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tested demonstrations The Platinum-Catalyzed Decomposition of Methanol: A Deceptive Demonstration Danielle L. Coffing and Jay L. Wile Ball State University Muncie, IN 47306
The platinum-catalyzed gas-phase decomposition of methanol can be used for classroom demonstration in an exciting, interesting fashion. This decomposition occurs according to the following chemical equation: CH,OH (g) + CHZO(g) + Hz ( g )
(1)
The platinum catalyst, after being heated until it glows, can be made to continue glowing for hours by suspending it over the methanol. Done properly, this demonstration can appear to be almost magical, and its explanation is more complicated than one might think. Demonstration Procedure Fill a 400-mL beaker with approximately 150 mL of methanol. Lay a glass rod across the top of the beaker. Attach a platinum foil so that it hangs from the glass rod slightly above and parallel to the surface of the methanol (see the figure). ARer the foil has been adjusted so that it hangs closely above the surface of the methanol, remove the rod and platinum from the beaker. Heat the platinum foil in a flame until it is glowing a bright orange color. ARer the platinum is removed from the flame, it will quickly cease glowing. However, when the glass rod is placed on the beaker so that the platinum is again suspended over the methanol, the platinum will promptly resume glowing. In laboratory tests, the glow has been sustained for almost four hours. ARer that amount of time, the platinum foil was no longer positioned directly over the methanol, be-
GEORGE L. GILBERT Denisan Univesiw Granville. OH 43023
cause a significant quantity of the methanol had evaporated or decomposed into formaldehyde. This demonstration can be used over a longer period of time by preserving it with an expanded polystyrene cup. First, invert the cup so that it covers the top of the beaker; then cut notches in the cup for the ends of the glass rod to stick through. In this way, the cup will fit the top of the beaker tightly, and the evaporation of the methanol will be slowed down. However, a small hole should he leR in the cup to prevent an explosion due to high gas pressure. The glow will decrease soon after the expanded polystyrene cup is placed on top of the beaker. At the same time, vapor will form inside of the beaker. The better the seal between the cup and the beaker, the less the platinum will glow. If the seal is good enough, the glow will cease. After the cup is removed, the orange glow will resume a t the original intensity within a minute even if the cup has been leR covering the beaker for several hours. The inside of the beaker remains hot during the entire demonstration. In laboratory tests, the platinum glow has resumed after being "turned off' for more than 30 hours. Demonstration Explanation The first explanation that comes to mind to describe this demonstration is that the reaction given by eq 1must be very exothermic, heating the platinum foil enough to cause it to glow a bright orange color. However, the calculated value of AiT for this decomposition reaction is 85.27 kJ1mol (I).This value indicates that the reaction is endothermic, not exothermic. In addition, the AG" of the decomposition is 52.6 kJ/mol (1)which shows the reaction is not even spontaneous a t 298.15 K. Also, if the decompositionof the methyl alcohol was indeed causing the platinum to glow, then the platinum foil should glow even brighter when the expanded polystyrene cup was used to cover the beaker than when the beaker was open. An uncovered beaker would allow more methanol and formaldehyde to escape, lowering the surface level of the methanol. This would reduce the rate of methanol decomposition. However, the glow fades each time the expanded polystyrene cup is placed over the beaker. In addition, covering the beaker with the cup would increase the gas pressure. Higher pressure would cause the reaction to proceed at a faster rate, making the platinum glow more brightly. Because the glow fades, another reaction must be taking place. When the expanded polystyrene cup is covering the beaker, the platinum gradually stops glowing. This indicates that another gas is involved in this demonstration and is being depleted when the beaker is covered. As this gas is exhausted. the elow from the ~ l a t i n u mfades. Because the gas must be coging from the Lir surrounding the beaker, it is most likelv oxveen. Therefore. second reaction involved is probably
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2H2 ( g ) + 0 2 (g) + 2H2O (g)
The aemonstration setap. After ntially being heated, plat num w I glow rea hot wnen s~spendeoover metnanol. Erhano wore equaly well
(2)
equal to -241.82 kJ/mol (I)and AGO Equation 2 has a equal to -228.6 kJlmol ( I ) . The negative enthalpy means the reaction is very exothermic and explains the platinum glow. The negative value for the Gibbs Free Energy shows Volume 70 Number 7 July 1993
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that the reaction will proceed spontaneously when both the oxygen and hydrogen are present. In order to test whether or not this reaction is really responsible for the platinum's glow, the platinum foil was held over a stream of hydrogen gas in an oxygen atmosphere. The platinum glowed a bright orange, indicating an exothermic reaction. Thus, the reaction represented by eq 2 is really responsible for the platinum's glow, and the decomposition of methanol (eq 1)merely serves as a source of hydrogen. Some of the water vapor formed during the demonstration condenses onto the wall ofthe beaker and explains the vapor observed when the expanded polystyrene cup was used to contain the gases in the beaker. This demonstration works because these two reactions, both the formation of water vapor and the decompositionof methanol, are mutually dependent. One final question remains. Why must the platinum foil be heated in a flame before placing it over the methanol? According to the equation
the decomposition of methanol is spontaneous only above 778 K. eati in^ the platinum foil gives the methanol enough energy to decompose slightly into formaldehyde and hvdroeen. The hvdrGen oroduced is then used in the " formakonlof water vapor. Because the hydrogen is being depleted, the equilibrium between methanol, formaldehyde, and hydrogen is shiRed to the right, causing more hydrogen to be produced. The formation of water vapor (eq 2) is exothermic. The heat produced from this reaction is used to sustain the thermodvnamic favorabilitv of the decomposition of methanol. M e n the expanded polystyrene cup is covering the beaker, a small hole has to exist to orev e k the g a ~ ~from & beciming too pressurized. This hole allows a small amount of oxygen to enter the beaker. This oxygen keeps eq 2 occurring at a much slower rate than when the beaker is open. Because the rate is significantly slower. less heat is oroduced. and the olatinum cannot glow. When the exp'anded pdlystyrenecup is removed, more oxygen enters the beaker. The reaction rate of eq 2 then increases, producing more heat and causing the platinum to resume glowing. This demonstration is useful in a classroom setting for several reasons. First, it is more complicated than it appears initially, involving a reaction that is not immediately obvious and is, therefore, more challenging for students to understand. Second, the platinum illustrates the phenomenon of exothermic reactions in a distinctive and memorable wav. Because the ~latinumfoil has to be heated before the reactions will proceed, this demonstration also is a perfect example of the temperature dependence of AG in determining the spontaneity of a reaction. Finally, this demonstration can be used to ex~lainthe mutual interaction of two reactions. Because an explanation of this demonstration requires the use of many chemical concepts, it is an ideal activity for stimulating synthesis among students near the end of the course.
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Literature Cited 1. Lide, D. R, Ed. CRC H a n d b d of Chemiaoy and Physics: CRC h a s : Boea Reton. n,1 9 ~ ~ 9 9 1 .
586
Journal of Chemical Education
A Visual Illustration of Oxidation Numbers and Moles Using Balloons To Demonstrate Moles of Electrons Submitted by
Wilbur Bergquist BSCS, Innovative Science Education 830 N: Tejon, Suite 405 Colorado Springs, CO 80903 Checked by
James Niewahner Nothern Kentucky University Highland Heights, KY 41076 The connection between moles of electrons and oxidation number can be illustrated maohicallv " bv " usine ballwns to capture hydrogen gas during an oxidation-reduction reaction. The amount of H2produced as sodium, magnesium, and aluminum undergo oxidation is mole-dependent and can be collected quickly in a ballwn. The volume occupied by the collected gas is directly related to the moles of electrons transferred. In the discussion phase following the demonstration, write and balance the equations (eqs 1-31 for the reactions using 2 moles of each reactant and H+ for the hydrogen source. This seems to help students undestand that the observed differences in the size of each balloon is related to the amount of electrons transferred in the reaction. Also emphasize that the same number of moles of each metal was used so the relative volumes of the gas generated is the same as the ratio of the oxidation numbers of the metals.
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Materials three 250-mL volumetric flasks three identical balloons 150 mL water 300 mL 6M hydrochloric acid 0.23 g sodium 0.24 g magnesium 0.27 g aluminum
Procedure Put 150 mL of water in one flask and 150 mL of the acid in eachofthe other two flasks. Place themetals in separate balloons and flatten them. Be sure to label the balloons and put the metal in the c o m t balloon Slip the neck of each balloon onto the flask. Check that the sodium balloon is attached to the flask containing the water. Line the three flasks in a row. Start with the aluminum system and tilt the balloon up to allow the metal to enter the flask. Swirl as necessary to ensure that all of the metals react. As the balloons inflate, have the students predict which balloon will become the largest. Dlsposal The collected hydrogen gas can be released to the atmosphere. The acid solutions can be mixed with the sodium hydroxide solution to neutralize the base produced during the reaction. Any excess acid can be neutralized with additional base, and the neutralized solutions can be flushed with copious amounts of water.
