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The elegant experiment "Rubber Elasticity: A Simple Method for Measurement of Thermodynamic Properties" by John P. Byrne (J. Chem. Educ. 1994, 71, 531) can be made even simpler if one uses an inexpensive toploading electronic balance to measure the tension force of the rubber band, instead of the triple-beam balance Byrne employed, and an old coat hanger to fashion a wire harness and
hook to stretch the rubber band. I have repeated Byrne's experiment using a 200-g capacity (±0.01 g accuracy) electronic balance (Acculab V-200) supported on a ring stand and
ring, above a water bath consisting of a 0.5-L tall-form beaker
of water heated by a 550-W stirrer-hotplate. A schematic
of the apparatus is shown below.
The rubber band, prepared as described by Byrne,
was stretched between the wire harness supported by the
pan of the balance and an adjustable hook held in position by
a three-finger clamp; since electronic balances use
magnetic forces during weighing to maintain the pan at a fixed
null position, the rubber band is held at a constant length
during the experiment. The values of the thermodynamic
functions, fu, the contribution of changes in internal energy
with length to the restoring force, and fs, the entropic
contribution to the restoring force, that I obtained are
consistent with those of Byrnes:
(dU/dl)T = fu = 0.40 ± 0.02 N
and
(dS/dl)T = fs = 0.78 ± 0.02 N
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