Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

146
VIII Group 17 (halogen) compounds and complexes
are tabulated in Appendix A. There was an anomaly noted starting around 150 K and
ending at 162 K depending on the thermal history of the sample, the heat-capacity data
showing a small inflexion in this temperature range. The cause of this anomaly has not
been investigated. A phase transition occurs around 218 K as shown in Figure VIII-6.
This is ascribed due to ordering of the hydrogen position. The total entropy change of
the transition was estimated to be 4.6 J·K –1·mol –1 , whereas the exact calculation resulted
in 3.13 J·K –1·mol –1 .
The value of the heat capacity selected by this review is:
ο
C p,m (SnCl 2·2H 2 O, cr, 298.15 K) = (166.4 ± 0.5) J·K –1·mol –1 .
Figure VIII-6: Heat-capacity data of [1974MAT/OGU] showing an anomaly around
150 K and a Phase transition at 218 K.
/ J·K–1·mol
–1
200
150
100
∫
0
∫
∫
∫
∫
∫
ln 11.7
C o d ln(T/K) = 1.283
p, m
d ln (T/K) = 113.828
d ln (T/K) = 8.277
d ln (T/K) = 35.682
d ln (T/K) = 8.926
d ln (T/K) = 44.273
ln 149
ln 11.7 Co p, m
ln 160
ln 149 Co p, m
ln 211
ln 160 Co p, m
ln 223
ln 211 Co p, m
ln 298
ln 223 Co p, m
–1
S° m
(SnCl 2·2H 2
O, cr, 298.15) = 212.27 J·K
–1·mol
C o p, m
50
exp. data [1974MAT/OGU]
extrapolated, assuming C o (12K) = a·T3
p, m
0
2 3 4 5 6
ln (T / K)
Due to the complex thermal behaviour of the compound, a calculation of the
entropy is not straight forward and implies uncertainties. Using the data from Matsuo et
al. [1974MAT/OGU] the standard entropy
ο
–1
S m (SnCl 2·2H 2 O, cr, 298.15 K) = (212.27 ± 1.50) J·K
–1·mol
can be determined by plotting Cp
( T ) against ln(T/K), see Figure VIII-6. This value is
selected. Thus, hydration reaction would be accompanied by
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012