Vapor Pressure Demonstrations Using a Butane Lighter R. Del Delumyea Millar Wilson Laboratory for Chemical Research, Jacksonville University, Jacksonville, Florida 32211
The concept of the change of state ofmatter and particularly the volatility of liauids is a n important concept taught in introductbry che&try eourses:~emonstrati&s or experiments involving these concepts presented in this ~ o u r n a l have ' ~ ~ involvedvariations ti the "classical" measurement of vapor pressure of a volatile liquid in a sealed container attached to a manometer. An exception is the experiment by Marzzacco and Speckha~d,~ wkch evaluated butane and Freon 114 as demonstration gases. The latter was preferred due to its ease of liquefaction and the fact that common butane is a mixture of gases. However, due to environmental concerns surrounding the use of CFCs and the wide use of butane liehters. an experiment usine these devices has been developed. The modem lighter is relatively safe, clean-burning, inexpensive, small, lightweight, and disposable. Although these lighters are commonly referred to as "butane lighters", the actual fuel is a mixture of two butane isomers and a small amount of a lower molecular weight hydrocarrv Thev are eases a t ambient bon. in a ~ r o ~ r i e t a blend. pre&ure and temperature. This mixtureofgases obeys the ideal gas law to a first approximation. Since liauids are appro~imately1,000 tin% more dense than gases, it would seem that the ideal fuel would be a liquid. However, since only gases bum, the liquid must be readily converted to the gas phase. Alternatively, a n even better fuel would he a gas converted to a liquid under a pressure greater than its vapor pressure at a given temperature. Both ethane (CzH6,bp: -88.6 'C) and propane (C3H8,bp: -42.1 'C) are gases a t room temperature. The pressure exh erted bv DroDane a t room temperature is a ~ ~ r o x i r n a t e10 atm, a i i h i i h e r yet for ethane. These pre&;res are higher than an inexpensive plastic lighter can safely hold. Pentane (C5H12,bP: 36.1 %), on the other hand, is a liquid at room temperature with a vapor pressure too low (0.67 atm) to support a flame in a gas lighter. Butanes (n-Cali,, bp: 0.5 'C; i-C4Hl0,bp: -12 'C) are gases a t room temperature, exerting a vapor pressure of 2.4 atm and 3.4 a h , respectively. In the vapor phase, the gas mixture used in commercial lighters behaves like an ideal gas. The higher the temperature of the container, the greater the pressure inside the lighter. The fuel is stored as a liquid under pressure. Reducing the pressure causes the liquid to "boil", releasing vapor that is burned in the flame of the lighter. At a vapor pressure above 1atm, gas will be forcibly expelled from a container thmugh the small orifice of the lighter. The rate of gas release depends on the size of the orifice and the difference in pressure between the ambient pressure and the pressure inside the container, which in turn depends on temperature. These facts form the bases for warnines not to itore butane lighters a t temperatures above 50 -C; an example would be inside a closed car on a hot dav. There are also warnings not to carry them on airlines because sudden depressurization could cause rupture. 'Richardson, W. S.; Jones, R. F. J. Chem. Educ. 1987, 64,968. 'Sears, J. A.; Grieve, C. J.; Mosher, M. J. Chem. Educ. 1990, 67, 477 .
3Woolf,A. A. J. Chem. Educ. 1990, 67,278. 4Marzzacco,C. J.; Speckhard, D. J. Chem. Educ. 1986, 63,436.
A schematic diagram of the experimentai apparatus to determine
mass loss and flame height.
Of more practical note, the utility of a butane lighter is in its flame. It has been observed that in particularly wld climates. butane liehters do not work unless carried inside a pocket: The second experiment verifies this observation bv measurine the height of the flame from a butane liehter a t various tcmperat&es. Flame height depends onYboth the release of fuel vanor and the complex kinetics of its combustion. This complexity results i n n linear relationship between the loearithm of flame heieht and the absolute temperature, rather than (11T). Procedures The experimental setup is shown in the figure. Using a rubber band to hold the components together, attach a lighter to a No. 8 stopper and a wood ruler. On the back, attach a thermometer that can measure temperatures to k1 'C. To the ruler, attach a smaller (plastic) ruler using another rubber band, and align the bottom of the scale on this mler with the top of the lighter orifice. These experiments should be conducted at four temperatures: in a refrigerator (typically 10 'C) at room temperature (approximately 25 'C) after heating to approximately 40 *C after cwling to approximately 30 'C For the warmest temperature, place the apparatus in a 1.000-mL beaker and cover with a laver of aluminum foil. Using a hot-air dryer, gently warm the container and its contents to approximately 40 T. .Caution: In accordance with the manufacturers' warning, do not heat aboue 50 'C,
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Maintain that temperature for 5 min to ensure the contents have equilibrated. To lower the temperature, reduce the heat from the air dryer by moving it farther from the beaker, and allow the temperature to stabilize around 30 'C. Optionally, either experiment may be tried aRer storage in a freezer to demonstrate the effect of low temperature on volatility. Experiment 1 Volatility as a Function of Temperature
Prepare a table with the following columns: Initial Weight (g) Final Weight (g) Absolute Temperature (K) Elapsed Time (min) Change in Weight (mg) Using wtton gloves, weigh the lighter on a top-loading balance, if available, to the nearest centigram. The experiment is to be conducted after equilibration a t a minimum of three temperatures. (The 30 'C temperature may be difficult to maintain.) Release the gas by depressing the lever on the lighter without striking the igniter for approximately 1 min. The time should be accurately measured using a stopwatch that is held in the other hand. Allow the lighter to reach room temperature, wipe off any condensation, and reweigh the lighter to determine the mass of fuel released. To determine the rate of release in mglmin, divide column 5 in the table by column 4. Prepare a plot of
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rate of release versus absolute temperature, and draw the best straight line through the data. Experiment 2 Temperature Dependence of Flame Height
Prepare a table with the following columns: Temperature ('C) Absolute Temperature (K) Inverse Temperature (1K) Flame Height (cm) Log (flame height) Place the apparatus in the refrigerator, in a beaker, or in the ambient air. and record the tem~erature.After eauilihration, move the apparatus to a relHtively calm air space, and proceed as rapidly as practical. With the plastic ruler a t the zero mark and the apparatus standing vertically, flick the lighter to ignite the flame, and hold the lever, controlling gas flow to its full-down position. Read the height of the flame a t the top of the yellow portion of the flame. Although wind currents will cause some flicker, the stable zone of the flame is usually quite apparent. The measurement should be performed three times at each temperature. Caution:In accordance with the manufacturer's directions, the temwrature o f the liehter should not exceed 50 'C. In the fin1expenmrnt, onlv n snrall ~ ' d u m of c ,Y released o f o trme ~ l r s sthnn 0 15 g nun,, co there 1.i Ilttle danger ofrxplosion or fire. 'Butane is a flammable gas. The release of its vapor should he done in a hood or in a large mom with no open flames.
