tested demonstrotions An Oscillating Reaction as a Demonstration of Principles Applied in Chemistrv and ' chemical Engineering Submined by Jeffrey J. Weimer University of Alabama in Huntsville Huntsville,AL 35899
GEORGE L. GILBERT Denison Univemity Granville,OH 43023
the T. The best method is to spot-weld the stainless steel pieces together. This will not work with comer. however. u s e clean copper electrodes and work from i6w power settings until the wires are joined firmlv. Alternatively, the wires can be joined by ancpoxy or an; other method that fives a solid joint, such as by folding and crimpinfi the two wires toeether. ThcT also can he made bv iudicious foldine of a longer piece of Stainless Steel or copper wire. Flatten the Pt wire with a hammer andjoin the flattened Pt wire to the lona end of the T. Anv method used a t the Pt-Stainless ~ t e e (or i Pt-copper) joint must eventually withstand heating in a Bunsen burner flame. Spot-welding is highly recommended (using lower power than at the Stainless Steel joint). Alternately, the Stainless Steel or copper can be c&mped tightly around the Pt wire. Bend the flattened Pt wire in a spiral or S shape so as to fit into the mouth of the flask and remain within a horizontal plane above the bottom of the flask. d "
Checked bv Wayne i.Smith Colby College Watewille. ME 04901 This demonstration has been used successfully in the first lecture for a freshman class on basic chemistry. It was repeated in the last lecture to show the calculations for the heats of reaction, and the basic tenets of transport processes were mentioned to point out additional material chemical engineers would confront in their future studies. The demonstration also has been used in conjunction with lectures given to high school seniors interested in careers in chemistry and chemical engineering. The visual effects may be lost in the back rows of large lecture halls (greater than 150 students). They will be more clearly visible if the room is darkened before the demonstration. Materials one 500-mL Erlenmeyer flask -200 mL of methanol 3-4 em of a 0.25-mm diameter Pt wire 21 an of a l-mm diameter Stainless Steel or copper wire Bunsen burner or large candle flame Procedure The objective is to form a T with the Stainless Steel or copper so as to be able to hang the Pt wire about 1 4 mm above the 150-mL line in the Erlenmeyer flask. See the schematic in the figure. Cut the Stainless Steel wire into two pieces about 6 cm and 15 cm.Join these pieces to form
-
Demonstration 'Caution: The demonstrator should stand hack from the flask when inserting the Pt, because the initial ex. ~ l o s i o nout the too of the flask can he forceful The demonstrator may choose to wear goggles a t this point. Subsequent explosions are not as violent. No danger exists of the explosion continuing outside the flask so that a safety shield is not necessary. The demonstrator should not put the nose near the flask. The demonstrator should wave his or her hand over the top of the flask to waR the fumes of the products toward the nose for smelling. Fill the flask to about 150 mL with methanol. The level should be such that the Pt wire will hang about 1 4 m m above the liquid surface. Holding the short end of the T, heat the Pt wire to glowing hot in the Bunsen or a candle flame. For a small Pt wire, a large candle flame will suffice. The objective is to heat the entire Pt wire. Quickly transfer the hot wire to the flask and drop the T in place. If done properly the first time after pouring the methanol, a n immediate explosion should occur within the flask. This is due to homogeneous
after inserting in flask
,flattened
Pt wire
Pt wire coiled as shown in top view below
-=Fbefore inserting in flask Schematic of the Stainless Steel or copper T with the flattened Pt wire attached in a view before inserting in the flask and after inserting in the flask (drawing not to scale).
Volume 71 Number 4 A ~ r i1994 l
325
burning of the methanol vapor and air in the flask and will produce a loud "pop". The explosion generally does not happen at later attempts. Depending on conditions (i.e., this may not work the first time), the Pt wire should start to glow shortly after being dropped into the flask and aRer the first explosion. If this does not happen, remove the wire and repeat the procedure starting from the beating step. Be certain to transfer the wire while it is still warm. After a period where the wire has shown sustained glowing, another explosion should occur. A pale blue flame will burn momentarily inside the flask, and the F't wire will appear to "blow itself out". The system will recover and the Pt wire again will heat up, glow for an extended period, cause an explosion, and blow itself out. The cycle will repeat itself on average about 112 h or longer. Eventually, the frequency of the explosions will decrease and the Pt will not glow again after the explosion. This usually means that the methanol must be refilled and the process started over from the beginning. Discussion The glowing wire and oscillating cycles can be used to es in freshman chemistrv and demonstrate ~ r i n c i ~ ltaught sophomordj&ior chemicalengineering courses. In &icular, concepts developed in thermodynamics, catalysis, reaction kinetics. and transoort ohenomena are involved in the oscillating reaction. Each is discussed separately below in a format that can be used to present the information to students. A
.
Thermodynamics Thermodynamics deals in part with the production or use of energy during a chemical reaction. The Pt wire is glowing, so that something is clearly producing energy (heat). This must be due to a chemical reaction, since no outside mechanical connections are made to the wire. Methanol, CH30H,and air are the only possible reactants present. If the flask is sampled by smell while the wire is glowing, the distinct presence of formaldehyde, CH20, is detected. In addition, CH30H also can undergo complete combustion to CO, and H20. The students should be asked to draw up a list of possible reactions for CH30H with air to produce CH20, COz, and H2O. Since everything but 0 2 in air is inert, the list should appear as
Either eq 1 or eq 2 is producing the CH20. In order to determine whether one or both are producing the heat, the students must be familiar with concepts of formation enthalpy and reaction enthalpy. The students should be asked to calculate the reaction enthalpies for each of the above reactions based on the values for the standard enthalpy of formation given in the table. The values are 85 kJlmol for eq 1, -157 kJ1mol for eq 2, and -677 kJ/mol for eq 3. This clearly shows that direct decomposition of CH30H to CHzO cannot be the only reaction occurring, because this reaction requires heat (is endothermic). The reaction in eq 2 may be occurring alone to produce CH20. Since the COz and Hz0 products in eq 3 cannot be detected by sight or smell, the extent of this reaction is unfortunately unknown without further chemical analysis of the products, such as by gas chromatography or mass spectrosCOPY. 326
Journal of Chemical Education
Standard Enthalpies of Formation of the Compounds in the Reactions ( 1 ) Compound
AH? (kJ1mol)
Hz0
0 0 -201 -116 -242
coz
394
02
H2
CH30H CH20
Catalysis Heterogeneous catalysis deals with chemical reactions occnmng on solid surfaces. The reaction occurs on Pt surface, and the Pt is not consumed in the reaction. This is the d e f ~ t i o nof a catalyst. Other metals remain inert to the reaction, as can be demonstrated by using such materials as Cu, Al, Fe, or Ni in place of the Pt and by the fact that the Stainless Steel or copper support wire does not glow. The best conditions (the hottest Pt wires) are reached when the F't has the largest surface area for a given mass of material. This exposes the largest amount of the metal to the gas. The wire is flattened before inserting into the flask because a circular cross section presents a lower surface area per mass than a flat cross section. Flat Pt ribbons also work well, and thick F't rods would not work. As a note to the lecturer, noble metal catalysts such as Pt and Pd are oRen spread as a nearly monoatomic layer on inert substrates (ceramics like alumina) in practical systems such as the automotive catalytic converters to obtain the highest surface area to mass distribution. Transport Phenomena Transport phenomena deal with processes that carry heat and mass to and from a system. The reaction that produces heat at the Pt wire requires that CH30H vapor and O2meet at the wire to read. The CH30H vapor rises from the liquid to the wire by a transport process called d i f i sion. In the same way, OZcomes from the air in the room and is transported by diffusion through the top of the flask to the wire. If these mass transport processes become limited, the reaction will "shut down". The reaction stops aRer an explosion because the 0, has been depleted from the flask. The wire begins to glow again only after the CH30H vapor and 0 2 refill the flask and meet at the Pt wire. The reaction oscillates most effectively in a room where the air is not moving (with poor ventilation) and does not oscillate at all if set up in a well-ventilated hood. This is probably because air moving over the mouth of the flask increases the transport of fresh air (via convection processes)into the flask so that the vapor mixture in the flask remains below the critical explosion limit for CH30H or CHzO in air. As the reaction runs, the heat produced at the wire is transported by radiation, convection, and conduction processes to the walls of the flask, and they become warm as well. The heat also causes the CH3OH to evaporate more rapidly. Conduction of heat occurs down the support wire. A Stainless Steel wire is a better support than a Cu wire because the thermal conductivity of Stainless Steel is lower and the F't wire will glow hotter. The difference should be noticeable as a slightly cooler spot at the point where the Pt contacts the support Cu versus the Stainless Steel.
Reaction Kinetics
Reaction kinetics deals with how fast a reaction occurs and the products that will be produced. Reaction rates are generally exponential functions of temperature and expressed in a formulation commonly known a s the Arrhenius law
than the rate that heat leaves due to transport processes and the Pt wire will not glow. Because reaction rates increase with increasing temperature, the reaction will stop in this case. A cold Pt wire causes no reactions to occur because the initial reactions are too slow for the overall process to be self-sustaining. This is why the wire is heated first before being inserted into the flask.
-E
rate = A exp ($
where E is the activation energy of the process, A is the preexponential factor, R is the gas constant, and T is the temperature in kelvins. Energetic factors controlling the reaction, such as the energy required to break the bonds in the reactants. are contained in the activation enerw. A general rule df thumb is that the activation energy& a reaction is orooortional to the reaction enthalov. Based on the values In the table, eq 3 should be faster %an eq 2 a t the same temperature if the preexponential factors are equivalent. The ratio of reaction rates for eq 3 to eq 2 decreases with increasing temoerature. Steric factors controlling a reaction, such as ihe efficiency of a molecular collision for given molecular orientations, affect the mamitude of the Factor A in eq 4. ~ncreasing'sterichindrances decrease the magnitude of A. This may lead to situations where eq 2 is actually faster than eq 3 at low temperatures and eq 3 dominates a t high temperatures. The situation is furthir complicated because the reaction is a heterogeneous catalytic reaction. Indeed a function of a catalyst is to lower the activation energy barrier for a reaction and change the reaction pathway to remove steric hindrances. Steric considerations on a surface involve such things as the dissociation of a molecule when neighboring surface sites are occuoied. The reaction rate controls the rate offormation of heat in the reaction. I f reaction o~CHROH to oroduce CH,O or CO, and HzO goes too slowly, heatkill be produced leis rapidl;
General
Trial and error is required to obtain a satisfactory time for the onset of oscillations and the oeriod of oscillations. They appear to be controlled predo&inantly by an interplay between the size of the Pt wire (the larger the wire, the shorter the oscillation period) and the distance between the F't and the methanol (the close~thedistance, the shorter the oscillation period). In some cases the oscillations appeared to start sooner when a clean flask was used. The reaction flask can also sometimes be "primed to shorten the onset of oscillations by running for a short time prior to the demonstration, removing the Pt wire just before the demonstration, and starting immediately from the beginning. The initial explosion generally does not occur in this case. Anominal time of about 5-10 min after the first explosion before oscillations start can be expected, and an oscillation period of about one explosion per minute can be set up thereafter with some practice. Acknowledgment The author acknowledgesR. Imbihil a t the kitz-HaberInstitut in Berlin, Germany, who demonstrated this reaction during a lecture on surface science aspects of oscillating catalytic reactions. Literature Cited 1. Reid, R.C.;Praumitt,J. M.; PaLng,B. E. ~ P ~ o p e r t & s o f ~ e s & L i q u &hod.; iis, McGraw-Hill Book Company: New York, 1987.
Volume 71 Number4 April 1994
327
