K. F. O7Drircoll
Villanova University Villanova, Pennsylvania
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Demonstration of PV Work
by Balloon Inflation
Two recent interesting concerning the rheology and thermodynamics of inflating balloons have prompted us to set up an experiment for undergraduate physical chemistry students which, in the words of one of the students, "Made more clear the real, hut sometimes vague and abstract concept of pressure-volume work." The apparatus and the procedure for its use are essentially the same as that described by Condon and Fryd with the exceptiou that the outlet for displaced water is either even with or above the top of the water in the aspirator bottle. As a result of this the expansion of the balloon can be made to do a net work which can be calculated (neglecting friction) as the product of the mass of water displaced times the net head. The balloon used was of a heavy gum rubber (Orsat bulb) so that its stretching was reversible up to a t least 400 ml of volume. The gross work done under any net head of water can be determined by graphical integration of a plot of pressure versus volume. Since the stretching of the balloon was reversible the work
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done a t zero net head represents the work to stretch the balloon. If this work is subtracted from the gross work a t any particular head it will yield the net work a t that head, which may be compared with the net work calculated from the obvious equation: wnat = upgh. Typical student data for such a comparison yield a variance of 1-5%. There are some obvious ramifications of this experiment. One is a demonstration that the expansion of the halloon is reversible; this can be shown by working with the water outlet below the water level in the graduated cylinder. After expanding the balloon to a give11 volume the. water may be returned to the aspirator bottle by venting the halloon to the atmosphere in spurts. Comparison of different methods of graphical integration may also be obtained by having the students integrate by ~veighingthe cut-out curves or by summing geometric areas which approximate the curves.
' STFIN,R. S., J. CHEM.EDUC.,35, 203 (1958). a
CONDON, F. E., AND FRYD,M., Idem, 517(1958).
