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Entropy education 9.pdf (646.26 kB)
Explaining the entropy concept and entropy components
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Total entropy of a
thermodynamic system consists of two components: thermal entropy due to energy,
and residual entropy due to molecular orientation. In this article, a three-step
method for explaining entropy is suggested. Step one is to use a classical
method to introduce thermal entropy STM
as a function of temperature T and
heat capacity at constant pressure Cp:
STM = ∫(Cp/T) dT. Thermal entropy is the entropy due to
uncertainty in motion of molecules and vanishes at absolute zero (zero-point
energy state). It is also the measure of useless thermal energy that cannot be
converted into useful work. The next step is to introduce residual entropy S0 as a function of the
number of molecules N and the number
of distinct orientations available to them in a crystal m: S0 = N kB
ln m, where kB is the
Boltzmann constant. Residual entropy quantifies the uncertainty in molecular
orientation. Residual entropy, unlike thermal entropy, is independent of
temperature and remains present at absolute zero. The third step is to show
that thermal entropy and residual entropy add up to the total entropy of a
thermodynamic system S: S = S0 + STM. This
method of explanation should result in a better comprehension of residual
entropy and thermal entropy, as well as of their similarities and differences.
The new method was tested in teaching at Faculty of Chemistry University of
Belgrade, Serbia. The results of the test show that the new method has a
potential to improve the quality of teaching.