Improvements to Dernonstratlon of Vapor Pressure
To the Editor: I would like to sueeest some imnrovements t o the recentlv reported "~emonst%ion of vapor Pressure" .1987,64,%8j. First, in place of the 500-mL filter flask, I use a Kieldahl bulb (volumekt)out 50 mL), thus eliminating the onelbole rubher stopper (it is difficult to maintain a seal between this stopper and the flask during the demonstration); the rest of the-setup remains the same. Rather than introducing a large amount (50mL) of liouid. I draw about 1mL of ethvl acetate into a dropper (initiaily, some of the ethyl acetateail1 drop out: I discuss how this is related to the establishine of the equilibrium vapor pressure above the liquid in the dropper). With the hose clamp open, the dropper is inserted into the rubber tubing, the i m i of ethyl acetate is added, and the hose clamp is closed to prevent leakage around the dropper as the pressure builds. When the pressure has stabilized, I show the students, a t least those in the front rows, that some liouid ethvl acetate remains: calculations usine.. P V = nRT ~~rebeal tha; only about 0.014 of ethyl acetate evaporates to oroduce a oressure of 60 mm He - .lit is interestine to ask first ior their e h c a t e d guesses). Then I ask what thev think will happen if I do the exneriment with twice as m&h ethyl acetiti. Many students will predict a doubling of the pressure. The experiment is repeated in a different Kjeldahl bulb with about 2 mL of ethyl acetate. When the students see that the pressure is the same, I review the concepts of equilibrium vapor pressure of a liquid and the boiling point of a liquid (if increasing the amount of liquid increased the vapor pressure, then the water in a lake should be boiling).
g
AE = f hv7 To the Editor: Leo [1988,65,11]has unnecessarily complicated the AE = hv relation, which simply means that the magnitudes of the two energies in emission or absorption are balanced. By making them vector quantities he gets into deeper waters because the relation will then depend on the choice of sign convention and the position of the system boundary. The accepted thermodvnamic convention.. as onnosed to the old the;mochemical one, is to assign a negative sign to changes leading to a more stable state of a system. An exothermic, or exoergonic, change is considered negative on this convention. Leo used two adjacent systems, one containing an assembly of atoms or molecules, the other a radiation source or sink. Hence an emission of radiation, moving the assembly from an excited state to a more stable ground or intermediate level, should be considered negative; the surroundine svstem. which becomes excited bv whatever means on receiiing radiation, suffers a positive c6ange. A.
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-E,, = E,
For an absorption process the signs are reversed.
In either process the sum of energy changes is zero and energy is conserved in accordance with the first law. If the assembly and radiation were placed within a single system, as they would be in an experimental set-up, the system as a whole would be thermoneutral and AE and hv would also be of opposite, but indeterminate, sign. A. A. Woolf
Gordon F. Hambb
Bristol Polytechnic Bristol, UK
To the Editor
To the Editor
The incorporation of a calculation usine the ideal gas law as suggested by Hambly is a welcomed i&provementto my method for demonstratinn vapor ~. pressure. I would like to add, however, that somecdlege chemistry or science departments and most high school departments do not have access to a Kieldahl bulb. Since I have never had a nrohlem maintaining a seal between the rubber stopperLand the flask durine the demonstration. I would not be reluctant to recommend the use of a 5 0 - m ~ Erlenmeyer flask in place of the Kieldahl bulb. As Hamblv notes. however. usine 1 mL of a vdlatile solvent in a ~~~~~~~~~ml'containeoffer~ only those students in the first several rows an opportunity to observe the liquid. A larger, 600-mL flask with approximately 50 mL of liquid may be seen in a large classroom or lecture hall. For thatreason,I would suggestthe use of such a volume in all cases except a demonstration before a very small group. The lareer volume doesnot detract from a calculation to estimate the-amount of liquid that evaporates to produce the vapor pressure. Indeed, it may be more interesting to ask for an educated guess of the amount when a larger volume of liquid is present.
I agree with A. A. Woolfs assertion that if the boundary of a system is defined to encompass both an assembly of atoms and a radiation sink or source, the system as a whole would be thermoneutral upon photon emission or absorption. However, defining the system in this way does not resolve the problem addressed in the article. If convention is followed, a negative value is obtained for the AE of an atom or molecule unon . .hoto on emission. Since h and v are both positive, clearly, a negative sign has to be incorporated into the relation AE = hu. The problem then becomes justifying the negative sign using convention, as was done in the article. Granted, it is unusual to have the boundary of a system enclose only one atom or molecule, because a single particle cannot be treated thermodynamically. It is necessary to define the boundarv this wav. .. however. because AE in the equation refers to the energy change experienced by a single atom or molecule. not an assemblv of atoms or molecules and radiation as sugg&ed by ~ o o l f . . In retrosnect. I should have recommended the use of an absolute value $ign with AE to yield a single equation for emission and absorption, IAEI = hv. Even so, this would he meaningless to students unless the problem and the solution were first made clear.
W. S. Richardson DeoarmKIntof Physical Sciences sciooi 01sciences A u b ~ r ndiversity at Montgomery Monlgomery. A. 36193
278
Journal of Chemical Education
Howard R. Leo Onondaga Community College Syracuse. NY 13215
