Gas Reactions in Plastic Bags Relating Laboratory Observations to the Atomic-Molecular Model Maurice Robinson and Gordon M. Barrow' Royal Roads Military College, Victoria, BC, Canada VOSlBO Many of us introduce students to the atomic make up of matter by focussing on molecular substances, particularly those made up of small molecules. Most of these are gases under ordinary conditions. The value of the atomicmolecular model is most clearly shown to beginning students by using it to interpret the transformations produced by chemical reactions. Thus, in introductory chemistry courses i t is valuable to have students work with gasphase chemical reactions that they can actually see. Using Plastic Kitchen Bags that Zip Advantages To avoid the cost, danger, and technical demands of vacuum-line systems, bubbles and balloons have been suggested as vehicles for the early introduction of gas-phase reactions. To these simple alternatives we suggest the use of zipper-type plastic bags, such as Ziplock (Dow) bags. Such bags are cheap, safe, relatively durable, and easy to use as a vessel for chemical reactions. Also, these bags do not require the pressure that must be wed to blow up balloons. Using zip-type plastic bags, beginning students can carry out a great variety of chemical reactions, or teachers can carry out demonstrations for them. The focus can be on the chemicals and their reactions, rather than on the devices that are used to manage the system. In addition, these bags can be used to conveniently carry out some gas density studies t h a t support the molecular mass implications of formulas of small molecules. Procedure Caution: The usual care in handling hazardous chemicals must be exercised. When appmpriate, a fume hood should be used when filling the bags and carrying out the reactions.
Filling the Bags A collapsed zip-type plastic bag can be fdled with gas with very little admixture of air by inserting a gas inlet tube adjacent to the end of the "zipper", thus allowing the bag to inflate. The zipping is completed as the gas inlet tube is removed. This procedure can be used for a variety of gases that are available in cylinders on lecture bottles. Alternatively, gases can be generated by chemical reactions. These reactions can be carried out in the bag itself or in a bag using a test tube or flask with a delivery tube that can be inserted into the hag. Carrying Out the Chemical Reactions Achemical reaction between two gases can be carried out simolv bv one eas in a small bag (15 x 18 cm) and "outtina . then inserting &at bag into a larger (27 x 28cm), which is then partially filled with the second gas. With a little practice or by stapling tabs onto the zip of the smaller
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Journal of Chemical Education
bag, you can open the small bag so that its contents mix with the gas of the larger hag. Alternatively, a bag can be divided into two compartments by a bar or roller-clamp device. The first compartment is filled with one gas, and the clamp is applied to confine it to part of the bag. After the second gas is placed in the second compartment the clamp is removed to allow the gases to mix and the reaction to occur. Topics To Be Demonstrated Below are a few areas or topics that can be conveniently explored in a beginning chemistry course using these hags. Gas Densities and Buoyancy The relation between gas density and molecular mass can be explored using a top-loading balance to weigh bags filled with various gases. Weighing a bag before and after filling i t with air is a good preliminary to such studies. Students can easily see that a bag of air weighs no more than a collapsed empty bag. This leads to discussing the need for a buoyancy correction when gas samples are weighed. The volume of plastic bags can be estimated by filling them with water, and if the density of air is given, buoyancy corrections can be made. Bags filled with other gases can also be weighed, for example, hydrogen, helium, methane, carbon dioxide, or sulfur hexafluoride. Students can readily master the equipment and can come to a n appreciation of buoyancy and the masses of samples of different gases. Relating these easily determined values to molecular formulas and atomic weights then follows. Given such information, Avogadro's hypothesis can be drawn out. Conservation of Mass .Caution: Highly exothermic reactions melt the plastic bag and must be avoided. The conservation of mass law, which is often merely stated for students, can be conveniently illustrated using reactions in plastic bags. The system is clearly defined, and the transformation is easily observed. The mass that is measured is primarily that of the reaction system, not the container. Reactions involving solids, liquids, or gases can be used. Reactions between gases require a background in buoyancy effects. Many reactions are suitable, and those that complement classroom topics can be used. Examples include the following. Precipitation reactions e.g., the formation of silver chloride Colorful oxidation reactions ex., permanganate reactions Gas-formingreactions e.g., adding zinc to hydrochloric acid Vapor Pressure and Boiling Points Vapor pressure and boiling points are two other physical properties that can be investigated. Avolatile liquid (about
20 mL) is placed in a bag, and excess air is removed before the bag is sealed. The bag and its content can then be gently warmed in a water bath until, suddenly, the temperature is such that vapor rather liquid is formed. This boiling process helps students relate vapor pressures to the boiling phenomena because it is so different from the usual bubble formation that occurs when a liquid is heated in an open container. The Chemical Reactions The principal instructional value of using plastic bags is seen in demonstrations or laboratory studies of gas-phase chemical reactions. The following reactions offer a few examples of successful applications.
id-~ase Reactions Caution: Much heat, enough to melt the plastic, is evolved.
The traditional reaction between hydrogen chloride and ammonia, to form ammonium chloride, is often used to illustrate the rates of diffusions of the two gases. The chemical aspects of this reaction are much more striking when the reaction is carried out in plastic bags: Much solid ammonium chloride is formed, and the bags collapse. The exothermic nature of this, and other gas-phase reactions, is usually not evident when the gases are mixed in the atmosphere. Oxidation-Reduction Reactions
The reaction between hydrogen sulfide and sulfur dioxide gives an impressive collapse of the container and the formation of much yellow sulfur. However, this only occurs if the reaction is catalyzed by a small amount of water. Such easily observed reactions between small-molecule substances help beginning students to make the atomicmolecular model part of their own understanding. Again the thermal aspects of this reaction are evident as the bag becomes warm. Photochemistry
Plastic hags facilitate the demonstration of many photochemical reactions. Caution: The
following reaction is explosive. Only small amounts of reactents should be used. This reaction should be carried out in a fume hood. Reaction between Hz and Clz. The traditional reaction between hydrogen and chlorine is a good example. A mixture of the two gases can be prepared, and the reaction can be started by the light from a camera flash. The chlorine is put in the smaller bag (15 x 18em) or one compartment of a larger bag just until the bag is inflated to a thickness of 0.5 to 1 cm. A quite adequate bang will result from the reaction of this much chlorine with excess hydrogen. The flash output depends on the make and the model of the camera flash. Some flashes are not powerful enough to
induce reaction. In any case, i t is best to set up a remote control to trigger the flash in a fume hood. Reactions between Clz and Alkane. Many related free radical reactions can be explored. When simple alkanes, such as methane or propane, are used in place ofhydrogen, the photoflash produces a noticeable crack, but no appreciable products. The photochemical process is heard but not seen! Addition Reactions. Gas-phase addition reactions can also be illustrated, for example, the addition of chlorine to ethylene or acetylene. Smog Reactions Caution: Nitrogen
oxide is a particularly noxious gas, and it requires great care in handling. It is not suitable for student use.
More and more students have seen photochemical smog. However, for most students, smog is only a set of formulas and equations that, involve oxides of nitrogen. The nasty color of smoe is dramaticallv ~roducedwhen colorless ni) a smali plastic bag is allowed to mix trogen o x i d e ( ~ 0in with air or-even better-with pure oxygen. A dark redbrown color immediately develops due to the following reaction. 2NO + 0, + 2N0, The release of heat is noticeable when the reaction is carried out in the baa. This is not obvious when traces of the gases are mixed o;when the reaction iscarried out in glass vessels. Exposing the mixture u)a photoflash produces the crack of a ~hotoihemicaldeconi-o&ition.. ~ r i u m a b l vthe one that leads to the ozone formation of the smog reaction. NOz+02~N0+03
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Reactions of Gases with Liquids and Solids
Plastic bags are also convenient when only one of the reactants is a gas and the other is a liquid or solid. The following are examples of such reactions that can be conveniently carried out. absorption of ammonia inta water absorption of carbon didxide into limewater absorption of carbon dioxide on the surface of sodium hydroxide pellets ereaction of sulfur dioxide with a potassium permanganate solution The addition of a small amount of the absorbing or reacting liquid or solid to a gas-filled bag produces a rapid, memorable collapse of the bag. Summary Many reactions involving gases can be brought into the introductory chemistry course using zip-type plastic bags. Working with the reactions of these small-molecular substances helps beginning students to appreciate the meaning and value of describing substances by chemical formulas.
Volume 69 Number 12 December 1992
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