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

128
VII Tin oxygen and hydrogen compounds and complexes
techniques (see Table A-30). Seetharaman and Staffansson [1977SEE/STA] determined
the equation for the standard Gibbs energy of formation of SnO 2 (tetragonal) as:
ο 1373K
fGm 990
[ Δ ] K (SnO 2, tetragonal, T ) = − [575.066 − 0.207376 (T /K)] ± 0.920 kJ·mol –1 .
These data were shown to be closely consistent with a number of other studies.
For the calculation of the standard state quantities, these data cannot be used but it may
be possible to check the consistency of the finally selected values at 298 K with a heatcapacity
equation with the several data based on the cell-potential measurements (e.g.
Petot-Ervos et al. [1975PET/FAR]). Gorbachev and Nekrasov [1976GOR/NEK] made
an assessment of the Gibbs energy of formation of SnO 2 (tetragonal) in the system with
Cu-Fe-Sn-S-H 2 O. These are phase equilibrium experiments and cannot be used here
except again for checking the consistency with our finally selected data. As an example,
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Δ G (SnO 2 , tetragonal, 673 K) = − (438.19 ± 0.20) kJ·mol –1 .
f
m
The values recommended by Lamoreaux and Hildenbrand [1987LAM/HIL] for
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Δ fHm(SnO 2 , tetragonal, 298.15 K) and S m (SnO 2 , tetragonal, 298.15 K) are
− (577.574 ± 0.166) kJ·mol –1 and (49.011 ± 0.081) J·K –1·mol –1 respectively.
Mallika et al. [2001MAL/EDW] showed that SnO(cr) disproportionated into
Sn(l) and SnO 2 (cr) at 800 K at a controlled rate of heating. They measured the potential
of a galvanic cell with SnO/SnO 2 as the test electrode and ( p ≈ 10 –3 O atm)|Pt or
2
Fe|Fe x O as the reference electrodes using x YO = 0.15 of YSZ (yttria stabilised
1.5
zirconia) as the electrolyte over a temperature range from 772 to 1206 K. The data
yielded the following expression for the Gibbs energy of formation of SnO 2 as:
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Δ (SnO 2 , tetragonal, T ) = − (568.9 − 0.2007 T/K) kJ·mol –1 fGm
. To calculate the
enthalpy of formation, Mallika et al. performed a third-law analysis of the 60 data
points making use of the free energy functions for SnO 2 and the elements compiled by
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Lamoreaux et al. [1987LAM/HIL]. Their calculated value is Δ fHm(SnO 2 , tetragonal,
298.15 K) = − (578.3 ± 4.0) kJ·mol –1 .
VII.2.3.2
Heat capacity of SnO 2 (cr)
There are a number of studies on the heat capacity of SnO 2 (cr), cassiterite, with the
latest study of Gurevich et al. [2004GUR/GAV2] and [2004GUR/GAV3]. Zhogin et al.
[1980ZHO/KOS] measured the heat capacity of crystalline tin dioxide in a vacuum
adiabatic calorimeter. Thermodynamic functions ( HT − H0K)
/ T , ( S ,
T
− S0K )
−( GT
−G0K )/
T were calculated based on heat capacity of SnO 2 (cass) measured at 58
points in the temperature range 10 to 300 K. No anomalies on the heat-capacity curve
were observed. Deviations of experimental points from smooth curve are: 20% in the 10
to 17 K range, less then 10% in the 17 to 30 K range, less then 3% in the 30 to 100 K
range, less than 0.3% in the 100 to 200 K range and less then 0.2% in the 200 to 300 K
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range. Equation C (SnO 2 , cass, T )/J·K –1·mol –1 = 2.51 × 10 –5 × (T/K) 3 p,m
was used to
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extrapolate C p,m (SnO 2 , cass, T ) to 0 K. The data of this study are given in Appendix A
(Table A-70).
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012