Enthalpies of formation and lattice enthalpies of alkaline metal acetates

Abstract

The standard (p° = 0.1 MPa) molar enthalpies of formation in the crystalline state of the alkaline metal acetates CH₃COOM (M = Li, Na, K, Rb, Cs), at T = 298.15 K, were determined by reaction-solution calorimetry as: ${\Delta }_{\text{f}}{H}_{\text{m}}°(\text{C}{\text{H}}_3\text{COOLi},\text{cr})=-(741.40\pm 0.95)$ kJ mol⁻¹, ${\Delta }_{\text{f}}{H}_{\text{m}}°(\text{C}{\text{H}}_3\text{COONa},\text{cr})=-(711.01\pm 0.51)$ kJ mol⁻¹, ${\Delta }_{\text{f}}{H}_{\text{m}}°(\text{C}{\text{H}}_3\text{COOK},\text{cr})=-(722.36\pm 0.49)$ kJ mol⁻¹, ${\Delta }_{\text{f}}{H}_{\text{m}}°(\text{C}{\text{H}}_3\text{COORb},\text{cr})=-(722.31\pm 1.09)$ kJ mol⁻¹, ${\Delta }_{\text{f}}{H}_{\text{m}}°(\text{C}{\text{H}}_3\text{COOCs},\text{cr})=-(726.10\pm 1.07)$ kJ mol⁻¹. These results, taken together with the enthalpies of formation of the haloacetates XCH₂COOM (M = Li, Na; X = Cl, Br, I) and chloropropionates ClCH(CH₃)COOM (M = Li, Na) re-evaluated from literature data were used to derive a consistent set of lattice energies, and discuss some general trends of the structure–energetics relationship for the CH₃COOM, XCH₂COOM, and ClCH(CH₃)COOM compounds, based on the Kapustinskii approximation. It was found that the lattice energies of the haloacetates are essentially independent of the halogen and ca. 17–25 kJ mol⁻¹ smaller than those of the corresponding acetates. In addition, no significant difference between the lattice enthalpy values of the haloacetates and chloropropionates was observed. Finally, linear correlations of very similar slope were obtained by plotting the MO bond distances derived from the Kapustinskii equation against the published experimental MO bond distances for alkaline metal acetates and alkoxides. The analysis of these relations suggests that the metal–oxygen bond distances for the lithium, potassium, and rubidium acetates, whose molecular structures in the solid state have not been determined, can be estimated as 214, 288, and 304 pm, respectively.

Introduction

The alkaline metal acetates (CH₃COOM, M = Li, Na, K, Rb, Cs) have a wide range of synthetic, industrial, and medical applications [1], [2]. Potassium acetate, for example, is used as a diuretic and in the purification of penicillin; lithium acetate is employed as a component of catalysts for the production of polyesters or as an additive to improve their physical properties.

A number of X-ray diffraction [3], [4], [5], [6] and thermophysical studies [7], [8], [9], [10], [11], [12] have tried to elucidate the structures and phase transitions occurring in these salts at different temperatures. Although some conflicting results exist [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], the emerging picture suggests that the structures adopted in the solid state at p = 0.1 MPa and T = 298 K strongly depend on the metal. Thus CH₃COOLi is triclinic, with V_cell/Z = 0.0876 nm³ [3], where V_cell is the unit cell volume and Z the number of asymmetric units it contains. For CH₃COONa two orthorhombic modifications are known, with V_cell/Z = 0.0901 (form I) and 0.0887 nm³ (form II), respectively [4]. Form II seems to be the thermodynamically stable one [4], [9]. The potassium and rubidium acetates are monoclinic with V_cell/Z = 0.1055 and 0.1108 nm³, respectively [5], [6]. Finally, CH₃COOCs adopts a hexagonal structure [5], [6] with V_cell/Z = 0.1467 nm³ [6]. Single crystal X-ray diffraction results available for the sodium [4], [13] and caesium [6], [13] derivatives indicate that on average the metal–oxygen short contacts [14] are d_NaO = 246 (form I) and 245 pm (form II), and d_CsO = 322 pm.

In addition to their industrial importance, these compounds are frequently used in discussions involving the ionic bond model in solids [15], [16], [17], [18], [19], [20], [21]. It is therefore surprising that no systematic investigation of their enthalpies of formation in the crystalline state, from which the corresponding lattice enthalpies can be derived, has been made up till now. These data were obtained in this work from reaction–solution calorimetry experiments and used to analyse the structure–energetics relationship in the alkaline metal acetates based on the Kapustinskii approximation.