overhead projector demonstrations
DORIS edited KOLB by Bradley University Peoria, IL61625
A Simple Demonstration of the Law of Combining Volumes Donald B. DuPr6 University of Louisville Louisville, KY 40292
The law of combining volumes may be easily demonstrated on the overhead pmjcctor using two glass syringes connected by a smdl plcceoftygon tubing, as illustrated in part a ofthe figure. The volume &pduations and the colors will project nicely with the use of glass syringes, such a s (The contents 5-ml. I%Ureusable. friction-free syrin~e!~s. syringes do notproject well.) Test and markings of the visibility of the volume graduations, the freedom of travel of the plungers, and the leakage of the assembly beforehand by forcing air back and forth between the syringes. This may be done a s part of the demonstration to illustrate the mechanics of the device to the audience. The demonstration is based on the reaction of two volume.; ofnltrk oxide reacting with one volume of molecular oxygen to produce two volume^ of nitrogen dioxide, via the following reaction, which proceeds almost to a~mpletion. 2N0 + 0, + 2N02 colorless + red-brown Experimental .Caution: The oxides of nitrogen produced in this demon-
stration are irritating and noxious even in small quantities. Have a bucket of water handy for disposal To begin the demonstration, the syringes are disconnected and a convenient volume (e.g., 1.5mL for 5-mL capacity syringes) of 0 2 is sucked into the syringe on the right. Twice this volume (e.g., 3.0 mL) of colorless NO is sucked into the syringe on the left. Oxygen and NO may be obtained from lecture bottles or prepared on site a s described by Alyea (I) and transferred to the projection syringes. Oxygen is easily generated from hydrogen peroxide using MnOz a s a catalyst, and NO is readily prepared from nitric acid and copper (I). The syringes are reconnected with the tubing and laid on the overhead. The volumes and lack of color of the two gas
(a)Glass syringes with NO and O2joined with asmall piece of plastic tubing. (b) Displacement of the left syringe barrel immediately after oxygen is injected. (c) After the reaction is complete, the plunger of the left syringe retracts to the original volume. NO and 0, are both colorless gases. The product of the reaction, NO,, is redarown. samples are noted. Before the gases diffuse together, quickly depress fully and hold in the plunger of the oxygen syringe. The plunger of the NO syringe will move outward (see figure, part b), approaching the mark ofthe sum of the two gaseous volumes (i.e., 4.5 mL for the 5-mL syringe). The gaseous mixture immediately turns red-brown a s the above reaction takes place. he reaction is vely fast and the gaseous volume in the left s.yringe quickly contracts baek to the original point, showing that two vdumes of colorless NO and one volume of 0 2 produce two volumes of red-brown NOz. (Because the reaction is very fast, the maximum travel of the left plunger may be gauged by having the top of the barrel push back a small block of balsa wood or polystyrene foam.) This demonstration of Gay Lussac's famous law is easier to set up and explain than one previously described in this Journal (2). The role of NO and NOz in smog formation may be discussed, a s well a s recent evidence of the functioning of the simple NO molecule a s a neurotransmitter and vasodilator (3). Literature Cited
Volume 70 Number 10 October 1993
849
