Standard Molar Enthalpies of Formation of Sodium Alkoxides
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J. Chem. Thermodynamics 39 (2007) 449–454 www.elsevier.com/locate/jct Standard molar enthalpies of formation of sodium alkoxides K. Chandran a, T.G. Srinivasan a, A. Gopalan b, V. Ganesan a,* a Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, TN, India b Industrial Chemistry Department, Alagappa University, Karaikudi 630 006, TN, India Received 14 April 2006; received in revised form 9 July 2006; accepted 20 July 2006 Available online 12 August 2006 Abstract The molar enthalpies of solution of sodium in methanol, ethanol, and n-propanol and of sodium alkoxides in their corresponding alcohols were measured at T = 298.15 K using an isoperibol solution calorimeter. From these results and other auxiliary data, the stan- dard molar enthalpies of formation Df H mðRONa; crÞ of sodium methoxide, sodium ethoxide, and sodium n-propoxide were calculated and found to be {(À366.21 ± 1.38), (À413.39 ± 1.45), and (À441.57 ± 1.18)} kJ Æ molÀ1, respectively. A linear correlation has been found between Df H mðRONaÞ and Df H mðROHÞ for R = n-alkyl, enabling the prediction of data for other sodium alkoxides. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sodium methoxide; Sodium ethoxide; Sodium n-propoxide; Solution calorimeter; Enthalpy of formation 1. Introduction viz. methanol [2], ethanol [3], Jaysol SS [4], propanol [5,6], butyl cellosolve [7], ethyl carbitol [8,9], etc., have been used Liquid sodium is used as coolant in fast reactors (FRs). world wide for sodium removal purposes. During the operation of these reactors, a thin layer of There were reports of run-away reactions leading to sodium adheres to the steel components in the coolant cir- accident when ethyl carbitol was used for cleaning sodium cuit due to wetting. During reactor maintenance, some of storage tanks in France [8] and Germany [9]. Measure- these components need to be replaced or maintained peri- ments of thermochemical data such as enthalpy of forma- odically. Exposure of such sodium wetted components to tion, enthalpy of solution, heat capacity, thermal air could lead to fire and possible hydrogen explosion as decomposition behaviour, etc., of sodium alkoxides are the reaction between sodium and moisture present in air useful for better understanding of sodium–alcohol reaction is highly exothermic in nature. In addition to posing safety and to avoid recurring of such eventualities during sodium related problems, this reaction also adversely affects the cleaning processes. The preparation and characterization mechanical properties of the steel components due to the of sodium alkoxides namely, sodium methoxide, sodium formation of sodium hydroxide. These problems could be ethoxide, and sodium n-propoxide were reported elsewhere tackled by using suitable solvent to dissolve sodium under [10]. Heat capacity measurements, crystal structure elucida- controlled conditions and avoiding residual sodium tion, thermal decomposition, and kinetic analysis of all hydroxide on the components. Lutton et al. [1] have these sodium alkoxides were studied and reported else- described several sodium removal techniques. The highly where [11–13]. Since data on enthalpy of solution of reactive sodium metal can be converted into its less reactive sodium alkoxides in alcohol and enthalpy of formation of compound either by using an aqueous or a non-aqueous sodium n-propoxide are not available, the present work reagent. Among the non-aqueous reagents, several alcohols was taken up. The enthalpy of formation of sodium meth- oxide, and sodium ethoxide was also measured in the pres- * Corresponding author. Tel.: +91 44 27480098; fax: +91 44 27480065. ent study, compared with the reported data [14–19] as E-mail address: [email protected] (V. Ganesan). given in table 1 and discussed. 0021-9614/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2006.07.024 450 K. Chandran et al. / J. Chem. Thermodynamics 39 (2007) 449–454 2. Experimental calorimetric vessel was made up of a double walled, bright silvered, vacuum-sealed glass Dewar having a capacity of 2.1. Chemicals 400 cm3 and fitted with a perspex flange with an O-ring. The top flange has appropriate provisions to facilitate the Nuclear grade sodium (mass fraction purity 0.995) from introduction of stirrer, a sample bulb, a calibration heater Alkali Metals, India, was further purified by vacuum distil- and a thermistor probe. The calibration heater used was a lation. The HPLC grade methanol (mass fraction purity PTFE-coated KARMA wire with a resistance of 4.35 X, 0.998) from Ranbaxy Fine Chemicals, India, absolute eth- wound non-inductively over a PTFE disc and enclosed in anol (mass fraction purity 0.999) from Hayman, UK, and a glass bulb. The bulb was filled with n-hexadecane AR grade n-propanol (mass fraction purity 0.995) from (>0.99 mass fraction purity) from Kasci Kogyo, Tokyo, SD Fine Chemicals, India, were further purified by distilla- enough to submerge the heating element for better heat tion [20] and were used for the preparation of their respec- transfer. The sample tube had a thin walled bulb at the tive sodium alkoxides. base, provision for introducing a plunger, which could move up and down through a leak tight fitting. Using this 2.2. Preparation of sodium alkoxides plunger the thin walled base could be broken in order to introduce the sample in to the solvent. The calorimetric The reaction of sodium metal with alcohol to yield the vessel was kept in a constant temperature water bath hav- respective sodium alkoxide was performed in dry argon ing a control accuracy of ±0.01 K. The temperature of the atmosphere using a gas-tight glass vessel. Solid sodium bath was maintained at 298.15 K throughout the experi- pieces weighing about 500 mg each were added to 150 cm3 ment. A thermistor probe with a sensitivity of 2 Æ 10À4 K of alcohol taken in the reaction vessel. When the alcohol was used to measure the temperature in the calorimetric 1 was nearly saturated, the addition of sodium was stopped. vessel. A HP 34401A, 62 digit, multimeter was used for pre- The excess alcohol was removed by vacuum distillation cise current measurement. A control unit consisting of a and the reaction product was dried at room temperature stabilised power-supply unit, a constant-current power for about 6 h. The temperature of the reaction vessel was supply unit with a quartz crystal based timer and glitch free subsequently raised to 353 K to remove the final traces of solid-state relay was used for switching the supply ON and alcohol. A free flowing milky white powder of sodium alk- OFF for a preset time during the electrical calibration. oxide was obtained and stored in the argon atmosphere Both temperature and time data were acquired through a glove box. The formation and phase purity of sodium alkox- personal computer. The calorimeter performance was ides were confirmed by IR spectroscopy and X-ray powder tested using KCl from NIST (SRM 1655), which had been diffraction studies and the chemical assay was determined by heated at T = 800 K to remove occluded water, cooled and Atomic Emission Spectroscopy and CHNS analyser. stored in a dry argon atmosphere glove box. Since KCl is moisture sensitive, weighing and loading operations were 2.3. Calorimeter performed in an argon atmosphere dry box. Each addition of KCl was carried out with 100 cm3 of distilled water. The The schematic of an iso-peribol solution calorimeter calorimeter performance was also tested with tris (hydroxy used in the present work was reported elsewhere [21]. The methyl) amino-methane (Tris). The enthalpy of solution TABLE 1 Thermochemical data on (sodium + alcohol) systems Serial No. Author Reaction scheme DsolH mðRONa=ROHÞ= Df H mðRONa; crÞ= ðkJ Á molÀ1Þ ðkJ Á molÀ1Þ 1 Forcrand and Berthelot [14] Na CH OH CH ONa 1=2H 200.96 þ 3 excess !½ 3 CH3OH þ 2 À Na C H OH C H ONa 1=2H 187.02 þ 2 5 excess !½ 2 5 C2H5OH þ 2 À Na n-C H OH C H ONa 1=2H 177.19 þ 3 7 exces !½ 3 7 C3H7OH þ 2 À 2 Blanchard et al. [15] 2C H ONa 0:1N H SO soln Na SO C H OH 490.8 ± 5.7 2 5 þ 2 4 !½ 2 4H2O þ½ 2 5 H2O À 3 Voice [16] Na C H OH C H ONa 1=2H 189a þ 2 5 excess !½ 2 5 C2H5OH þ 2 À 4 Leal et al. [17] CH ONa H O NaOH CH OH 58.3 ± 2.4 372.4 ± 2.4 3 þ 2 excess !½ H2O þ½ 3 H2O À À C H ONa H O NaOH C H OH 60.9 ± 1.8 411.6 ± 1.9 2 5 þ 2 excess !½ H2O þ½ 2 5 H2O À À i-C H ONa H O NaOH C H OH 54.0 ± 1.5 461.6 ± 1.7 3 7 þ 2 excess !½ H2O þ½ 3 7 H2O À À CH ONa 0:1 N HCl soln NaCl CH OH 113.6 ± 3.0 375.7 ± 3.1 3 þ !½ H2O þ½ 3 H2O À À 5 This work Na CH OH CH ONa 1=2H 203.95 ± 1.30 366.21 ± 1.38 þ 3 excess !½ 3 CH3OH þ 2 À À Na C H OH C H ONa 1=2H 190.43 ± 1.34 413.39 ± 1.45 þ 2 5 excess !½ 2 5 C2H5OH þ 2 À À Na n-C H OH n-C H ONa 1=2H 180.87 ± 0.84 441.57 ± 1.18 þ 3 7 excess !½ 3 7 C3H7OH þ 2 À À CH ONa CH OH CH ONa 76.84 ± 0.34 3 þ 3 excess !½ 3 CH3OH À C H ONa C H OH C H ONa 54.84 ± 0.39 2 5 þ 2 5 excess !½ 2 5 C2H5OH À n-C H ONa C H OH n-C H ONa 41.90 ± 0.66 3 7 þ 3 7 excess !½ 3 7 C3H7OH À a Value taken from figure since no raw data were available.