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Thermochemistry of aksaite
J. Chem. Thermodynamics 1999, 31, 1605–1608 Article No. jcht.1999.0563 Available online at http://www.idealibrary.com on
Thermochemistry of aksaite Jia Yongzhong, Li Jun, Gao Shiyang,a and Xia Shuping Department of Chemistry, Lanzhou University, Lanzhou 730000, P.R.C., and Xi’an Branch, Institute of Salt Lakes, Chinese Academy of Sciences, Xi’an 710043, P.R.C. The enthalpy of solution of aksaite MgB6 O10 · 5H2 O in approximately 1 mol · dm−3 aqueous hydrochloric acid was determined. With the incorporation of the standard molar enthalpies of formation of MgO(s), H3 BO3 (s), and H2 O(l), the standard molar enthalpy of formation of −(6063.65 ± 4.85) kJ · mol−1 of aksaite was obtained. The standard molar entropy of aksaite was also calculated according to the thermodynamic relation. c 1999 Academic Press
KEYWORDS: aksaite; standard molar enthalpy of formation; solution calorimetry; molar enthalpy of solution
1. Introduction Aksaite,(1, 2)
MgB6 O10 · 5H2 O, is a magnesium borate mineral with a structural formula of MgB6 O7 (OH)6 · 2H2 O and is found in several places in the world. Until now, there have been no reports on the thermodynamic properties of aksaite. Thermodynamic properties play very important roles in scientific research and industrial applications. As a part of the serial studies on the thermochemistry of hydrated borates,(3–6) we present here the standard molar enthalpy of formation 1f Hmo of aksaite. In addition, the standard molar entropy of aksaite was calculated.
2. Experimental Aksaite for calorimetric experiments was synthesized according to the modified procedure described in the literature.(2, 7) All the reagents were analytical grade (Beijing Chemical Factory, P.R.C.). The MgO was obtained by thermal decomposition of Mg(OH)2 · 4MgCO3 · 6H2 O in an electric furnace at T = 873 K for about 3 h. The MgO (20.0 g), H3 BO3 (186.0 g), MgCl2 · 6H2 O (1000.0 g), and H2 O (250.0 g) were transferred to a flask with a reflux condenser, the mixture was heated to the boiling a To whom correspondence should be addressed (E-mail: [email protected]).
0021–9614/99/121605 + 04 $30.00/0
c 1999 Academic Press
1606
J. Yongzhong et al. TABLE 1. The chemical composition of aksaite
(mass fraction, w) MgO
B2 O3
H2 O
Analytical
0.1189
0.6161
0.2650a
Theoretical
0.1188
0.6158
0.2654
a Calculated by difference.
temperature of 395.15 K and became a clear solution. After 15 d, Mg-borate crystallized from the clear solution. The solid was separated, washed thoroughly with distilled water, and then with alcohol and ether. Finally, the solid was dried at T = 333 K to constant mass. The synthetic sample was characterized by X-ray powder diffraction (recorded on a Rigaku D/MAX-2400, Ni-filtered and Cu-radiation), F.t.i.r. spectroscopy (recorded on a Nicolet 170SX FT-IR spectrometer with KBr pellets at room temperature), and Raman spectroscopy (recorded on a SPEX1403 spectrometer using the beam of 5145 nm at about 200 mW of argon ion laser with the sample being held in a pyrex tube). The sample was analyzed according to standard methods: magnesium was titrated with a standard solution of Na-EDTA in an alkaline buffer solution of (ammonium hydroxide + ammonium chloride), boron by a standard solution of NaOH in the presence of mannitol, and H2 O by difference. The chemical composition of aksaite is given in table 1. The mass fraction of aksaite is 0.9993. All the results showed that the synthetic sample is a pure compound and is suitable for calorimetric experiment. Impurity corrections were unnecessary. Aksaite can be regarded as the product of the following reaction: MgO(s) + 6H3 BO3 (s) = MgB6 O10 · 5H2 O(s) + 4H2 O(l).
(1)
The standard molar enthalpy of formation of aksaite could be obtained by solution calorimetry in combination with the standard molar enthalpies of formation of MgO(s), H3 BO3 (s), and H2 O(l). The solution calorimetric procedure used here is the same as that used in the previous paper.(3) The H3 BO3 and aksaite were dissolved in approximately 1 mol · dm−3 aqueous hydrochloric acid, and then the calculated amount of substance of MgO was dissolved in aqueous (hydrochloric acid + boric acid) which consisted of approximately 1 mol · dm−3 HCl(aq) and the calculated amount of H3 BO3 . The temperature of the calorimetric experiments was (298.15 ± 0.005) K. An LKB 8700 precision calorimeter was used and has been described in detail previously.(3) The calibrations were repeated before and after each experiment. The dissolution rate of aksaite in diluted aqueous hydrochloric acid is so slow that the aksaite crystals have to be reduced to powder to accelerate the solution rate. There were no solid residues observed after the reactions in all calorimetric experiments.
Thermochemistry of aksaite
1607
TABLE 2. The molar enthalpies of solution 1sol Hm of aksaite in approximately 1 mol · dm−3
aqueous hydrochloric acid at T = 298.15 Ka
No
m/mg
1T /K
ε/(J · K−1 )
1vap H/J
1sol Hm /(kJ · mol−1 )
1
208.6
−0.001243
10582.9
−0.050
21.47
2
199.0
−0.001111
11208.1
−0.050
21.31
3
204.6
−0.001255
10241.1
−0.050
21.39
4
200.7
−0.001140
11060.4
−0.050
21.40
5
207.8
−0.001286
10241.1
−0.050
21.58
6
203.0
−0.001139
11208.1
−0.050
21.42 h1sol Hm i = (21.43 ± 0.06) kJ · mol−1
b
a Determined with an LKB precision calorimeter; in each experiment, 100.10 cm3 of HCl(aq) was used. b Uncertainty is twice the standard derivation of the mean.
3. Results and discussion The results of the calorimetric experiments are given in table 2, in which m is the mass of sample, 1T is the temperature change of the calorimeter corrected for heat exchange with the environment, ε is the mean energy equivalent of the calorimetric system, 1vap H is the correction for saturating the free volume of the ampoules with water vapor, 1sol Hm is the molar enthalpy of solution of solute, and the uncertainty is twice the standard deviation of the mean. The enthalpy of solution of aksaite in approximately 1 mol · dm−3 aqueous hydrochloric acid is positive, while those of other magnesium hexaborates(3) in the same solvents are negative. In order to calculate 1vap H , the enthalpy of vaporization of water, 44.01 kJ · mol−1 ,(8) and the molar volume of 171.78 cm3 · mol−1 of aksaite(9) were required. The ampoules used for calorimetric experiments were all assumed to have an internal volume of 1 cm3 . The molar mass of aksaite is 339.24 g · mol−1 . Table 3 gives the thermochemical cycle for the derivation of the standard molar enthalpy of formation of aksaite. The molar enthalpies of solution of H3 BO3 (s) of (21.83 ± 0.08) kJ · mol−1 in approximately 1 mol · dm−3 HCl(aq), and of MgO(s) of −(146.20 ± 0.36) kJ · mol−1 in the mixture of HCl and H3 BO3 were taken from our previous work.(3) The standard molar enthalpies of formation of H2 O(l), MgO(s), and H3 BO3 (s) were taken from the CODATA Key Values,(10) namely −(285.830 ± 0.040) kJ · mol−1 , −(601.60 ± 0.30) kJ · mol−1 , and −(1094.8 ± 0.8) kJ · mol−1 , respectively. The enthalpy of dilution of HCl(aq) was calculated from the NBS tables.(11) From these data, the standard molar enthalpy of formation of aksaite was calculated to be −(6063.65 ± 4.85) kJ · mol−1 . For comparison, the 1f Hmo of −6007.00 kJ · mol−1 of aksaite was calculated using the group contribution method developed by Li Jun et al.(12) for the calculation of thermodynamic properties of hydrated borates. The calculated value is close to the experimental result. The relative error is 0.93 per cent. Because no experimental data on the 1f G om of aksaite are available, we used a group contribution method(12) to calculate 1f G om of aksaite to be −5495.64 kJ · mol−1 .
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J. Yongzhong et al.
o (aksaite, TABLE 3. Thermochemical cycle and results for the derivation of 1f Hm
298.15 K)a
Reaction 1.
6H3 BO3 (s) + 168.635(HCl · 54.664H2 O) =
2.
MgO(s) + 6H3 BO3 (aq) + 168.635(HCl · 54.664H2 O) =
3.
MgCl2 (aq) + 6H3 BO3 (aq) + 166.635(HCl · 54.326H2 O) =
6H3 BO3 (aq) + 168.635(HCl · 54.664H2 O) MgCl2 (aq) + 6H3 BO3 (aq) + 166.635(HCl · 55.326H2 O) MgB6 O10 · 5H2 O(s) + 168.635(HCl · 54.688H2 O) 4.
168.635(HCl · 54.688H2 O) =
5.
MgO(s) + 6H3 BO3 (s) =
168.635(HCl · 54.664H2 O) + 4H2 O(l) MgB6 O10 · 5H2 O(s) + 4H2 O(l)
1r Hm /(kJ · mol−1 )b
130.98 ± 0.48 −146.20 ± 0.36 −21.43 ± 0.06 0.08 ± 0.01 −36.57 ± 0.60
a Standard molar enthalpy of formation from the oxides. b Uncertainties are twice the standard deviation of the mean.
Finally, the standard molar entropy of aksaite was calculated to be 353.64 J · K−1 · mol−1 according to following reaction: MgB6 O10 · 5H2 O(s) = Mg(s) + 6B(s) + 5H2 (g) + 15/2O2 (g).
(2)
The standard molar entropies of the elements were taken from CODATA Key Values to be 32.67 J · K−1 · mol−1 , 5.90 J · K−1 · mol−1 , 130.571 J · K−1 · mol−1 , and 205.043 J · K−1 · mol−1 for Mg(s), B(s), H2 (g), and O2 (g), respectively. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Clark, J. R.; Erd, R. C. Am. Mineral. 1963, 48, 930–935. Lehmann, H. A.; Rietz, G. Z. Anorg. Allg. Chem. 1967, 350, 168–176. Li, J.; Gao, Sh. Y.; Xia, Sh. P.; Li, B.; Hu, R. Z. J. Chem. Thermodynamics 1997, 29, 491–497. Li, J.; Gao, Sh. Y.; Xia, Sh. P.; Li, B.; Hu, R. Z. J. Chem. Thermodynamics 1997, 29, 1071–1075. Li, J.; Gao, Sh. Y.; Li, B. J. Chem. Thermodynamics 1998, 30, 425–430. Li, J.; Gao, Sh. Y.; Li, B. J. Chem. Thermodynamics 1998, 30, 681–688. Gao, Sh. Y.; Xiao, G. Y.; Xia, Sh. P. Thermochim. Acta 1990, 169, 311–322. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey, S. M.; Schumm, R. H. Nat. Bur. Stand (U. S.) Tech. Note 270–3. 1968. Dal Negro, A.; Ungaretti, L.; Sabelli, C. Am. Mineral. 1971, 56, 1553–1566. Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics. Hemisphere: New York. 1989. Parker, V. B. Thermal Properties of Aqueous Uni-univalent Electrolytes. Natl. Stand. Ref. Data Ser.-NSRDS-NBS-2. 1965. Li, J.; Li, B.; Gao, Sh. Y. Phys. Chem. Mineral (submitted). (Received 5 February 1999; in final form 16 June 1999)
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Thermochemistry of aksaite Jia Yongzhong, Li Jun, Gao Shiyang,a and Xia Shuping Department of Chemistry, Lanzhou University, Lanzhou 730000, P.R.C., and Xi’an Branch, Institute of Salt Lakes, Chinese Academy of Sciences, Xi’an 710043, P.R.C. The enthalpy of solution of aksaite MgB6 O10 · 5H2 O in approximately 1 mol · dm−3 aqueous hydrochloric acid was determined. With the incorporation of the standard molar enthalpies of formation of MgO(s), H3 BO3 (s), and H2 O(l), the standard molar enthalpy of formation of −(6063.65 ± 4.85) kJ · mol−1 of aksaite was obtained. The standard molar entropy of aksaite was also calculated according to the thermodynamic relation. c 1999 Academic Press
KEYWORDS: aksaite; standard molar enthalpy of formation; solution calorimetry; molar enthalpy of solution
1. Introduction Aksaite,(1, 2)
MgB6 O10 · 5H2 O, is a magnesium borate mineral with a structural formula of MgB6 O7 (OH)6 · 2H2 O and is found in several places in the world. Until now, there have been no reports on the thermodynamic properties of aksaite. Thermodynamic properties play very important roles in scientific research and industrial applications. As a part of the serial studies on the thermochemistry of hydrated borates,(3–6) we present here the standard molar enthalpy of formation 1f Hmo of aksaite. In addition, the standard molar entropy of aksaite was calculated.
2. Experimental Aksaite for calorimetric experiments was synthesized according to the modified procedure described in the literature.(2, 7) All the reagents were analytical grade (Beijing Chemical Factory, P.R.C.). The MgO was obtained by thermal decomposition of Mg(OH)2 · 4MgCO3 · 6H2 O in an electric furnace at T = 873 K for about 3 h. The MgO (20.0 g), H3 BO3 (186.0 g), MgCl2 · 6H2 O (1000.0 g), and H2 O (250.0 g) were transferred to a flask with a reflux condenser, the mixture was heated to the boiling a To whom correspondence should be addressed (E-mail: [email protected]).
0021–9614/99/121605 + 04 $30.00/0
c 1999 Academic Press
1606
J. Yongzhong et al. TABLE 1. The chemical composition of aksaite
(mass fraction, w) MgO
B2 O3
H2 O
Analytical
0.1189
0.6161
0.2650a
Theoretical
0.1188
0.6158
0.2654
a Calculated by difference.
temperature of 395.15 K and became a clear solution. After 15 d, Mg-borate crystallized from the clear solution. The solid was separated, washed thoroughly with distilled water, and then with alcohol and ether. Finally, the solid was dried at T = 333 K to constant mass. The synthetic sample was characterized by X-ray powder diffraction (recorded on a Rigaku D/MAX-2400, Ni-filtered and Cu-radiation), F.t.i.r. spectroscopy (recorded on a Nicolet 170SX FT-IR spectrometer with KBr pellets at room temperature), and Raman spectroscopy (recorded on a SPEX1403 spectrometer using the beam of 5145 nm at about 200 mW of argon ion laser with the sample being held in a pyrex tube). The sample was analyzed according to standard methods: magnesium was titrated with a standard solution of Na-EDTA in an alkaline buffer solution of (ammonium hydroxide + ammonium chloride), boron by a standard solution of NaOH in the presence of mannitol, and H2 O by difference. The chemical composition of aksaite is given in table 1. The mass fraction of aksaite is 0.9993. All the results showed that the synthetic sample is a pure compound and is suitable for calorimetric experiment. Impurity corrections were unnecessary. Aksaite can be regarded as the product of the following reaction: MgO(s) + 6H3 BO3 (s) = MgB6 O10 · 5H2 O(s) + 4H2 O(l).
(1)
The standard molar enthalpy of formation of aksaite could be obtained by solution calorimetry in combination with the standard molar enthalpies of formation of MgO(s), H3 BO3 (s), and H2 O(l). The solution calorimetric procedure used here is the same as that used in the previous paper.(3) The H3 BO3 and aksaite were dissolved in approximately 1 mol · dm−3 aqueous hydrochloric acid, and then the calculated amount of substance of MgO was dissolved in aqueous (hydrochloric acid + boric acid) which consisted of approximately 1 mol · dm−3 HCl(aq) and the calculated amount of H3 BO3 . The temperature of the calorimetric experiments was (298.15 ± 0.005) K. An LKB 8700 precision calorimeter was used and has been described in detail previously.(3) The calibrations were repeated before and after each experiment. The dissolution rate of aksaite in diluted aqueous hydrochloric acid is so slow that the aksaite crystals have to be reduced to powder to accelerate the solution rate. There were no solid residues observed after the reactions in all calorimetric experiments.
Thermochemistry of aksaite
1607
TABLE 2. The molar enthalpies of solution 1sol Hm of aksaite in approximately 1 mol · dm−3
aqueous hydrochloric acid at T = 298.15 Ka
No
m/mg
1T /K
ε/(J · K−1 )
1vap H/J
1sol Hm /(kJ · mol−1 )
1
208.6
−0.001243
10582.9
−0.050
21.47
2
199.0
−0.001111
11208.1
−0.050
21.31
3
204.6
−0.001255
10241.1
−0.050
21.39
4
200.7
−0.001140
11060.4
−0.050
21.40
5
207.8
−0.001286
10241.1
−0.050
21.58
6
203.0
−0.001139
11208.1
−0.050
21.42 h1sol Hm i = (21.43 ± 0.06) kJ · mol−1
b
a Determined with an LKB precision calorimeter; in each experiment, 100.10 cm3 of HCl(aq) was used. b Uncertainty is twice the standard derivation of the mean.
3. Results and discussion The results of the calorimetric experiments are given in table 2, in which m is the mass of sample, 1T is the temperature change of the calorimeter corrected for heat exchange with the environment, ε is the mean energy equivalent of the calorimetric system, 1vap H is the correction for saturating the free volume of the ampoules with water vapor, 1sol Hm is the molar enthalpy of solution of solute, and the uncertainty is twice the standard deviation of the mean. The enthalpy of solution of aksaite in approximately 1 mol · dm−3 aqueous hydrochloric acid is positive, while those of other magnesium hexaborates(3) in the same solvents are negative. In order to calculate 1vap H , the enthalpy of vaporization of water, 44.01 kJ · mol−1 ,(8) and the molar volume of 171.78 cm3 · mol−1 of aksaite(9) were required. The ampoules used for calorimetric experiments were all assumed to have an internal volume of 1 cm3 . The molar mass of aksaite is 339.24 g · mol−1 . Table 3 gives the thermochemical cycle for the derivation of the standard molar enthalpy of formation of aksaite. The molar enthalpies of solution of H3 BO3 (s) of (21.83 ± 0.08) kJ · mol−1 in approximately 1 mol · dm−3 HCl(aq), and of MgO(s) of −(146.20 ± 0.36) kJ · mol−1 in the mixture of HCl and H3 BO3 were taken from our previous work.(3) The standard molar enthalpies of formation of H2 O(l), MgO(s), and H3 BO3 (s) were taken from the CODATA Key Values,(10) namely −(285.830 ± 0.040) kJ · mol−1 , −(601.60 ± 0.30) kJ · mol−1 , and −(1094.8 ± 0.8) kJ · mol−1 , respectively. The enthalpy of dilution of HCl(aq) was calculated from the NBS tables.(11) From these data, the standard molar enthalpy of formation of aksaite was calculated to be −(6063.65 ± 4.85) kJ · mol−1 . For comparison, the 1f Hmo of −6007.00 kJ · mol−1 of aksaite was calculated using the group contribution method developed by Li Jun et al.(12) for the calculation of thermodynamic properties of hydrated borates. The calculated value is close to the experimental result. The relative error is 0.93 per cent. Because no experimental data on the 1f G om of aksaite are available, we used a group contribution method(12) to calculate 1f G om of aksaite to be −5495.64 kJ · mol−1 .
1608
J. Yongzhong et al.
o (aksaite, TABLE 3. Thermochemical cycle and results for the derivation of 1f Hm
298.15 K)a
Reaction 1.
6H3 BO3 (s) + 168.635(HCl · 54.664H2 O) =
2.
MgO(s) + 6H3 BO3 (aq) + 168.635(HCl · 54.664H2 O) =
3.
MgCl2 (aq) + 6H3 BO3 (aq) + 166.635(HCl · 54.326H2 O) =
6H3 BO3 (aq) + 168.635(HCl · 54.664H2 O) MgCl2 (aq) + 6H3 BO3 (aq) + 166.635(HCl · 55.326H2 O) MgB6 O10 · 5H2 O(s) + 168.635(HCl · 54.688H2 O) 4.
168.635(HCl · 54.688H2 O) =
5.
MgO(s) + 6H3 BO3 (s) =
168.635(HCl · 54.664H2 O) + 4H2 O(l) MgB6 O10 · 5H2 O(s) + 4H2 O(l)
1r Hm /(kJ · mol−1 )b
130.98 ± 0.48 −146.20 ± 0.36 −21.43 ± 0.06 0.08 ± 0.01 −36.57 ± 0.60
a Standard molar enthalpy of formation from the oxides. b Uncertainties are twice the standard deviation of the mean.
Finally, the standard molar entropy of aksaite was calculated to be 353.64 J · K−1 · mol−1 according to following reaction: MgB6 O10 · 5H2 O(s) = Mg(s) + 6B(s) + 5H2 (g) + 15/2O2 (g).
(2)
The standard molar entropies of the elements were taken from CODATA Key Values to be 32.67 J · K−1 · mol−1 , 5.90 J · K−1 · mol−1 , 130.571 J · K−1 · mol−1 , and 205.043 J · K−1 · mol−1 for Mg(s), B(s), H2 (g), and O2 (g), respectively. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Clark, J. R.; Erd, R. C. Am. Mineral. 1963, 48, 930–935. Lehmann, H. A.; Rietz, G. Z. Anorg. Allg. Chem. 1967, 350, 168–176. Li, J.; Gao, Sh. Y.; Xia, Sh. P.; Li, B.; Hu, R. Z. J. Chem. Thermodynamics 1997, 29, 491–497. Li, J.; Gao, Sh. Y.; Xia, Sh. P.; Li, B.; Hu, R. Z. J. Chem. Thermodynamics 1997, 29, 1071–1075. Li, J.; Gao, Sh. Y.; Li, B. J. Chem. Thermodynamics 1998, 30, 425–430. Li, J.; Gao, Sh. Y.; Li, B. J. Chem. Thermodynamics 1998, 30, 681–688. Gao, Sh. Y.; Xiao, G. Y.; Xia, Sh. P. Thermochim. Acta 1990, 169, 311–322. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey, S. M.; Schumm, R. H. Nat. Bur. Stand (U. S.) Tech. Note 270–3. 1968. Dal Negro, A.; Ungaretti, L.; Sabelli, C. Am. Mineral. 1971, 56, 1553–1566. Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics. Hemisphere: New York. 1989. Parker, V. B. Thermal Properties of Aqueous Uni-univalent Electrolytes. Natl. Stand. Ref. Data Ser.-NSRDS-NBS-2. 1965. Li, J.; Li, B.; Gao, Sh. Y. Phys. Chem. Mineral (submitted). (Received 5 February 1999; in final form 16 June 1999)
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