overhead projector demonstrations
edited by
DOREKOLB Bradlsy University Peoria, IL61625
There must be enough water in the dish to allow the surface to be swept free of dust and other surface contarnination before the monolayer is formed. The solution to be spread is made by first dissolving 1.42 gstenric acid in 1 L Sally ~olornon'and Chinhyu Hur of petrolcurnethef then dilutina by 10, thus makinea mnDrexel University centration of 0.142 g/L. ~etroleumethkris chosen because Philadelphia, PA 19104 it evaporates completely leaving no surface-active contaminant. A Pasteur pipet is calibrated by adding enough Avogadro's number is conveniently approximated by dedrops of petroleum ether to reach the 2-mL mark on a 10termining the number of molecules in a monomolecular migradbated cylinder. Peform the calibration quickly to layer of stearic acid. This experiment, included in many diminish errors due to evaporation. Pasteur pipets may degeneral chrrnistry laboratory manuals ( 1 , 21, is adapted liver anywhere from 75 to 95 dropdmL, depending upon readily for the overhead projector. The dernon.itrution cun the anele of deliverv. If the oioet is held verticallv it delivhe oerformrd at the ooint in lecture iust before the ddiniers frLm 92 to 95 &opdmi. i f the pipet is heli a t a 45' tion of the mole and the discussion of Avogadro's number. anele the number of drops delivered covers a broader Students may be introduced to the demonstration by being 75-85. However, it may be easier to deliver the range, told that chemists need to know the number of molecules d r o ~ durinz s the demonstration while holdine the oioet a t (or units) oresent in one a a m formula weieht of a given an angle. he demonstrator must calibrate the pipet using kbstance:~here is an eaiy experimental method to find the same angle of delivery that will be used during the the number of molecules in a eiven mass of stearic acid. a overhead projector demonstration. ~ 0 acid ~ .is itcompound with the formula & ~ ~ Stearic Just before one makes the monolayer, the surface of the tracted to a water surface in such a wav that a sinele laver of molecules forms. The cross-section& area o c ~ ; ~ i e d " b ~ evaporating dish is swept with a metal or plastic straight edge. The small amount of water that is removed can be each stearic acid molecule is known to be 0.21 nm2from the absorbed by a Kimwipe placed near one side of the transFrom the area results of surface area measurements (3,4). parency. The sweeping should be done a t least three or four of the entire monolayer and of one stearic acid molecule, times or until no visible oarticles remain on the oroiected the number of stear& acid molecules that will fit in the image of the water surface. Add the stearic acid soiution area can be calculated. By finding out how many grams of one drop a t a time, pausing for a few seconds aRer each stearic acid are needed to form the monolayer, the number drop. Practice delivering drops using the same angle that of gram formula weights of stearic acid in the monolayer is vou used in calibrating the Pasteur oioet. The fust few found. From these two values Avogadro's number is calcudrops disappear instantaneously. As the monolayer aplated. oroaches the close-oacked state the droos soread a little Procedure more slowly. The fast drop that is needed to produce a close-packed monolayer shatters as it spreads. This is the Be sure that the overhead oroiector is leveled and that number of drops that should be used in the calculation of the stage is not too hot. It is important to practice this demthe Avogadro number and probably will be about 12 drops onstration under conditions identical to those that you plan to use when perfbrming it in lront of students. ~ ~ i t r i in a 14-cm dish. The very next drop persists on the water surface like a shrinking lens that &sippears within about dish bottom is positioncd on the overhead to allow enuugh 20 seconds. room for calcdations. Directions and expected results are You will find it easier to handle a 10-cm Petri bottom, given below for a 14-cm or 10-cm dish. The clean dish is which has roughly half the area. This smaller dish is easier filled with water (distilled or tap water will do) until it to remove from the overhead and less likely to create a wanearly overflows. tery mess on the stage. However, it has the disadvantage of allowing only about 5 drops to spread. If you use this 'Author to whom correspondence should be addressed. ZPetroleumether is a mixture of low boiling alkanes, mostly pensmaller sized dish, you will sometimes bypass the shattanes and hexanes. tered last drop. If this happens the last drop will become
Measuring Avogadro's Number on the Overhead Projector
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the lens drop instead, and is the one that should be used for the calculation of Avogadro's number. Discussion The number of stearic acid molecules that fit in a closepacked monolayer of known diameter is found readily using the area oecupied by a single molecule. This part of the calculation can be done before or after the demonstration. For a dish with a diameter of 14 cm: 1molecule = 7.3 x 10" moledes 3.14 x 49 cm2x 21 x 10-'%mz To find the number of gram formula weights of stearic acid the number of drops of solution needed to form the monolayer on the 14-cm dish is combined with the concentration of stearic acid. The results of 10 trials with a pipet that delivered 80 drops of solution (when held at an angle) is shown below. The average number of drops required to form the close-packed monolayer was 12 (the 13th was the one that produced the lens) . 1mL 0.142g 1gram formula weight 12 drops -x 80 drops 1000 mL 284 g 7.5 x lo4 gram formula weight Avogadro's number = 1°16- 9.7 x 10" moleeuledgram formula weight 7.5 x 10-~ -
Remind students that this is a remarkably good result for such a crude technique. Literature Clted 1. Milio, F. R.: Debye, N. W 0.;Me*, Jouanodch, 1989;p 67.
C. Erperimpnts in ckemisoy;Narmvt Brace
2. Roberts, J. L.,Jr.: Hollenbeg, J. I.:Pashna, J. M.k m l Chemiafry in thehboralory, 3rd ed.: Freeman, NY, 1991. p 115. 3. Smah, H.11,Hurley, R. 6.J. Phya Colloid Ckem.1949,53,p 1409. 4. Adamson,A.Physlml Ckemiaoy ofSurfmces; Wiley: New Yo*, 1976: Chapte.3.
Polarity, Miscibility, and Surface Tension of Liquids Todd P. Silverstein Willamene University Salem, OR 97301 To explain physical characteristics of liquids such as surface tension, capillary action, meniscus curvature, viscosity, and miscibility, one needs to understand the intermolecular interactions underlying cohesive and adhesive forces. A very simple overhead projector demonstration using water and ethanol gives a dramatic visual illustration of these various concepts. Materials distilled, deionized water 100%ethanol 2 droppers or Papteur pipets 2 square glass plates, about 3 in. x 3 in. Procedure With a marker label one plate "water" and the other "ethanol", and place them on the overhead projector. Place severa1 dropq of water and ethanol on the respective plates. The water forms a tight, stable drop that stands fairly high above the plate. The ethanol forms a drop that rapidly thins and spreads out across the plate's surface.
Now add just a couple of drops of ethanol on one side of the stable water drop. The water drop actually moves away from the ethanol when their interfaces meet! Finally, add ethanol all around the perimeter of the water drop. You will observe dramatic events at the mixing interface, resulting in the thinning and expansion of the water drop. Discussion The explanation for the physical basis of these observations besins with the polarities of water. ethanol. and the glass (sikate) surfacerwater (H-0-H), has two pblar 0-H bonds. Ethanol (CHxCHrO-H) has only one O-H bond: thus, it id significantly less polar than water. (Dipole mo: ments, in debyos at 25 'C, are 1.69 for ethanol and 1.87 for ~
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The silicate glass is quite polar, with many Si=O bonds protruding from the surface. Hence, the adhesive forces between either liquid and the glass surface are fairly strong. The cohesive forces holding the pure liquid molecules together are stronger for water, since it is more polar than ethanol and has a higher hydrogen-bonding capacity. Thus, it turns out that for HzO, whesive > adhesive forces, and the drop remains tight and stable. For ethanol, adhesive > cohesive forces, and the drop thins, spreading across the glass surface. When Hz0 molecules first encounter and mix with ethanol molecules, the strong whesive forces (high surface tension) in the water maintain cohesiveness in the water drop, moving the entire drop across the glass surface, away from the ethanol. Hydrogen bonding is significantly stronger within the water drop than within the ethanol drop, so water molecules are more tightly held within the water drop. Water's high surface tension creates a kinetic barrier for free mixing of the two drops. Well before the drops have a chance to mix substantially, the water drop moves across the plate, away from the ethanol drop. It is "propelled" by the disparity between the strong waterlglass adhesive forces and the weaker ethanollwater cohesive forces. A related experiment was reported recently in Science.' The authors demanstrated that by manipulating adhesive forces, a water droplet could actually be induced to travel uphill. against the force of mavitv. S~ecificallv.a water drop &a~-~laced on an inclined wafer (he surface of which was subsequently made hydrophobic on the lower side of the drop. The hydrophobicity of the lower surface and the hydrophilicity of the upper surface caused the water to move up the slope, defying gravity! Finally, in the demonstration, by ringing the water drop with ethanol, sufficient mixing occurs to decrease cohesive forces, while adhesive forces remain roughly constant. When adhesive > cohesive forces, the drop thins and spreads across the glass surface. This demonstration could be followed with a discussion of "legs" in a wine glass. Here the ethanollwater mixture has diminished cohesive forces. adhesive > cohesive forces. and the water actually climbs'up the inner surface of the class. As surface area increases, ethanol vaporizes (hieher Gapor pressure and lower than water), cohesive forces increase, and the liquid drops back into the glass, only to start the process over again. 'Chauduty, M. K.; Whitesides, G. M. Science 1992, 256, 15391541.
Volume 70 Number 3 March 1993
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