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

VI.2 Sn 2+ 95
As only three data pairs ( E ο , T ) over a temperature range of 20 K are available
the standard enthapy of formation, thus obtained, seems rather doubtful.
Vasil’ev and coworkers attempted to determine Δ fHm(Sn 2+ , 298.15 K)
calorimetrically. In a first step the enthalpy of the following reaction
β-Sn + 2 H 2 O 2 (sln, ∞ H 2 O) + 4 HCl(sln, 26.2 H 2 O) → SnCl 4 (sln) + 4 H 2 O(l)
was investigated by [1973VAS/KOK] using a calorimeter, with an isothermal jacket,
and with automatic recording of the temperature/time curves [1966VAS/LOB]. The
reaction vessel, stirrer-ampoule holder, and casing of the calibrating heater were made
of tantalum. It was demonstrated that this precision calorimeter works fine
[1966VAS/LOB2]. So this calorimeter was used in the following steps too.
The heats of dissolution of SnCl 2 (cr) and SnCl 2·2H 2 O(cr) in HCl solutions
containing H 2 O 2 have been measured at 25 °C according to Reactions (VI.14) and
(VI.15) [1973VAS/VAS2],
SnCl 2 (cr) + H 2 O 2 (sln, ∞H 2 O) + 2 HCl(sln, 26.3H 2 O) → SnCl 4 (sln) + 2 H 2 O(l)
ο
(VI.14)
SnCl 2·2H 2 O(cr) + H 2 O 2 (sln, ∞H 2 O) + 2 HCl(sln, 26.3H 2 O) → SnCl 4 (sln) + 4 H 2 O(l)
(VI.15)
the standard enthalpies of formation of SnCl 2 (cr) and SnCl 2·2H 2 O(cr) have been
calculated using the results of [1973VAS/KOK].
Finally the standard enthalpy of formation of tin(II) chloride in perchloric acid
solutions has been investigated calorimetrically [1973VAS/VAS], [1976VAS/KOK].
The dissolution of SnCl 2 (cr) in HClO 4 solutions can be represented by the following
reactions:
SnCl 2 (cr) → Sn 2+ + 2 Cl –
SnCl 2 (cr) → SnCl + + Cl –
SnCl 2 (cr) → SnCl 2 (aq).
(VI.16)
(VI.17)
(VI.18)
As the equations for dissolution of SnCl 2·2H 2 O(cr) are analogous, only
Eq. (VI.19) needs to be given:
SnCl 2·2H 2 O(cr) → Sn 2+ + 2 Cl – + 2 H 2 O(l).
Equations (VI.16) to (VI.18) contribute to the measured value of
SnCl 2 (cr) according to Eq. (VI.20):
Δ H (SnCl 2 , cr, 298.15 K) = α Δ 1 r H (VI.16) + α 2 Δ r H (VI.17) + α 3 Δ r H
s ln
(VI.19)
Δ H of
s ln
(VI.18)
(VI.20)
where α 1 , α 2 and α 3 in Eq. (VI.20) are the fractions of Sn 2+ , SnCl + and SnCl 2 ;
Δ r H (VI.16), Δ r H (VI.17) and Δ r H (VI.18) are the enthalpy contributions of Reactions
(VI.16) to (VI.18). For the calculation of α n the authors used the stability constants
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012