L. GILBERT Denisan University Granville,OH 43023
It is interesting to note that the corresponding reaction between sodium nitrite and ammonium chloride to produce nitrogen does not occur under these conditions even though the change in enthalpy (-221.7 kJ1mol) is more favorable. Ammonium chloride and sodium nitrite do not produce nitrogen readily from an aqueous solution without the addition of a mineral acid. It seems likely that the reaction actually occurs between nitrous acid and the m i n e . Hydroxylarnmonium ion is a much stronger acid than ammonium ion (ionization constants 2.2 x 10.' and 5.6 x 10-'', respectively) so is more capable of providing the nitrous acid needed for the reaction to proceed.
A Simple and Colorful Demonstration of Light-Catalyzed Bromination of an Alkane Submined by
Malcolm P. Stevens University of Hartford West Harlford. CT 0611 7
Checked by Leonard C. Grotz University of Wiswnsin Center, Waukesha Co. Waukesha, WI 53188-1 628 Light-catalyzed halogenation of alkanes is traditionally among the first react~o"~ students encounter in thr heg;nlng organic course Such a reactlon can be demonstrated in an extremely simple but colorful manner as follows Procedure Caution: The bromine solution is kept in
bottle.
a small dropping
Caution: If a sunlamp is used, precautions should be taken to shade the lamp so that the students cannot view the bulb directly. Caution:The organic compounds should be disposed of with other halogen-containingwaste solvents.
Add about 25 mL of water to each of two 25 x 200-mm test tubes. One drop of dilute (3 M) ammonia and 10 drops of phenol red indicator are added to impart a distinct violet color. Avolume of a liquid alkane equal to that of the water is then added to each test tube. (2,2,4-Trimethylpeutaneis recommended because it reacts rapidly and is relatively inexpensive, but any liauid alkane should suffice.) of a 5%(vlv)bromine solution in a n ~ ~ ~ r o x & a2t m e li ~ inert solvent, such as methylene chloride, is then added to one of the tubes slowly so that the bromine mixes only with the upper layer. Both tubes are then corked. Upon irradiation of both test tubes with a 275-W sunlamp, the brown color of the bromine-containing solution fades r a ~ i d l vto colorless. and a narrow vellow band anpears i;thi aqueous layer at the liquid;nterface. u p i n agitation of both test tubes, the aqueous layer in the brominated sample turns yellow throughout, thus indicating the generation of the acidic (hydrogen bromide) by-product. ~
~~
~
~
