Low temperature heat capacities and standard molar enthalpy of formation of sodium benzoate C6H5COONa (s)

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Abstract

Sodium benzoate was synthesized by the method of liquid phase synthesis, in which benzoic acid and anhydrous sodium carbonate were chosen as the reactants. The structure and composition of the compound were characterized by FTIR, chemical analysis, elemental analysis and X-ray powder diffraction techniques. Low temperature heat capacities of the compound were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 400 K. A polynomial equation of the heat capacities as a function of the temperature was fitted by least square method. The smoothed heat capacities and the thermodynamic functions of the compound relative to 298.15 K have been calculated based on the equation. In accordance with Hess law, the standard molar enthalpy of formation of the title compound C6H5COONa (s) was determined to be ΔfH°m[C6H5COONa,s]=(642.56±0.64)kJmol1 by using an isoperibol solution-reaction calorimeter.

Introduction

Sodium is an essential element which exists in the form of positive ion in human body. It has the important physiological functions and nutritional values, and participates in the metabolism of water in the biological body. It can maintain the acid–base equilibrium in the human body and is also one of the important components in human body's muscle and nerve cells. The shortage of sodium may result in many kinds of diseases, such as nausea, cataplexy, low blood pressure, severe convulsion and so on. Benzoic acid has prominent physiological and chemical activity as a fine chemical, medical intermediate and food additive in the fields of medicine, cosmetic and nutrient, which can be used to synthesize a series of important products. The compounds of benzoic acid with many biological metals are the food preservatives and anti-microbial agents widely used in many foods from soup to cereals because they can inhibit microbial growth [1], [2]. In addition, it can greatly increase the solubility of medicament in body fluids, and obviously enhance the absorption ratio of useful drug in the human body [3].

The synthesis and characterization of sodium benzoate have been reported in literature [4]. However, until now, thermodynamic properties of the compound have not been found, which restricted the progress of relevant theoretical study and application development. The purpose of the present study is to measure low temperature heat capacities of the compound by adiabatic calorimetry and determine the dissolution enthalpies of the reactants and the products of the liquid phase reaction of sodium acetate with benzoic acid by an isoperibol solution-reaction calorimetry. Finally, some thermodynamic parameters such as the enthalpy change of the reaction and the standard molar enthalpy of formation of the product C6H5COONa (s) were derived from these experimental results.

Section snippets

Synthesis and characterization of the sodium benzoate C6H5COONa (s)

The compound was synthesized according to the method reported in the literature [4]. Benzoic acid and anhydrous sodium carbonate were chosen as the reactants, accurately weighed at the molar ratio of n(C6H5COOH):n(Na2CO3) = 2:1, put anhydrous sodium carbonate (about 5.30 g, 0.05 mol) into a beaker with a volume of 250 mL, and add 20 mL double deionized water. When the solution was heated up nearly to boil, then benzoic acid (about 12.2 g or 0.1 mol) dissolved in the water beforehand was slowly dripped

Low-temperature heat capacities

The experimental molar heat capacities were listed in Table 2 and plotted in Fig. 2. It was found out from Fig. 2 that the heat capacity curve of the compound was smoothly increased with the temperature, which showed that the structure of the compound was stable over the temperature range between T = 78 K and 400 K, no phase change, association nor thermal decomposition occurred. The 106 experimental points in the temperature region between T = 78 K and 400 K were fitted by the least square method, and

Conclusions

  • (1)

    This paper reports low temperature heat capacities measured by adiabatic calorimetry and the dissolution enthalpies of the reactants and the products of the liquid phase reaction of sodium acetate with benzoic acid by isoperibol solution calorimetry. Furthermore, the thermodynamic functions and standard molar enthalpy of formation of the product C6H5COONa (s) were derived from these experimental results.

  • (2)

    The enthalpy change of the liquid phase reaction (1) obtained by solution calorimetry was

Acknowledgement

This work was financially supported by the National Science Foundation of China under the contract NSFC No. 20673050.

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