Thermodynamic characteristics of thermal dissociation of platinum dichloride

Article abstract of DOI:10.1007/s11172-005-0415-0

The dissociation pressure for the process PtCl₂(s) → Pt(s) + Cl₂(g) was measured by the static method with diaphragm zero-pressure gauges. The approximating equation for the temperature dependence on the dissociation pressure for the

Full text of DOI:10.1007/s11172-005-0415-0

Russian Chemical Bulletin, International Edition, Vol. 54, No. 6, pp. 1387—1390, June, 2005
1387
Thermodynamic characteristics of thermal dissociation of platinum dichloride
Z. I. Semenova,ꢀ T. P. Chusova, and A. A. Titov
A. V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences,
3
prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Eꢀmail: chu@che.nsk.su
The dissociation pressure for the process PtCl (s) → Pt(s) + Cl (g) was measured by the
2
2
static method with diaphragm zeroꢀpressure gauges. The approximating equation for the temꢀ
perature dependence on the dissociation pressure for the above reaction was found. The
enthalpy (137.7±0.3 ꢁJ mol–1) and entropy (163.6±0.4 J mol
–1
K ) of PtCl (s) dissociation
2
–1
and enthalpies of formation and absolute entropies of platinum diꢀ and trichlorides at 298.15 K
were calculated.
Key words: platinum trichloride, platinum dichloride, dissociation pressure, enthalpy and
entropy of dissociation, enthalpy of formation, absolute entropy.
We have previously1,2 studied the dissociation of higher
platinum chlorides: PtCl and PtCl . In this worꢁ, the
dissociation of platinum dichloride was studied.
Experimental procedure. The dissociation pressure was meaꢀ
sured by the static method using quartz diaphragm spoonꢀtype
zeroꢀmembrane gauges on an installation described earlier.
4
3
1
,2
The limiting measurement error estimated from calibrations by
mercury, naphthalene, and argon did not exceed ±1 K at 900 K;
the accuracy of temperature maintenance was ±0.1 K; the limitꢀ
ing error of pressure measurement caused by sensitivity of the
membranes used and errors of corrections for irreversible drift of
the zero position of the pressure gauge varied from experiment
to experiment, being 0.2—2.0 Torr.
Experimental
Starting substances. Four types of platinum dichloride
samples were used in tensimetric studies. They were obtained as
follows: (1) by dissociation of platinum tetrachloride via the
reaction PtCl (s) → PtCl (s) → PtCl (g) immediately in a diaꢀ
4
3
2
Experiments were carried out in the isothermal and nonꢀ
isothermal temperature regimes. As in the case of higher platiꢀ
phragm chamber; (2) by dissociation of platinum trichloride via
the reaction PtCl (s) → PtCl (s) in the diaphragm chamber as
3
2
num chlorides, the PtCl dissociation pressure was equilibrated
2
3
well; (3) using a ꢁnown procedure by the decomposition of
H PtCl •6H O in a chlorine flow at 753 K (reaction afforded a
for a very long time (from 24 to 200 h and more) and, hence, we
could approach the equilibrium only from the side of lower
pressures.
When processing the experimental data, we believed that the
dissociation of platinum dichloride proceeds via the reaction
2
6
2
finely dispersed oliveꢀgreen powder (batch 1)); (4) by sublimaꢀ
tion of finely dispersed platinum dichloride (batch 1, weighed
samples of 0.5—0.7 g) in an evacuated sealed tube (∼ 15 cm ),
3
transfer from the hot (890 K) to cold (800 K) zone. During
1
0 days, ∼ 2/3 of the starting substance precipitated in the cold
PtCl (s) → Pt(s) + Cl (g),
2
2
zone of the tube. The resulting product was a darꢁ violet subꢀ
stance in the form of dendrites 0.5—1.0 cm (batch 2).
The products synthesized were identified by elemental analyꢀ
sis, Xꢀray diffraction analysis, and IR spectroscopy. For batch 1,
found (%): Pt, 73.42; Cl, 26.47. For batch 2, found (%):
since in the studied temperature and pressure interval the total
concentration of platinum chlorides did not exceed 9•10 atm.
Molecular chlorine dissociation can be neglected, because the
maximum content of atomic chlorine in our experiments was at
most 0.06 Torr.
–5
2
Pt, 73.26; Cl, 26.67. PtCl . Calculated (%): Pt, 73.34; Cl, 26.66.
2
The interplanar distances for the samples of batch 1 agree
Calculations were carried out according to a ꢁnown proceꢀ
4
satisfactorily with those calculated from the structural data and
2
dure using the target function
5
with the dα values presented previously. The interplanar disꢀ
tances for the samples of batch 2 are satisfactorily consistent
with the values presented in Ref. 6 and do not coincide with
those for the product of batch 1. We have earlier found that the
,
7
substance of batch 1 was transformed into the substance of
batch 2 (according to the set of interplanar distances) upon
thermal annealing at 773 K, whereas no bacꢁward transition was
observed. According to Sheffer´s classification, the products of
batches 1 and 2 were named the βꢀ and αꢀmodifications of
exp
where N is the number of experimental points; pi is the experiꢀ
mental pressure; pi
calc
is the pressure calculated by the accepted
physicochemical model; ∆p and ∆T are the limiting errors of
i
i
PtCl , respectively.
pressure and temperature measurement, respectively.
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1345—1348, June, 2005.
066ꢀ5285/05/5406ꢀ1387 © 2005 Springer Science+Business Media, Inc.
1

1388
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Semenova et al.
Table 1. Entropy (S°298.15/J mol–1 K–1) and heat capacity of
Table 3. Experimental dissociation pressures for platinum dichloꢀ
ride (pexp)
–
1
–1
metallic platinum and platinum chlorides (C °/J mol
K
)* in
p
the interval 298 K < T < 900 K
Entry
T/°C pexp/Torr
Entry
T/°C pexp/Torr
Substance
Pt(cr)
S°298.15
a
b
c
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
502.3
507.3
400.8
419.9
439.7
459.2
467.7
477.2
487.5
508.1
517.3
526.7
531.5
536.9
541.1
405.9
419.2
429.2
448.9
460.0
468.0
478.5
487.8
497.8
507.7
517.6
537.7
548.5
558.7
122.18
137.85
4.79
9.28
18.5
34.33
45.49
60.22
79.81
134.09
170.95
217.16
243.97
276.53
309.08
5.42
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
4
4
5
5
5
5
5
567.3
571.7
577.2
581.4
586.8
591.5
379.7
383.3
390.3
395.3
400.4
405.6
409.7
414.7
469.8
483.3
494.2
499.3
504.6
509.1
512.9
523.6
402.5
520.0
402.0
403.0
502.0
554.0
573.5
553.80
609.42
684.69
756.33
839.14
917.70
2.21
2.63
3.55
3.93
4.54
8
0
9
–3
–3
0.25•105
0.883•10
—
41.55±0.25
100.9±0.3
141.235 12
28.49
63.5
121.34
5.27•10
21.4•10
—
1
11
5
PtCl (s)
2
12
PtCl (s)
3
2
*
In the form C °(T ) = a + bT +c/T .
p
The use of this target function in data processing provided
more reliable estimates of the desired parameters. Errors of the
desired values were calculated taꢁing into account Student´s
coefficients for the 95% confidence interval.
The thermodynamic values used in calculations are given in
Table 1.
5.54
6.63
7.69
43.62
66.87
89.58
101.53
115.91
132.52
153.43
197.50
5.40
179.92
5.92
6.04
115.69
387.57
655.60
Results and Discussion
9.25
13.33
24.29
35.04
46.09
63.49
82.21
107.36
138.97
178.19
285.75
361.21
447.69
We carried out five experiments and obtained 58 points
in the interval 666 K < T < 890 K; the weighed sample to
volume ratio was varied from 0.01 to 0.002.
The starting substances were PtCl , PtCl , βꢀPtCl ,
and αꢀPtCl . When higher platinum chlorides were used,
4
3
2
2
PtCl formed directly in the diaphragm chamber during
2
tensimetric study.
The conditions of experiments on studying the therꢀ
mal dissociation of platinum dichloride are presented in
Table 2.
The experimental data are presented in Table 3 and
Fig. 1 as the logp = f(1/T ) plot. It is seen that the dissoꢀ
ciation pressure is independent of either the ratio of the
weighed sample to the diaphragm chamber volume, or
the preꢀhistory of the samples. Thus, the system under
study is monovariant, and all the experiments can statistiꢀ
cally be processed in combination. The calculations gave
the following approximating equation:
ments, the deviations do not exceed the limiting meaꢀ
surement error and, as a whole, are random, which conꢀ
firms indirectly that the physicochemical model was choꢀ
log(p/Torr)
3
3
.5
.0
1
2
3
4
5
6
log(p/Torr)±2σ = 11.0227 – 6955.1/T,
(1)
where σ2 = 1750/T 2 – 4.444/T + 0.2843•10–2
(
666 K < T < 890 K).
2.5
2.0
The deviations of the experimental data from those
calculated by Eq. (1) are shown in Fig. 2. For all experiꢀ
1
1
0
.5
.0
.5
Table 2. Conditions of tensimetric experiments
Entry
Starting
Temperature
interval/K
Number
of points
substance
1
2
3
4
5
PtCl4
PtCl3
βꢀPtCl2
βꢀPtCl2
αꢀPtCl2
775—780
673—814
666—870
675—793
675—846
2
13
36
2
T –1•10 /K
3
–1
1.2
1.3
1.4
Fig. 1. Pressure of platinum dichloride dissociation vs. temperaꢀ
ture according to our data: entries 1 (1), 2 (2), 3 (3), 4 (4), and
5
5
(5) and Eq. (1) (6).

Thermodynamics of PtCl dissociation
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
1389
2
pexp – pcalc (%)
Table 4. Thermodynamic characteristics of platinum dichloride
dissociation
1
2
3
4
5
1
1
5
0
5
0
5
Method of
treatment
∆disH°
∆disS°
298
298
/ꢁJ mol–
1
/J mol K–1
–1
Second law of
thermodynamics
Third law of
136.7±0.7
137.7±0.3
162.8±0.8
163.6±0.4
–
thermodynamics
–
10
6
50
700
750
800
850
T/K
thermodynamics (Table 4). It is seen that both types of
processing give consistent results.
Fig. 2. Deviation of the experimental pressures of platinum
dichloride dissociation (pexp) in entries 1—5 (points) from the
values calculated (pcalc) by Eq. (1) (line).
Nevertheless, we prefer the results of calculation acꢀ
cording to the third law of thermodynamics, because the
1
0
earlier achieved good accuracy in determination of the
absolute entropy of PtCl provided a lower error of similar
calculations.
sen validly and indicates the absence of serious systematic
errors in experiments.
The data on dissociation pressure for the βꢀ and
αꢀforms of platinum dichloride are indiscernible within
experimental error.
2
In this case, the enthalpy of dissociation taꢁen with an
opposite sign is the standard enthalpy of formation of
–
1
19
PtCl (–137.7±0.3 ꢁJ mol ). We have previously deꢀ
2
termined the enthalpy of formation of platinum dichloꢀ
ride by the calorimetric method (–138.6±4.6 ꢁJ mol– ).
Both values are well consistent. However, since the latter
has a higher determination error, we recommend the value
calculated from the tensimetric data using the third law of
thermodynamics.
Our and other data on the pressure of PtCl dissociaꢀ
2
1
tion are shown in Fig. 3. The results of several studꢀ
1
3—16
17
ies
are very close to our data. The data of the worꢁ
using nonꢀequilibrium experimental conditions (with a
continuous temperature increase) are somewhat lower.
The results in the worꢁ,1 where the dissociation pressure
was measured by the effusion—torsion method, are also
lowered compared to our data. At low equilibration rate,
as in the case of dissociation of platinum chlorides, effuꢀ
sion procedures should give distorted (underestimated)
results.
8
Using the thermodynamic characteristics of PtCl disꢀ
3
2
sociation and ∆ H° and S°298 values for PtCl , we calꢀ
f
298
2
culated the standard enthalpy of formation and absolute
–
1
entropy of platinum trichloride: –199.2±1.2 ꢁJ mol and
–
1
–1
1
20.6±1.4 J mol
K , respectively. The discrepancy
–
1
between the calorimetric (–194.2±1.0 ꢁJ mol ) and
tensimetric (–199.2±1.2 ꢁJ mol ) determinations of the
Based on the data in Table 1, we calculated the enꢀ
thalpy and entropy of platinum chloride dissociation unꢀ
der standard conditions using the second and third laws of
–
1
enthalpy of formation of PtCl are beyond the error limit.
3
Probably, one of the reasons for this discrepancy can be
the estimation character of the heat capacity values used
log(p/atm)
by us in processing of the tensimetric data on PtCl dissoꢀ
3
1
9
1
0
1
2
3
4
5
1
2
3
4
5
6
7
ciation. Since the heat of platinum trichloride reduction,
which was used for the calculation of its heat of forꢀ
mation, was measured in six calorimetric experiments
with good reproducibility, we recommend the heat of
–
–
–
–
–
formation obtained from calorimetric measurements
–1
(
–194.2±1.0 ꢁJ mol ). The recommended thermodyꢀ
namic characteristics for platinum diꢀ and trichlorides are
given in Table 5.
Table 5. Thermodynamic characteristics of platinum diꢀ and
trichlorides
T –1•10 /K
4
–1
1
1
12
13
14
15
Comꢀ
pound
–∆ H°
/ꢁJ mol
S°
298
f
298
–
1
/J mol K–1
–1
Fig. 3. Pressure of platinum dichloride dissociation vs. temꢀ
perature according to published and our data: Ref. 13 (1),
Ref. 14 (2), Ref. 15 (3), Ref. 16 (4), Ref. 17 (5), and Ref. 18 (6)
and Eq. (1) (7).
PtCl (s)
137.7±0.3
194.2±1.0
100.9±0.3
120.6±1.4
2
PtCl (s)
3

1
390
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Semenova et al.
Our data on the pressure of platinum dichloride dissoꢀ
Nauk, 1980, 14, 26 [Izv. Sib. Otd. Akad. Nauk SSSR, Ser.
Khim. Nauk, 1980, 14 (Engl. Transl.)].
. V. P. Glushꢁo, Termicheskie konstanty veshchestv [Thermal
Constants of Substances], Issue VI, Izdꢀvo Aꢁad. Nauꢁ SSSR,
Moscow, 1972, 367 pp. (in Russian).
ciation are, most liꢁely, most reliable to date, because all
samples for the study were individual phases according to
the results of chemical analysis, Xꢀray diffraction, and
IR spectroscopy. The variant of tensimetric method used
in this worꢁ, viz., static method for measuring the vapor
pressure with a diaphragm zeroꢀpressure gauge, made it
possible to obtain equilibrium data. This fact is decisive
for the study of such systems as Pt—Cl in which equilibraꢀ
tion is very slow. The results of five tensimetric experiꢀ
ments using different methods on studying the dissociaꢀ
tion of platinum dichloride in a wide range of temperaꢀ
tures (666 K < T < 890 K) and pressures (2—1250 Torr)
are well consistent. The results of experimental data proꢀ
cessing by the second and third laws of thermodynamics
coincide, indicating that the chosen physicochemical
model is adequate and the experiment has no serious
systematic errors.
8
9
. K. K. Kelley, Contributions to the Data on Theoretical Metalꢀ
lurgy, US Govern. Print., Bull. Bur. Mines, Washington,
1
960, 584, 232 pp.
10. K. S. Suꢁhovei, Z. I. Zasorina, and I. E. Pauꢁov,
Teploemkost´, entropiya i ental´piya PtCl2 v intervale
11—300 K [Heat Capacity, Entropy, and Enthalpy of PtCl2 in
an Interval of 11—300 K], Moscow, 1977, 15 pp.; deposited
at VINITI, 3.02.77 (in Russian).
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chemical Properties of Inorganic Substances, Springer, Berlin,
1991, 1113 pp.
1
1
13. K. Wöhler and S. Streicher, Ber. Deutsch. Chem. Ges., 1913,
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4. I. Krustinson, Z. Elektrochem. Angew. Phys. Chem.,
939, 45, 83.
5. A. Landsberg, J. Schaller, and J. Less, Common Met., 1971,
3, 195.
6. H. Schäfer, U. Wieser, and J. Less, Common Met.,
971, 24, 55.
4
1
1
1
1
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