THERMODYNAMICS OF THE V206(sOR 1)-H20(g) SYSTEM
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The Thermodynamics of the Vanadium Pentoxide (Solid or Liquid)-Water Vapor System by L. N. Yannopoulosla U.S. Army Nuclear Defense Laboratory, Edgewood Arsenal, Maryland
(Received December ii, 1967)
The reaction between V2006(sor 1) and HzO(g) has been investigated between 639.0 and 899.0" by the vapor transfer method. The presence of a volatile vanadium hydroxide, VO (OH)a(g),has been indirectly deduced from isothermal vapor pressure data. These data indicate the existence of the heterogeneous equilibrium, OB VzOs(s or 1) 3HzO(g) e 2VO(OH)3(g), if one vanadium atom is assumed in the gaseous molecule. The determined heat and entropy for the reaction of steam with liquid and solid vanadium pentoxide, within the experimental uncertainty, are 44.0 0.5 kcal/mol and -1.03 =!C 0.10 cal/mol deg, respectively.
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Introduction Experimental work on the formation and stability of several gaseous metallic hydroxide compounds at elevated temperatures has been reviewed and critically evaluated.lbI2 It is of interest to note that experimental information on many of these gaseous compounds, especially when supplied by various experimental methods, may provide a basis for correlating the conditions and chemistry of their formation to their analogous existing gaseous oxyhalides. Glemser and MullerS present some va,por pressure data on the system V205(s)HzO(g) in an oxygen atmosphere between 500 and 650". They report all of their data a t a flow rate of 100 cc/min and conclude that the formation of the complex species V,0,(OH)4(g) if1 the product. Neugebauer4 also makes a brief reference to the increased volatility of liquid vanadium pentoxide in the presence of steam a t 900". The transpiration study of Glemser and Muller3 lacked flow rate data demonstrating equilibrium in the Vz06(s) HZO(g) reaction. Since there was also an absence of quantitative vapor pressure data on the Vz06(1)-HzO(g) system, a study of the V205 (s or 1)H2O(g) system was undertaken to ascertain the conditions of formation of any volatile vanadium hydroxides and to determine their thermodynamic properties.
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Experimental Section Quantitative vapor pressure data were obtained by the vapor transfer method. The experimental arrangement consisted of a 10-in. long quartz cell (1/2-in. bore tube) placed in the hot zone of a furnace. The cell contained a constricltion in the thermalgradient zone to minimize thermal diffusion of the gases. Cell volume was minimized t o 15 3c by the insertion of l/d-in. sealed quartz or Vycor tubes. Insertion and removal of the oxide sample, contained in a quartz boat, was facilitated by the use of a quartz taper joint close to the downstream end of the hot zone. The joint was tested to 900" for
expansion and gas leakage and was found to function satisfactorily. The temperature of the Kanthal wire-wound resistance furnace was monitored by a chromel-alumel (C/A) thermocouple positigned next to the reaction cell opposite the quartz sample boat and was controlled to within f.2" by an automatic controller-recorder and a dual powerstat relay-type arrangement. I n addition, the temperature was periodically checked by a calibrated C/A thermocouple, positioned next to the control thermocouple, and a potentiometer. A plateaushaped curve of temperature vs. position along the hot zone ensured the absence of any longitudinal thermal gradients. Oxygen-enriched saturated steam, generated in a small glass vessel by bubbling oxygen through water, was introduced into the reaction cell via a '/4-in. Pyrex tube and Pyrex-quartz graded seal. The glass vessel and its contents were maintained a t constant temperature by submerging the vessel in a constantly stirred glycerol bath thermostated to =!C0.025". The steam, passing over the sample in the hot zone, reacted with the solid or liquid vanadium pentoxide (Fisher certified reagent of 100.1% total assay) to form the gaseous product. The product, transported downstream to the cooler part of the I/4-in. exit tube, decomposed and condensed on the walls of the tube as the oxide, V206. The steam was then condensed in a 0" trap. The inlet and outlet tubes were maintained a few degrees above 100" by separate heating tapes (extending to the level of the glycerol a t the inlet side) to prevent steam condensation. The carrier gas flow rate was checked (1) (a) Address all correspondence to Westinghouse Electric Corporation, Research and Development Center, Pittsburgh, Pa., 15235; (b) 0 , Glemser and H. G. Wendlandt, Advan. Inorg. Chem. Radiochem., 5 , 215 (1963). (2) S. A. Jordan, Ph.D. Thesis, University of Pennsylvania, 1965. (3) 0. Glemser and A. Muller, Z . Anorg. Allg. Chem., 325, 220 (1963). (4) J. Neugebauer, Acta Chem. Hung., 37, 247 (1963).
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L. N. YANNOPOULOS
3294 intermittently at both the inlet and outlet of the cell by a pinchcock buret and continuously monitored by a calibrated flowrator tube at the inlet. The flow rate from the gas cylinder was controlled by a Nupro bellows metering valve. The microgram quantities of vanadium pentoxide condensate were analyzed for vanadium by utilizing a colorimetric method based on the formation of phosphotungstovanadic acid c ~ r n p l e x . ~The , ~ vanadium pentoxide deposit was clearly defined in the outlet tube and was removed by heating the tube with an electric heat gun while HC1 and distilled water were poured through the tube t o dissolve the deposit. The solution was directly collected in a 10- or 25-ml volumetric flask. Vanadium concentrations varied from 3.0 to 15.0 pg/ ml. Absorbance measurements were read on a Beckman DU-2 Spectrophotometer at 410 mp using a l-cm quartz cell. The total pressure (barometric), the temperature of the carrier gas at its measured position, the gas flow rate, the quantity of water collected, the amount of Vz06deposited on the condensation tube, and the duration of each experiment were recorded and used to calculate the apparent vapor pressure of vanadium pentoxide, the water vapor pressure, and the total gas flow rate a t STP. Possible effects on the vapor pressure, such as impurities in the oxygen carrier gas, the reuse of the vanadium pentoxide sample for succeeding runs, the duration of the experiment, the surface area of the powdered vanadium pentoxide and its liquid oxide, and the lowering of oxygen pressure by the introduction of Ar-02 mixtures were checked.
Results Measurements were taken under oxidizing conditions in the temperature range of 639.0-899.0". Preliminary experiments with only oxygen flowing over the Vz05(sor 1) sample revealed no visible deposits of vanadium pentoxide on the walls of the outlet condensation tube. The introduction of saturated HzO(g)-Oz(g) mixtures resulted in distinct deposits of VzOb, indicative of its increased volatility in the presence of steam. When steam was introduced alone the deposits of VzOa were also observed. The water vapor pressures were in excess of 100 mm because of the low volatility of vanadium pentoxide a t lower water vapor pressures. The apparent vapor pressures of V&(s or 1) were calculated and analyzed as a function of the water vapor pressure, PH20. A linear log-log plot of the pressures of the deduced hydroxide species VO(OH)s(g) as a function of the water vapor pressure, PHso,is presented in Figure 1. The slopes of the lines from least-squares 0.06, analysis are 1.47 0.06, 1.57 i= 0.15, 1.50 1.47 i 0.15, 1.44 0.13, and 1.54 =k 0.17 for 639.0, 718.5, 749.0, 800.5, 850.0, and 899.0", respectively. These slopes are close to 1.5, which indicates that the heterogeneous vanadium pentoxide-water vapor reacThe Journal of Physical Chemistru
Figure 1. Experimental isotherms for the VaOs (8 or 1)-HzO(g) system. A refers to equilibrium points with Ar-03 mixtures.
tion takes place according to the following chemical equilibrium if one vanadium atom in the gaseous hydroxide molecule is assumed
VZOF,(Sor 1) f 3HzO(g)
2 2VO(OH),(g)
The points A a t 639.0 and 800.5" were obtained with an
Ar-O2 mixture as a carrier gas of 6.1 and 25,4% oxygen, respectively, of the total gas in the system. It is expected that any rate of change of the VzOb-condensed phase activity is low enough to be insignificant since the volatility of VzOb(sor 1) with the Ar-02 mixture did not deviate. I n addition, the vapor pressures were unaffected by different purity levels of oxygen (99.5 and 99.99%), changes of the sample surface area, or different reaction times of the experiment. Furthermore, even if a polymer species, such as V4010(g), were considered in the vaporization process, it can safely be assumed that, in the absence of steam and in the presence of oxygen carrier gas, there is no contribution of a volatile species of VzObto the total apparent vapor pressure. Therefore, the interpretation of the data appears to be valid and the vapor pressure of the hydroxide species VO(OH)3(g) is twice the apparent vapor pressure of ( 6 ) M. D. Cooper and P. K, Winter, Anal. Chem., 12, 605 (1949). (6) 8. Killingbeck, Ph.D. Thesis, University of Kansas, 1964.
THERMODYNAMICS OF THE VzO&
E
0
8
Q
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I
A'Jg pH,O (rnrnHg1
639.0 639.0 718.5 718.5
398.7 526.6 310.6 513.7
749.0 800.5
522.5
hr
I10
I
I
0
iz
Q
61
IOXlC
3295
1)-H20(g) SYSTEM
-'C
8 0
OR
.
291.1
5XlC
1x10
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I
110
ILO
FLOW RATE
I
It,
I ,bo
(cc/min 01 STPI
'
21,
Figure 2. Flow rate dependence of the vandium oxide trihydroxide equilibrium pressures.
I 2b
t kl430
1 0.L
aL
0.a
!.I, (%')
'4
1s:,
o!
1.b
Figure 3. Temperature dependence of the equilibrium constant. A refers to the VzOs(s)-HzO(g)system.
V206(sor l), Figure 1, in accordance with the deduced A least-squares analysis of all the points of Figure 3 stoichiometry for the reaction. yields the equation Equilibrium attained during the experiments is demonstrated in Figure 2 by the nearly constant vapor pres-9619.1 f 116.0 log K , = (-0.225 f 0.023) sures of VO(OR)a(g) as a function of the STP flow T rates. There is less than 5% deviation of P v o ( o H ) ~ ( ~ ) over the water vapor pressure range covered while The heat and entropy of the reaction, with their probat each datum point the reproducibility of the total able errors, are then calculated as 44.0 f 0.5 kcal/mol number of moles and of the water vapor pressures is and -1.03 f 0.10 cal/mol deg, respectively. If the better than 1.0%. data for the VzOj(l) H20(g) reaction are considered The equilibrium constants were calculated directly alone, a similar treatment gives 43.6 f 1.0 kcal/mol from the vapor pressure for each point on the basis of and -1.03 f 0.13 cal/mol deg for the respective heat the stoichiometry of the formulated reaction. The and entropy. Therefore, the data in this temperature mean values, with standard deviations, in reciprocal range can be represented by one equation within the atmospheres, are (1.68 f 0.17) X 1O-l1, (1.45 f 0.25) experimental uncertainty and with the assumption that (2.24 k:0.13) X lO-'O, (5.34 f 0.92) X X any heat capacity effects are negligible. (1.39 f 0.14) X and (4.60 f 0.63) X for Discussion 639.0, 718.5, 749.0, 800.5, 850.0, and 899.0', respectively. Examination of the vapor pressure data reveals an Figure 3 is a typical van't Hoff plot of the data. agreement with the literature3r4with regard to the volaThis plot shows that the lower point A a t 639.0', which tility of the solid or liquid vanadium pentoxide in the represents the equilibrium constant of the solid vanapresence of steam and oxidizing atmosphere, but a disdium pentoxide reaction with steam, is linear with the agreement as to the identity of the gaseous hydroxide other points of the liquid vanadium pentoxide-steam species formed. reaction. Within the uncertainty of the measureTaking the value for the heat of sublimation of Vz05ments, this point is linear because of the proximity of (s) to (VzO&(g) and the value for the depolymerization this temperature to the melting point (656') of V206(s). of (VzO&(g), reported by Berkowitz, Chupka, and
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Volume 7?2, Number 9 September 1068
L. N. UANNOPOULOS
3296 Inghram' and also quoted by Glemser and a value of 91.0 kcal/mol for the change V205(s) Tzt VzOS(g) is calculated. Combining this value with our value of 44.0 kcal/mol for the heterogeneous reaction yields a heat value of -47.0 kcal/mol for the corresponding homogeneous reaction V206(g)
+ 3&0(g)
2VO(OH)a(g)
With K e l l e y ' ~ ~entropies ,~ of formation for V205(s), v205(1), and HzO(g),plus our entropy of reaction, the values for the entropy of formation of the gaseous species, VO(OH)a(g), a t 800 and 1000"Kwere calculated as 114.1 and 130.4 eu, respectively. Similarly, from literature data8pgand from the free energies of reaction, the free energy function value, (GoT- H02g8.1a)/T, of VO(OH)a(g) is estimated to be -66.4 and -72.8 cal/mol deg a t 800 and lOOO"K, respectively. The corresponding calculated8 values for the analogous gaseous oxyhalide species, VOCla(g), are 105.4 and 111.0 eu for their entropy of formation, and -90.5 and -94.5 cal/mol deg for their free energy function at the respective temperatures of 800 and 1000"K. The gaseous species isostructural to voc13(g), i e . , POC13(g) and POFs(g), have values for their entropies of formation8rg a t 800°K of 100.2 and 88.2 eu, respectively. These values compare reasonably with the value of 105.4 eu for VOC&(g) a t this temperature. From the entropy value, -15.0 eu, of Glemser and Mullera for their reported reaction VzOj(s) 2HzO(g) 0% + VZO,(OH)~(~), a value of 161.0 eu for the entropy of formation of V203(OH)4(g)at 800°K is obtained. There is no information available in the literature on any gaseous oxyhalide compound, V203X4(g), analogous to this pyrovanadic-type gaseous oxide hydroxide species. Furthermore, if a least-square analysis of the vapor pressure data tabulated by Glemser and Muller3 is performed, the values for the slopes of the isothermal 0.38, 1.96 f. 0.24, and plots are 1.65 rt 0.07, 2.11 2.05 rt 0.18 for 500, 550, 600, and 650", respectively.
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The Journal of Physical Chemistry
The authorsa assumed an average slope of 2.0 to arrive at the stoichiometry of the V205(s) H20(g) reaction which justifies their conclusion of the existence of a V203(OH)4gaseous species. I n view of (a) their low 0.07, which corresponds closer to our value of 1.65 slope of 1.5, (b) the larger scattering of their data as evidenced by the deviations of the slope values at 550 and 600", (c) the absence of flow rate data on their part to demonstrate the existence of equilibrium, and (d) the absence of an analogous gaseous oxyhalide, such as V203x4, the presence of such a complex gaseous hydroxide as a major product of the high temperature Vz05(sor 1) H20(g) reaction is considered doubtful. The proximity of the values for the entropies of formation of VO(OH),(g) and VOCL(g) and Wells'lO slightly distorted tetrahedral structure of the latter molecule derived from electron diffraction data, suggests an isostructural configuration, such as
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0
II
7\ 1 OH
HO
OH for the gaseous vanadium oxide trihydroxide, VO(0H)s.
Acknowledgment. The author wishes to thank Dr. W. S. Koski of the Johns Hopkins University and Dr. G. R. B. Elliott of the Los Alamos Scientific Laboratory for their encouragement and helpful suggestions. This work was supported in part b y ' t h e U. S. Atomic Energy Commission. (7) J. Berkowitr, W. A. Chupka, and M. G. Inghram, J. Chem. Phys., 27, 87 (1957). (8) K. K. Kelley, "Contributions to the Data on Theoretical Metallurgy," U. s. Bureau of Mines Bulletin 684, U. s. Government Printing Office, Washington, D. C., 1960. (9) K. K. Kelley, "Contributions to the Data on Theoretical Metallurgy," U. 8. Bureau of Mines Bulletin 692, U. 8. Government Printing Office, Washington, D. C., 1961. (10) A. F. Wells, "Structural Inorganic Chemistry,'! 2nd ed, Oxford University Press, London, 1950.
