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The dissociation energy of lithium hydroxide
193
The Dissociation Energy of Lithium Hydroxide In recent years there have been a number of measurements of the energy of dissociation of LiQH into Li and OH at OrKi D, fLiOH) [I-IO]. The older values were obtained by using tentative data for molecufar parameters [I, 2, 5, 6, 71. Recently new estimates of the molecular parameters have been published [IO], Only one of the reported D,, (LiOH) values was obtained by using this new set of estimates [8]. Besides, the published values are scattered over a range of 5 kcdjmole. Therefore it is surprising that un~il now no critical comparison of the reporled data has been published. A COTrect value of D, (LiOH) is of importance in order to apply the Li/LiUH method of mcasuring the free H-atom concentration in flames [ll J. Here we report the value obtained by one of us in C,H,-air flames [6], and we present a critical comparison of al1 reported data in order to deduce the most probable D, value and its error limit. The value of D, (LiOH) is determined from vapor pressure measurements [2, 101 or from measurements of the equilibrium constant, K, of the reaction Li + H,O eLiOH + H [ll]. The slope of a plot of In K against I/T yields a value of the enthalpy change at temperature T, AH, (second-law method), or AHo may be derived from a single absolute value of K at a known temperature (third-law method). By using tabulated values of sz cF JT fOF the reaction partners, Aff, can be converted into
A,H, (cP is specific heat). Then D,(LiOH) found from
is
D, (LDH) = ID, (H&I) - A& in which D,iH,O) stands for the dissociation energy of H,O into H and OH. Table 1 presents the values of D, (LiOH), as recalculated by us from the newest thermodynamic data published by JANAF [IO]. The values originally reported are given in parentheses. If only K values have been reported, we derived DO(LiOH) therefrom, using the JANAF data. The error limits of tlx values of Gurvich and Ryabova and of Cotton and Jenkins have been estimed by us. The other error limits are those reported by the iiuthoxs. The following comments may be made. I. From measurements of the ratio [LiOH], [Li] in a HI-air flame Smith and Sugden [i] calculated the dissociation constant of LiOH (h’, = [Li][OH]/[LiOH]), using the OH equi!ibriumconcentration calculated from the known flame temperature and gas composition. It is now we.11known that the concentration of flame radicals such as H and OH may vary strongly with height shortly above the reaction zone hecause of a relaxation in the establishment of thermodynamic equilibrium. This would n&kc: the D, value calculated by Smith and Sugden systematicalty too low. McEwan and PhilIips [5] have come to the same conclusion from their comparison of the Li/LiOH and the NaCl
P. J. Th. Zeegers and C. Th. J. Aikemade
$34
Flame Measurements --__“,
_--...
__---Second La\v
Third Law _l_.ll__-l”--_.-_:99 k i;
i03.h + 2 105.3 ?C3
105s i 2 103 I 2 107 51 103.7 * I
._I. Vupor Pressure -~-.-_---..-~. ~--- ._-
(102) 105 +4
(115)
105 +s
(105)
11041 !lOS) (101)
104.1 f 1
estimation they used an lr-parameter value of 0.3, reported by Hollander [12], which was measured in a CQ-air flame at 1964 K. HOWever, Hofmann and Kohn [I 3] have reported A = 0.73 for thtiir C&%,-air flame at 2500 K, Cotton and Jenkins do not discuss their reasons for preferring Hollander’s value for their HZ-OL-N, flame at 2370 K. However, we think that the real value of their a-parameter map well lie in between the two values. A higher value of the a-parameter yields a lower value of the absorption coefficient, which leads to a lower value of D,(LiOH) and consequently :n a better agreement between both vaiues reported by Cotton and Jenkins. If, for example, the a-parameter is assumed to be 0.5, the absorption coefficient is lower by a factor of 1.2. In turn, this results in a D,(LiOH) value that is lower by 0,P kcal/mole. Therefore, we prefer their lower value. However, we increased their error limit from 0.3 to 1 kcal,&nole, since their figure was based only on an error in the temperature. The vibration frequencies, the bond angle, and the bond distances of LiOH are not known very accurately. They are estimated from the corresponding quantities for LiF, H,O, DHO,
THO, and Li2U, determirted from infrared spectra or from microwave 3tuJics.. An error in the m&Mar constants will influence mainly the D, values calculated by the third-law method. This influence will be the more important, the hjghcr the temperature of the medium in which K is measured. The secondlaw values are Iess dependent on molecular constants. It is illterestiq to note that reasonable agreement exists between the average second- and third-law valuec: in Table I. On the basis of the experimental data available. at this morne~~t, we recommend for the dissociation energy of LiOH into Li and OH the value D,(LiOH) = 104.5 * 1 kcdl/mo!e. The error limit claimed includes the uncertainty in the molecular parameters.
References
1 _
4 5. h. 7. s. 4.
t0. 1:. 12. 13.
F>Gscli LaboialOrium Kijksuniversiteir Utrec:lt Utrecht. The Netherlands
The Dissociation Energy of Lithium Hydroxide In recent years there have been a number of measurements of the energy of dissociation of LiQH into Li and OH at OrKi D, fLiOH) [I-IO]. The older values were obtained by using tentative data for molecufar parameters [I, 2, 5, 6, 71. Recently new estimates of the molecular parameters have been published [IO], Only one of the reported D,, (LiOH) values was obtained by using this new set of estimates [8]. Besides, the published values are scattered over a range of 5 kcdjmole. Therefore it is surprising that un~il now no critical comparison of the reporled data has been published. A COTrect value of D, (LiOH) is of importance in order to apply the Li/LiUH method of mcasuring the free H-atom concentration in flames [ll J. Here we report the value obtained by one of us in C,H,-air flames [6], and we present a critical comparison of al1 reported data in order to deduce the most probable D, value and its error limit. The value of D, (LiOH) is determined from vapor pressure measurements [2, 101 or from measurements of the equilibrium constant, K, of the reaction Li + H,O eLiOH + H [ll]. The slope of a plot of In K against I/T yields a value of the enthalpy change at temperature T, AH, (second-law method), or AHo may be derived from a single absolute value of K at a known temperature (third-law method). By using tabulated values of sz cF JT fOF the reaction partners, Aff, can be converted into
A,H, (cP is specific heat). Then D,(LiOH) found from
is
D, (LDH) = ID, (H&I) - A& in which D,iH,O) stands for the dissociation energy of H,O into H and OH. Table 1 presents the values of D, (LiOH), as recalculated by us from the newest thermodynamic data published by JANAF [IO]. The values originally reported are given in parentheses. If only K values have been reported, we derived DO(LiOH) therefrom, using the JANAF data. The error limits of tlx values of Gurvich and Ryabova and of Cotton and Jenkins have been estimed by us. The other error limits are those reported by the iiuthoxs. The following comments may be made. I. From measurements of the ratio [LiOH], [Li] in a HI-air flame Smith and Sugden [i] calculated the dissociation constant of LiOH (h’, = [Li][OH]/[LiOH]), using the OH equi!ibriumconcentration calculated from the known flame temperature and gas composition. It is now we.11known that the concentration of flame radicals such as H and OH may vary strongly with height shortly above the reaction zone hecause of a relaxation in the establishment of thermodynamic equilibrium. This would n&kc: the D, value calculated by Smith and Sugden systematicalty too low. McEwan and PhilIips [5] have come to the same conclusion from their comparison of the Li/LiOH and the NaCl
P. J. Th. Zeegers and C. Th. J. Aikemade
$34
Flame Measurements --__“,
_--...
__---Second La\v
Third Law _l_.ll__-l”--_.-_:99 k i;
i03.h + 2 105.3 ?C3
105s i 2 103 I 2 107 51 103.7 * I
._I. Vupor Pressure -~-.-_---..-~. ~--- ._-
(102) 105 +4
(115)
105 +s
(105)
11041 !lOS) (101)
104.1 f 1
estimation they used an lr-parameter value of 0.3, reported by Hollander [12], which was measured in a CQ-air flame at 1964 K. HOWever, Hofmann and Kohn [I 3] have reported A = 0.73 for thtiir C&%,-air flame at 2500 K, Cotton and Jenkins do not discuss their reasons for preferring Hollander’s value for their HZ-OL-N, flame at 2370 K. However, we think that the real value of their a-parameter map well lie in between the two values. A higher value of the a-parameter yields a lower value of the absorption coefficient, which leads to a lower value of D,(LiOH) and consequently :n a better agreement between both vaiues reported by Cotton and Jenkins. If, for example, the a-parameter is assumed to be 0.5, the absorption coefficient is lower by a factor of 1.2. In turn, this results in a D,(LiOH) value that is lower by 0,P kcal/mole. Therefore, we prefer their lower value. However, we increased their error limit from 0.3 to 1 kcal,&nole, since their figure was based only on an error in the temperature. The vibration frequencies, the bond angle, and the bond distances of LiOH are not known very accurately. They are estimated from the corresponding quantities for LiF, H,O, DHO,
THO, and Li2U, determirted from infrared spectra or from microwave 3tuJics.. An error in the m&Mar constants will influence mainly the D, values calculated by the third-law method. This influence will be the more important, the hjghcr the temperature of the medium in which K is measured. The secondlaw values are Iess dependent on molecular constants. It is illterestiq to note that reasonable agreement exists between the average second- and third-law valuec: in Table I. On the basis of the experimental data available. at this morne~~t, we recommend for the dissociation energy of LiOH into Li and OH the value D,(LiOH) = 104.5 * 1 kcdl/mo!e. The error limit claimed includes the uncertainty in the molecular parameters.
References
1 _
4 5. h. 7. s. 4.
t0. 1:. 12. 13.
F>Gscli LaboialOrium Kijksuniversiteir Utrec:lt Utrecht. The Netherlands