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

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VII Tin oxygen and hydrogen compounds and complexes
Tin(II) oxide decomposes at ambient pressure at temperatures above 500 K
into metallic tin and an intermediate tin oxide either according to disproportionation
Reactions (VII.11) and (VII.12) [1947SPA/KOH]
4 SnO → Sn 3 O 4 + Sn (VII.11)
Sn 3 O 4 → 2 SnO 2 + Sn
(VII.12)
or according to disproportionation Reactions (VII.13) and (VII.14) [1973MUR/TRO]
3 SnO → Sn 2 O 3 + Sn (VII.13)
2 Sn 2 O 3 → 3 SnO 2 + Sn. (VII.14)
The intermediate reaction products could not be separated as pure phases due
to their nano-crystalline nature. As indicated by Reactions (VII.11) and (VII.13) the
stoichiometry of the intermediate phase is still under discussion. Sn 3 O 4 , Sn 2 O 3 , Sn 2 O 6 or
Sn 5 O 8 are considered as possible intermediates [2005GIE/POR], [2008SEK/TOG],
[2010TAN/SEK]. A first structural study by electron diffractometry yielded a triclinic
unit cell for a suggested stoichiometry Sn 3 O 4 [1967LAW]. From a recent precession
electron diffraction study [2010WHI/MOR] the complete crystal structure was derived
confirming the unit cell of [1967LAW] and the epitaxial relationships between the
various oxide structures of tin. From coulometric titrations in a high-temperature
galvanic cell Yang et al. [1994YAN/SUI] derived a temperature function for the Gibbs
energy of formation of Sn 3 O 4 , which was used subsequently in phase diagram modeling
of the system SnO-SnO 2 [2003CAH/DAV]. Although stoichiometry and structure of the
elusive intermediate tin oxides Sn 3 O 4 and Sn 2 O 3 have been studied and an attempt was
made to ascertain thermodynamic properties of Sn 3 O 4 , the latter do not qualify for
selection of standard values in this review.
SnO 2 occurs naturally as the mineral cassiterite and is the main ore of tin.
Cassiterite has the rutile-type structure [1964WYC] (tetragonal, space group
P4(2)/mmm, a = 4.7373 Å, c = 3.1864 Å, Z = 2, unit cell volume = 71.51 Å 3 ).
Cassiterite dissolves easily in alkali hydroxides; from these solutions brucite like
hydroxides such as K 2 Sn(OH) 6 [1976HON/ZUC] can be crystallised. Double oxides
may form by combination of K 2 O and SnO 2 (K 4 SnO 4 , K 2 Sn 3 O 7 and K 2 SnO 3 ). A mixed
valence oxide Sn 3 O 4 has also been reported [1984GRE/EAR]. The structure is triclinic
with space group P1(2) and V cell = 233.56 Å 3 .
VII.2.2
SnO(cr)
VII.2.2.1 Enthalpy of formation of SnO(cr)
Humphrey and O'Brien [1953HUM/OBR] determined the enthalpy of formation of
ο
SnO(cr) comparatively early. They obtained a value of Δ fHm(SnO, cr, 298.15 K) =
− (286.00 ± 1.34) kJ·mol –1 from the heat of combustion of tin.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012