214
Thermochemistry
(
Cell \
1 -—;) (25°C - T°C)
& ^ '
(
cal \
31 —) (25°C - 7°C)
Total calories lost = (231)(25.0 - T)
Going on the principle that
total calories needed = total calories lost
we have
2870 + 36.07 = (231)(25.0 - T)
267 T = 2905
T = 10.9°C, the resultant temperature
ENTROPY OF TRANSITION
The enthalpy of transition, divided by the absolute temperature at which it
occurs, is a common measure of the change in molecular order that occurs
during the transition. We refer to this change in molecular order as the "change
in entropy" (A5 T ), or the "entropy of transition." If A5 T is positive, the change
results in an increase in molecular disorder. Changes in state offer some of the
simplest examples from which one can obtain a feeling for the relationship
between changes in entropy and changes in molecular order. Crystals have a
very high degree of order; in them, the movement of atoms, ions, or molecules
is restricted primarily to vibration about their locations in the crystalline lattice.
When crystals melt, the component atoms, ions, or molecules can move fairly
independently of each other in the liquid, slowly changing their neighbors by
diffusion; the molecular order represented by the lattice disappears. When
liquids vaporize, the component atoms or molecules, now in the gaseous phase,
move about independently in a chaotic, random manner. Each stage, melting
and vaporization, represents an increase in molecular chaos, and is described in
terms of an increase in entropy.
Just as A//T represents the difference in enthalpies between "state 2" and
"state 1," so does A5 T represent the difference between the entropies in "state
2" and "state 1":
A5 T = S 2 ~ Si
cal
A5x ~ T mole °C
Thermochemistry
(
Cell \
1 -—;) (25°C - T°C)
& ^ '
(
cal \
31 —) (25°C - 7°C)
Total calories lost = (231)(25.0 - T)
Going on the principle that
total calories needed = total calories lost
we have
2870 + 36.07 = (231)(25.0 - T)
267 T = 2905
T = 10.9°C, the resultant temperature
ENTROPY OF TRANSITION
The enthalpy of transition, divided by the absolute temperature at which it
occurs, is a common measure of the change in molecular order that occurs
during the transition. We refer to this change in molecular order as the "change
in entropy" (A5 T ), or the "entropy of transition." If A5 T is positive, the change
results in an increase in molecular disorder. Changes in state offer some of the
simplest examples from which one can obtain a feeling for the relationship
between changes in entropy and changes in molecular order. Crystals have a
very high degree of order; in them, the movement of atoms, ions, or molecules
is restricted primarily to vibration about their locations in the crystalline lattice.
When crystals melt, the component atoms, ions, or molecules can move fairly
independently of each other in the liquid, slowly changing their neighbors by
diffusion; the molecular order represented by the lattice disappears. When
liquids vaporize, the component atoms or molecules, now in the gaseous phase,
move about independently in a chaotic, random manner. Each stage, melting
and vaporization, represents an increase in molecular chaos, and is described in
terms of an increase in entropy.
Just as A//T represents the difference in enthalpies between "state 2" and
"state 1," so does A5 T represent the difference between the entropies in "state
2" and "state 1":
A5 T = S 2 ~ Si
cal
A5x ~ T mole °C