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Vapour pressure of condensed HgSe from torsion-effusion measurements
M-1675 J.
c‘hem. Thennodynamic,s
1984,
16, 949-954
Vapour pressure of condensed HgSe from torsion-effusion measurements P. IMPERATORJ
and V. PJACENTF
Dipartimento di C&mica, 0018.5 Roma, Italy
Universitb
de,$i Studi “La Sapienza”,
(Received 3 January 1984; in revised form 3 April I984 j The vapour pressure p of condensed HgSe was measured by torsionand Knudsen-effusion techniques and its temperature dependence can be expressed by the equation: log(p/kPa) The sublimation
of this compound
= (8.5+0.3)-(62&O.?) occurs according HeSe(s)
The associated derived as an treatments of A,1&(298.15 K)
= Hg(g)+
x 103(KU?. to the process: :Se,(g).
standard molar enthalpy change AH,(298.15 K) = (186+ 7) kJ ‘mot- ’ was average of the corresponding values obtained by second- and third-law the results. The standard molar enthalpy of formation of HgSe(s), = -55 kJ.mol-‘, was also derived.
1. Introduction The vaporization of mercury selenide has been investigated by several authors. Isakova et al.,“) analysing by X-ray diffraction and by chemical analysis the vapour condensed on the cold finger in transpiration measurements between 615 and 725 K, report that HgSe(g) is the predominant species in the vapour. Silina and Khachaturyan”’ report that sublimation of this compound occurs according to HgSe(s) = HgSe(g).
(1)
Studying (mercury + selenium) by optical absorption at high pressures Brebrickt3’ found that no HgSe(g) is present in the vapour but that this is practically constituted by Hg(g) and molecules involving selenium. Also, mass-spectrometric workP”’ agree with Brebrick’s observation even though different ratios between the selenium molecules were found. Two sets of vapour pressures are reported in the literature: one at high pressures, measured by transpiration”.7,8J and by optical absorption,‘3’ and another at low pressures measured by the Knudsen-effusion method.‘2*8.9’ Mills’lo’ summarized and re-interpreted all the literature vapour pressures considering that sublimation occurs by the reaction: HgSe(c) = Hg(g) + f: (x;/i)Se,(g). i= 1 0021
9614:84/100949+06
$02.00/O
t‘ 1984 Academrc
(2) Press Inc. (London)
Limited
950
P. IMPERATORI
AND
V. PIACENTE
where xi is the mole fraction of the gaseous selenium species Sei. Since the published p(T) equations are not in agreement, Mills selected as more reliable the equation reported by Brebrick. (j’ At present apparently no other results have been reported so that as part of our research program we have deemed it suitable to investigate the vaporization of HgSe and to obtain new vapour pressures using two different techniques.
2. Experimental and results Samples of HgSe supplied by Koch-Light (99.9 moles per cent pure) were vaporized from a conventional torsion graphite crucible and the temperature dependence of its vapour pressure was measured by the torsion-effusion method. The basis of this method and the experimental apparatus have been described elsewhere.” ” 12) From this method at each temperature the vapour pressure was calculated from the torsion angle SI of the effusion cell by the equation: P = 2W(a,
ll.fi fQzI,f,)
= &a,
(3)
where K is the torsion constant (0.346 +0.003) x lo- 5 N * m of the torsion tungsten wire, a,, u2, 1,, and 1, are the areas of the effusion orifices and their distances from the rotation axis respectively, and jI and f2 are Freeman and Searcy’s geometrical factors.” 3, To check the reliability of the instrument constant K,, derived from direct measurements of the cell parameters, vapour pressures of pure lead were measured and compared with the values selected by Hultgren et al.‘14’ In the first stage of the vaporization of HgSe(s) in the range 316 to 503 K the loss of about 2 to 3 per cent of the original mass was noted. During this stage the vapour is practically constituted by only Hg(g) and its pressures are slightly lower than the pure-element” 4, vapour pressures. This mercury can be present in the sample as impurity (probably in amount higher than that reported by the supplier) and in the same compound by analogy with HgTe, where a narrow homogeneity range (Z 1 per cent) was located close to mole fraction O.S.“5’ At higher temperatures the vapour pressures measured during the vaporization of HgSe in the overall temperature range 541 to 602 K are given in table 1 and plotted in figure 1 as log,,(p/kPa) against l/T. The best straight line obtained as an average of independent least-squares treatments of the results determined in two runs, is given by: log,,(p/kPa) = (8.84_+0.12)-(6350_+70)(K/T), where the associated errors are estimated on the basis of the corresponding standard deviations. Some vapour pressures were also measured by the Knudsen-effusion method by measuring the mass-loss of the sample heated in a conventional Knudsen graphite cell by the well known equation:“6’ p/kPa = 2.29(m/g)(cm2/A)K;
’ {(T/K)(g
mol- ‘)/M} 1’2(s/t),
(4)
where A is the area of the effusion hole, K, the Clausing’s correction factor,“” M the molar mass of the vapour, and m the mass loss at a fixed temperature T during
VAPOUR
PRESSURE
951
OF HgSe(s)
IO3 K/T FIGURE I. Vapour pressure p measured over HgSe(s) by torsion Torsion method: 0. run 1; 0. run 2; Knudsen method: A. The straight equation.
and Knudsen-effusion methods. line represents the selected p(T)
the experimental time t. The experimental apparatus has been described previously. (18) It was calibrated by vaporizing pure zinc as a standard. In the calculation of the vapour pressure of HgSe(s), Hg(g) and Se,(g) were assumed to be the only constituents of the vapour. The Knudsen vapour pressures are summarized in table 2 and reported in figure 1. The values are slightly higher than those measured by the torsion method because of some probable systematic errors. This disagreement could also be due to the presence of molecules Se,(g) (x > 2) in the vapour phase in addition to Se,(g); however, the discrepancy cannot be ascribed exclusively to this. The least-squares treatment of the vapour pressures gives the equation: log,,(p/kPa) = (8.53+_0.71)-(6039+39)(K/T). During some of these measurements the vapour was condensed on a tantalum target and observed by a scanning-electron-microscope analysis. It was noted that after about 2 or 3 min, the time necessary for the preparation and starting in the SEM analysis, most of the mercury and selenium condensed on the target has recombined; this observation clarifies the error of Isakova et al.“) and Silina and Khachaturyan’2’ who have interpreted the HgSe sublimation process by the observation only of its condensed vapour. 3. Discussion The temperature dependence of the vapour pressure of the condensed HgSe measured by the torsion method and practically confirmed by the Knudsen results is expressed by the selected equation: log,,(p/kPa)
= (8.79 kO.23) - (6300 +_70)(K/T),
952
P. IMPERATORI
AND
V. PJACENTE
obtained after giving higher weight to the torsion results. From the slope of this equation the second-law enthalpy change of the process: HgWs) = W&d + 4%(g), (5) AHi(572 K) = 181 kJ.mol-‘, or ANk(298.15 K) = 185 kJ.mol-’ by using enthalpy increments reported in the literature,“‘. 14) was derived. The associated error should not exceed 5 kJ ’ mol- ’ considering the uncertainties only in the temperature determinations. TABLE
I. Experimental
Run 1
!og,,(p/kPa) 2
log,,(p/kPa)
T
torsion angle, va-pour solid HgSe determined
180a
it
__ n
P pa
541 543 545 541 549 552 554 555 557 559 561 562 565 561 568 570 572 574 581 597 599 602
12 13 14 15 16 17 18 20 22 24 21 30 33 36 39 42 45 48 69 135 153 173
1.32 1.41 1.55 1.66 1.73 1.86 1.99 2.19 2.40 2.63 2.95 3.31 3.63 3.98 4.26 4.57 4.90 5.25 7.59 14.80 17.02 19.10
= (8.75+0.12) 548 554 559 564 570 573 516 580 582 589 590 592 593 597
-(6320 20 25 31 36 53 63 71 80 91 119 132 134 143 173
= (8.99*0.13)-(6397&73)(K/T)
pressure, and third-law by the torsion-effusion
-{G:(T)-
sublimation method
HL(298.15 K)}IT
J.K-‘.mol-’
AHz(298.15
AH;(298.
186.9 187.3 187.3 187.4 ! 87.8 188.3 188.5 188.1 188.3 188.3 188.2 187.6 188.0 187.9 187.7 187.9 188.1 188.3 187.7 187.8 187.5 187.6
-__ Average:
2.19 2.75 3.39 3.98 5.75 6.92 7.76 8.71 10.02 13.20 14.51 14.83 15.80 19.12
I5 K)
kJ.mo!!’
193.8 193.8 193.8 193.8 193.8 193.7 193.7 193.7 193.7 193.6 193.6 193.6 193.6 193.6 193.6 193.5 193.5 193.5 193.4 193.2 193.2 193.2
5- 69)(K/T)
K) for
193.8 193.7 193.7 193.6 193.5 193.5 193.4 193.4 193.4 193.3 193.3 193.3 193.3 193.2
187.8 kO.4 185.8 186.2 186.4 186.9 186.3 185.9 186.0 186.5 186.1 186.3 185.9 186.3 186.2 186.0
Average:
186.2 +0.3
VAPOUR
PRESSURE
953
OF HgSe(s)
At each experimental temperature the AHk(298.15 K) associated with the sublimation process was also calculated by third-law treatment of the vapour pressures. The obtained values are reported in tables 1 and 2. The necessary functions -[(GE(T)-Hi(298.15 K)jjTJ for the condensed HgSe are taken from Mills”“) and those for Hg(g) and Se,(g) from Hultgren et ~1.~‘~’ The absence of any evident temperature trend in the calculated third-law AHkt298. I5 K) values and the very good agreement of the average third-law value, AHi(298.15 K) = 186 kJ . molt I, with the corresponding second-law value can be taken as evidence that the vaporization of HgSe occurs predominantly according to equation (5) and that in the vapour phase other molecules Se,(g) (X # 2) are minor. The agreement shows also that the uncertainties in the temperature measurements and in the instrument calibrations are negligible so that the vapour pressures measured by both methods are reliable. Our results and those found in the literature, some ‘l*” of them now corrected on the basis of the vaporization process (5), are summarized in table 3. The comparison shows that, except for the results obtained by the transpiration method,(‘*” all the slopes of the reported p(T) equations can be regarded as being in agreement within TABLE
2. Vapour
pressure
and third-law
sublimation Knudsen
T
m
f
P
K
i
5
Pa
2400
2.09 2.78 3.x0 4.90 4.07
540 544 549 554 556 559 565 570
0.058 0.048 0.119
0.106 0.112 0.204 0.202 0.197
1500 2700 1860 2400 3780 2760
AH,(2Y8.15 method
HgSe determined
- :G,,(T,--H;(298.15 K$'T -.-_--~- ~ ~--
kJ,mol
193.X IP3.8 193.7 193.7 103.7 lY3.6
9.55
182.4
I x2.2 184.2 184.1
1x3.9 181.7
Average: TABLE
Reference Pashinkin
and Salamatints’
Reznyakov and Isakova”’ Brebrick’“’ Isakova et ul.“’ Shakhtakhtinskii”’ Silina and Karapct’yants”’ Silina
and Khachaturyan’2’
This uorh
3. Comparison Tl K
of vapour
.~
TL 7 k
613
1043
673 723 613 523 610 51s 523
923 873 7'3 600 755 606 750
540
602
pressure
over condensed
Method Knudsen effusion and transptratron Transpiration Optical ahsorption Transpiration Knudsen effusion Transpiratton Knudsen efTusion i Knudsen effustonand transpiration Torsion efuston and Knudsen effusion
’ 1x3.5 182.9
193.6 193.5
6.46
by the
AH,n(298.15 K) _. ..--.~~
J,K-‘.mol-’
4.68
lRo0
K) for solid
183.3 +0.X
HgSe log,,(p/kPa) ‘4
= - -
A -~ B(k T -~
B
.~
X.hS
6445
6.44 8.22
4681
10.18
5900 7522
8.19
h245
8.52
6250
x.1 5
2976
8.79
6300
954
P. IMPERATORI
AND
V. PIACENTE
the experimental errors so that we propose as the standard molar vaporization enthalpy of HgSe(s) our value AHm(298.15 K) = 186 kJ. mol- ’ obtained as an average of the second- and third-law results. The associated error should not exceed 7 kJ. mol- ‘. For p(T) of the vapour in equilibrium with HgSe(s) we propose the equation: log,,(p/kPa)
= (8.5 *0.3)-(6.2f0.2)
x 103(K/T),
obtained after taking into account all the results summarized in table 3. The associated errors have been estimated. From the standard molar vaporization enthalpy of HgSe, the standard molar enthalpy of formation of this compound: A,H,?,,(298.15 K) = - 55 kJ. mol- ‘, has been calculated by using the standard molar enthalpy of vaporization of Hg: 61.3 kJ~mol-‘,(‘9’ and the standard partial molar sublimation enthalpy of Se,(g): 139.2 kJ. mol-‘.“O) In addition to the enthalpies of formation obtained by evaluation of vapour-pressure measurements, in the literature only two values are reported: one calorimetric, -21 kJ.mol-1,‘20’ and another, -45 kJ.mol-‘,t2’) obtained as the average of second- and third-law e.m.f. values. These are higher than those obtained from the evaluation of the vapour pressure but the difference could not be due to a systematic error in the pressure values derived from having neglected the presence of Se, (x # 2) molecules in the vapour as suggested by Mills, but is probably due to the inaccurate early calorimetryC2” and to the uncertainties associated with our and the e.m.f. experimental measurements. REFERENCES I. Isakova. R. A.; Nesterov, N.; Yesiutin, V. S. Tr. Insl. Mel. i Obogashcheniya Akad. Nauk Kazakh. SSR 1%3, 8, 6. 2. Silina, E. Y.; Khachaturyan, T. A. Trudy Mosk. Khim.-Tekhnol. 1ns1. im Mendeleeva 1964, 44, 20. 3. Brebrick. R. F. J. Chem. Phys. 1965, 43, 3846. 4. Goldfinger, P.; Jeunehomme, M. Trans. Faraday Sot. 1963, 59, 2851. 5. Berkowitz, J.; Chupka, W. A. J. Chem. Phys. 1%6,45, 4289. 6. Grade, M.; Hirschwald, W. Z. anorg. allg. Chem. 1980, 460, 106. I. Reznyakov, A. A.; lsakova, R. A. Zhur. Neorg. Khim. 1%8, 13, 625. 8. Pashinkin, A. S.; Salamatin, B. Zhur. Fiz. Khim. 1967,41, 239.5. 9. Shakhtakhtinskii. M. G. as reported by Silina, E. Y.; Karapet’yants, M. Kh. Zhur. Fiz. Khim. 1964, 38. 2907. IO. Mills, K. C. Thermodynamic Data .for Inorganic Sulphides, Selenides. and Tellurides. Butterworths: London. 1974. I I. Freeman. R. D. The Characterization of High Temperature Vapour. Margrave, J. L.: editor. Wiley: New York. 1967. 12. Piacente, V.; De Maria, G. Ric. Sci. 1969, 39, 549. 13. Freeman, R. D.; Searcy, A. W. J. Chem. Phys. 1954, 22. 762. 14. Hultgren. R.; Orr, R. L.; Kelley, K. K. Selected Values of Thermodynamic Properties c~f Metals and AUOJX Wiley: New York. 1963. 15. Blue, M. I).; Kruse, P. W. J. Phys. Chem. Solids 1962, 23, 577. 16. Knudsen, M. Ann. Phys. 1909, 28, 75. Il. Dushman. S. ScientiJic Foundation of Vacuum Technique. Wiley: New York. 1958. 18. Pelino, M.; Ferro, D.; Piacente, V. Thermochim. Acla 1980, 41, 297. 19. JANAF Thermochemical Tables. 2nd Edition. Stull. D. R.; Prophet, H.: editors. NRDS-NBS 37. Washington, D.C. 1971. 20. Fabre, C. Ann. Chim. Phys. 1887, 10, 472. 21. Ratajczak. E.; Terpilowski, J. Roczn. Chem. 1968, 42. 433.
c‘hem. Thennodynamic,s
1984,
16, 949-954
Vapour pressure of condensed HgSe from torsion-effusion measurements P. IMPERATORJ
and V. PJACENTF
Dipartimento di C&mica, 0018.5 Roma, Italy
Universitb
de,$i Studi “La Sapienza”,
(Received 3 January 1984; in revised form 3 April I984 j The vapour pressure p of condensed HgSe was measured by torsionand Knudsen-effusion techniques and its temperature dependence can be expressed by the equation: log(p/kPa) The sublimation
of this compound
= (8.5+0.3)-(62&O.?) occurs according HeSe(s)
The associated derived as an treatments of A,1&(298.15 K)
= Hg(g)+
x 103(KU?. to the process: :Se,(g).
standard molar enthalpy change AH,(298.15 K) = (186+ 7) kJ ‘mot- ’ was average of the corresponding values obtained by second- and third-law the results. The standard molar enthalpy of formation of HgSe(s), = -55 kJ.mol-‘, was also derived.
1. Introduction The vaporization of mercury selenide has been investigated by several authors. Isakova et al.,“) analysing by X-ray diffraction and by chemical analysis the vapour condensed on the cold finger in transpiration measurements between 615 and 725 K, report that HgSe(g) is the predominant species in the vapour. Silina and Khachaturyan”’ report that sublimation of this compound occurs according to HgSe(s) = HgSe(g).
(1)
Studying (mercury + selenium) by optical absorption at high pressures Brebrickt3’ found that no HgSe(g) is present in the vapour but that this is practically constituted by Hg(g) and molecules involving selenium. Also, mass-spectrometric workP”’ agree with Brebrick’s observation even though different ratios between the selenium molecules were found. Two sets of vapour pressures are reported in the literature: one at high pressures, measured by transpiration”.7,8J and by optical absorption,‘3’ and another at low pressures measured by the Knudsen-effusion method.‘2*8.9’ Mills’lo’ summarized and re-interpreted all the literature vapour pressures considering that sublimation occurs by the reaction: HgSe(c) = Hg(g) + f: (x;/i)Se,(g). i= 1 0021
9614:84/100949+06
$02.00/O
t‘ 1984 Academrc
(2) Press Inc. (London)
Limited
950
P. IMPERATORI
AND
V. PIACENTE
where xi is the mole fraction of the gaseous selenium species Sei. Since the published p(T) equations are not in agreement, Mills selected as more reliable the equation reported by Brebrick. (j’ At present apparently no other results have been reported so that as part of our research program we have deemed it suitable to investigate the vaporization of HgSe and to obtain new vapour pressures using two different techniques.
2. Experimental and results Samples of HgSe supplied by Koch-Light (99.9 moles per cent pure) were vaporized from a conventional torsion graphite crucible and the temperature dependence of its vapour pressure was measured by the torsion-effusion method. The basis of this method and the experimental apparatus have been described elsewhere.” ” 12) From this method at each temperature the vapour pressure was calculated from the torsion angle SI of the effusion cell by the equation: P = 2W(a,
ll.fi fQzI,f,)
= &a,
(3)
where K is the torsion constant (0.346 +0.003) x lo- 5 N * m of the torsion tungsten wire, a,, u2, 1,, and 1, are the areas of the effusion orifices and their distances from the rotation axis respectively, and jI and f2 are Freeman and Searcy’s geometrical factors.” 3, To check the reliability of the instrument constant K,, derived from direct measurements of the cell parameters, vapour pressures of pure lead were measured and compared with the values selected by Hultgren et al.‘14’ In the first stage of the vaporization of HgSe(s) in the range 316 to 503 K the loss of about 2 to 3 per cent of the original mass was noted. During this stage the vapour is practically constituted by only Hg(g) and its pressures are slightly lower than the pure-element” 4, vapour pressures. This mercury can be present in the sample as impurity (probably in amount higher than that reported by the supplier) and in the same compound by analogy with HgTe, where a narrow homogeneity range (Z 1 per cent) was located close to mole fraction O.S.“5’ At higher temperatures the vapour pressures measured during the vaporization of HgSe in the overall temperature range 541 to 602 K are given in table 1 and plotted in figure 1 as log,,(p/kPa) against l/T. The best straight line obtained as an average of independent least-squares treatments of the results determined in two runs, is given by: log,,(p/kPa) = (8.84_+0.12)-(6350_+70)(K/T), where the associated errors are estimated on the basis of the corresponding standard deviations. Some vapour pressures were also measured by the Knudsen-effusion method by measuring the mass-loss of the sample heated in a conventional Knudsen graphite cell by the well known equation:“6’ p/kPa = 2.29(m/g)(cm2/A)K;
’ {(T/K)(g
mol- ‘)/M} 1’2(s/t),
(4)
where A is the area of the effusion hole, K, the Clausing’s correction factor,“” M the molar mass of the vapour, and m the mass loss at a fixed temperature T during
VAPOUR
PRESSURE
951
OF HgSe(s)
IO3 K/T FIGURE I. Vapour pressure p measured over HgSe(s) by torsion Torsion method: 0. run 1; 0. run 2; Knudsen method: A. The straight equation.
and Knudsen-effusion methods. line represents the selected p(T)
the experimental time t. The experimental apparatus has been described previously. (18) It was calibrated by vaporizing pure zinc as a standard. In the calculation of the vapour pressure of HgSe(s), Hg(g) and Se,(g) were assumed to be the only constituents of the vapour. The Knudsen vapour pressures are summarized in table 2 and reported in figure 1. The values are slightly higher than those measured by the torsion method because of some probable systematic errors. This disagreement could also be due to the presence of molecules Se,(g) (x > 2) in the vapour phase in addition to Se,(g); however, the discrepancy cannot be ascribed exclusively to this. The least-squares treatment of the vapour pressures gives the equation: log,,(p/kPa) = (8.53+_0.71)-(6039+39)(K/T). During some of these measurements the vapour was condensed on a tantalum target and observed by a scanning-electron-microscope analysis. It was noted that after about 2 or 3 min, the time necessary for the preparation and starting in the SEM analysis, most of the mercury and selenium condensed on the target has recombined; this observation clarifies the error of Isakova et al.“) and Silina and Khachaturyan’2’ who have interpreted the HgSe sublimation process by the observation only of its condensed vapour. 3. Discussion The temperature dependence of the vapour pressure of the condensed HgSe measured by the torsion method and practically confirmed by the Knudsen results is expressed by the selected equation: log,,(p/kPa)
= (8.79 kO.23) - (6300 +_70)(K/T),
952
P. IMPERATORI
AND
V. PJACENTE
obtained after giving higher weight to the torsion results. From the slope of this equation the second-law enthalpy change of the process: HgWs) = W&d + 4%(g), (5) AHi(572 K) = 181 kJ.mol-‘, or ANk(298.15 K) = 185 kJ.mol-’ by using enthalpy increments reported in the literature,“‘. 14) was derived. The associated error should not exceed 5 kJ ’ mol- ’ considering the uncertainties only in the temperature determinations. TABLE
I. Experimental
Run 1
!og,,(p/kPa) 2
log,,(p/kPa)
T
torsion angle, va-pour solid HgSe determined
180a
it
__ n
P pa
541 543 545 541 549 552 554 555 557 559 561 562 565 561 568 570 572 574 581 597 599 602
12 13 14 15 16 17 18 20 22 24 21 30 33 36 39 42 45 48 69 135 153 173
1.32 1.41 1.55 1.66 1.73 1.86 1.99 2.19 2.40 2.63 2.95 3.31 3.63 3.98 4.26 4.57 4.90 5.25 7.59 14.80 17.02 19.10
= (8.75+0.12) 548 554 559 564 570 573 516 580 582 589 590 592 593 597
-(6320 20 25 31 36 53 63 71 80 91 119 132 134 143 173
= (8.99*0.13)-(6397&73)(K/T)
pressure, and third-law by the torsion-effusion
-{G:(T)-
sublimation method
HL(298.15 K)}IT
J.K-‘.mol-’
AHz(298.15
AH;(298.
186.9 187.3 187.3 187.4 ! 87.8 188.3 188.5 188.1 188.3 188.3 188.2 187.6 188.0 187.9 187.7 187.9 188.1 188.3 187.7 187.8 187.5 187.6
-__ Average:
2.19 2.75 3.39 3.98 5.75 6.92 7.76 8.71 10.02 13.20 14.51 14.83 15.80 19.12
I5 K)
kJ.mo!!’
193.8 193.8 193.8 193.8 193.8 193.7 193.7 193.7 193.7 193.6 193.6 193.6 193.6 193.6 193.6 193.5 193.5 193.5 193.4 193.2 193.2 193.2
5- 69)(K/T)
K) for
193.8 193.7 193.7 193.6 193.5 193.5 193.4 193.4 193.4 193.3 193.3 193.3 193.3 193.2
187.8 kO.4 185.8 186.2 186.4 186.9 186.3 185.9 186.0 186.5 186.1 186.3 185.9 186.3 186.2 186.0
Average:
186.2 +0.3
VAPOUR
PRESSURE
953
OF HgSe(s)
At each experimental temperature the AHk(298.15 K) associated with the sublimation process was also calculated by third-law treatment of the vapour pressures. The obtained values are reported in tables 1 and 2. The necessary functions -[(GE(T)-Hi(298.15 K)jjTJ for the condensed HgSe are taken from Mills”“) and those for Hg(g) and Se,(g) from Hultgren et ~1.~‘~’ The absence of any evident temperature trend in the calculated third-law AHkt298. I5 K) values and the very good agreement of the average third-law value, AHi(298.15 K) = 186 kJ . molt I, with the corresponding second-law value can be taken as evidence that the vaporization of HgSe occurs predominantly according to equation (5) and that in the vapour phase other molecules Se,(g) (X # 2) are minor. The agreement shows also that the uncertainties in the temperature measurements and in the instrument calibrations are negligible so that the vapour pressures measured by both methods are reliable. Our results and those found in the literature, some ‘l*” of them now corrected on the basis of the vaporization process (5), are summarized in table 3. The comparison shows that, except for the results obtained by the transpiration method,(‘*” all the slopes of the reported p(T) equations can be regarded as being in agreement within TABLE
2. Vapour
pressure
and third-law
sublimation Knudsen
T
m
f
P
K
i
5
Pa
2400
2.09 2.78 3.x0 4.90 4.07
540 544 549 554 556 559 565 570
0.058 0.048 0.119
0.106 0.112 0.204 0.202 0.197
1500 2700 1860 2400 3780 2760
AH,(2Y8.15 method
HgSe determined
- :G,,(T,--H;(298.15 K$'T -.-_--~- ~ ~--
kJ,mol
193.X IP3.8 193.7 193.7 103.7 lY3.6
9.55
182.4
I x2.2 184.2 184.1
1x3.9 181.7
Average: TABLE
Reference Pashinkin
and Salamatints’
Reznyakov and Isakova”’ Brebrick’“’ Isakova et ul.“’ Shakhtakhtinskii”’ Silina and Karapct’yants”’ Silina
and Khachaturyan’2’
This uorh
3. Comparison Tl K
of vapour
.~
TL 7 k
613
1043
673 723 613 523 610 51s 523
923 873 7'3 600 755 606 750
540
602
pressure
over condensed
Method Knudsen effusion and transptratron Transpiration Optical ahsorption Transpiration Knudsen effusion Transpiratton Knudsen efTusion i Knudsen effustonand transpiration Torsion efuston and Knudsen effusion
’ 1x3.5 182.9
193.6 193.5
6.46
by the
AH,n(298.15 K) _. ..--.~~
J,K-‘.mol-’
4.68
lRo0
K) for solid
183.3 +0.X
HgSe log,,(p/kPa) ‘4
= - -
A -~ B(k T -~
B
.~
X.hS
6445
6.44 8.22
4681
10.18
5900 7522
8.19
h245
8.52
6250
x.1 5
2976
8.79
6300
954
P. IMPERATORI
AND
V. PIACENTE
the experimental errors so that we propose as the standard molar vaporization enthalpy of HgSe(s) our value AHm(298.15 K) = 186 kJ. mol- ’ obtained as an average of the second- and third-law results. The associated error should not exceed 7 kJ. mol- ‘. For p(T) of the vapour in equilibrium with HgSe(s) we propose the equation: log,,(p/kPa)
= (8.5 *0.3)-(6.2f0.2)
x 103(K/T),
obtained after taking into account all the results summarized in table 3. The associated errors have been estimated. From the standard molar vaporization enthalpy of HgSe, the standard molar enthalpy of formation of this compound: A,H,?,,(298.15 K) = - 55 kJ. mol- ‘, has been calculated by using the standard molar enthalpy of vaporization of Hg: 61.3 kJ~mol-‘,(‘9’ and the standard partial molar sublimation enthalpy of Se,(g): 139.2 kJ. mol-‘.“O) In addition to the enthalpies of formation obtained by evaluation of vapour-pressure measurements, in the literature only two values are reported: one calorimetric, -21 kJ.mol-1,‘20’ and another, -45 kJ.mol-‘,t2’) obtained as the average of second- and third-law e.m.f. values. These are higher than those obtained from the evaluation of the vapour pressure but the difference could not be due to a systematic error in the pressure values derived from having neglected the presence of Se, (x # 2) molecules in the vapour as suggested by Mills, but is probably due to the inaccurate early calorimetryC2” and to the uncertainties associated with our and the e.m.f. experimental measurements. REFERENCES I. Isakova. R. A.; Nesterov, N.; Yesiutin, V. S. Tr. Insl. Mel. i Obogashcheniya Akad. Nauk Kazakh. SSR 1%3, 8, 6. 2. Silina, E. Y.; Khachaturyan, T. A. Trudy Mosk. Khim.-Tekhnol. 1ns1. im Mendeleeva 1964, 44, 20. 3. Brebrick. R. F. J. Chem. Phys. 1965, 43, 3846. 4. Goldfinger, P.; Jeunehomme, M. Trans. Faraday Sot. 1963, 59, 2851. 5. Berkowitz, J.; Chupka, W. A. J. Chem. Phys. 1%6,45, 4289. 6. Grade, M.; Hirschwald, W. Z. anorg. allg. Chem. 1980, 460, 106. I. Reznyakov, A. A.; lsakova, R. A. Zhur. Neorg. Khim. 1%8, 13, 625. 8. Pashinkin, A. S.; Salamatin, B. Zhur. Fiz. Khim. 1967,41, 239.5. 9. Shakhtakhtinskii. M. G. as reported by Silina, E. Y.; Karapet’yants, M. Kh. Zhur. Fiz. Khim. 1964, 38. 2907. IO. Mills, K. C. Thermodynamic Data .for Inorganic Sulphides, Selenides. and Tellurides. Butterworths: London. 1974. I I. Freeman. R. D. The Characterization of High Temperature Vapour. Margrave, J. L.: editor. Wiley: New York. 1967. 12. Piacente, V.; De Maria, G. Ric. Sci. 1969, 39, 549. 13. Freeman, R. D.; Searcy, A. W. J. Chem. Phys. 1954, 22. 762. 14. Hultgren. R.; Orr, R. L.; Kelley, K. K. Selected Values of Thermodynamic Properties c~f Metals and AUOJX Wiley: New York. 1963. 15. Blue, M. I).; Kruse, P. W. J. Phys. Chem. Solids 1962, 23, 577. 16. Knudsen, M. Ann. Phys. 1909, 28, 75. Il. Dushman. S. ScientiJic Foundation of Vacuum Technique. Wiley: New York. 1958. 18. Pelino, M.; Ferro, D.; Piacente, V. Thermochim. Acla 1980, 41, 297. 19. JANAF Thermochemical Tables. 2nd Edition. Stull. D. R.; Prophet, H.: editors. NRDS-NBS 37. Washington, D.C. 1971. 20. Fabre, C. Ann. Chim. Phys. 1887, 10, 472. 21. Ratajczak. E.; Terpilowski, J. Roczn. Chem. 1968, 42. 433.