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

260
A Discussion of selected references
“Sn(OH) 2 (s)” Sn(OH) 2 (aq); log10 K s,2
≈ − (4.87 ± 0.10) (A.4)
“Sn(OH) 2 (s)” + OH – Sn(OH) − 3 ; log10 K s,3
≈ − (0.34 ± 0.18).
The compound “Sn(OH) 2 (s)” was precipitated from SnCl 2 solutions with
NaOH. The equilibrium constant, K s,2
, of Reaction (A.4) was determined by
equilibrating precipitated “Sn(OH) 2 (s)” with H 2 O. The equilibrium concentration of
Sn(II) was determined by iodometric titration. No attempt was made to characterise
“Sn(OH) 2 (s)” stoichiometrically and/or structurally. Consequently it is not quite
justified to ascribe the value log10 K s,2
((A.5), 298.15 K) = − 4.87 to hexatin
tetrahydroxide tetroxide, Sn 6 (OH) 4 O 4 , as in Reaction (A.5), but lacking any
thermodynamic information on the latter phase it is suggested we do so.
1 6
Sn 6 (OH) 4 O 4 (cr) + 2 3 H 2O(l) Sn(OH) 2 (aq) (A.5)
Garret and Heiks [1941GAR/HEI] reported log10 K s,2
((A.6), 298.15 K) =
− (5.30 ± 0.13) for Reaction (A.6) which differs by 0.43 log 10 units from
log K ((A.5), 298.15 K) although it was obtained by a similar method.
10 s,2
SnO(cr) + H 2 O(l) Sn(OH) 2 (aq)
(A.6)
It is again not quite justified to ascribe this difference to Reaction (A.7), but
ο
Δ rGm
((A.7), 298.15 K) = − (2.45 ± 1.05) kJ·mol –1 is at least qualitatively in line with
the observation that white “Sn(OH) 2 (s)” transforms spontaneously into blue-black
SnO(cr)
1 6
Sn 6 (OH) 4 O 4 (cr) SnO(cr) + 1 3 H 2O(l). (A.7)
For Reaction (A.8) or (A.9)
“Sn(OH) 2 (s)” + OH – Sn(OH) −
3
1 6
Sn 6 (OH) 4 O 4 (cr) + OH – + 2 3 H 2O(l) Sn(OH) −
3
(A.8)
(A.9)
leading to * K s,3
the sum of the stoichiometric coefficients of the participating ionic
species vanishes, Σ ν B = 0 , see Eq. (II.38).
Thus * K s,3
does not need to be converted when the composition scale is
changed from molarity to molality. Moreover, Δz 2 = 0 for Reactions (A.8) and (A.9) and
thus the Debye-Hückel term vanishes and likewise in the course of ionic strength
*
corrections. In Figure A-3 log10 K s,3
(A.9) is plotted vs. m +
Na
. The range of sodium
ion concentrations covered by the experiments is too narrow to allow a reliable
estimation of Δε.
* ο
The mean value of log10 K s,3
(A.9) is (0.50 ± 0.18) log 10 units higher than
* ο
log10 K s,3
evaluated from [1941GAR/HEI] which refers to SnO(cr). This leads to
ο
Δ rGm
((A.7), 298.15 K) = − (2.85 ± 1.03) kJ·mol –1 which agrees surprisingly with the
value (− (2.45 ± 0.91) kJ·mol –1 ) derived from comparison of log10 K s,2
values for
Reactions (A.5) and (A.6), although in the experiments of [1906GOL/ECK] an
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012