Abstract:

Benzoic acid and lithium hydroxide with the analytical grade were chosen as the reactants, and the lithium benzoinate was synthesized by means of hydrothermal synthesis. XRD, FTIR and elementary and chemical analyses were applied to characterize its composition and structure. Low-temperature heat capacities of the compound were measured via a precision automated adiabatic calorimeter over the temperature range from 80—400 K. A polynomial equation of the heat capacities as a function of the temperature was fitted by the least square method. A reasonable thermochemical cycle was designed on the basis of the preparation reaction of the substance, 0.1 mol/L of HCl solution was chosen as the calorimetric solvent, and the standard molar enthalpies of dissolution for the reactants and products of the hydrothermal reaction in the selected solvents were measured by an isoperibol solution-reaction calorimeter, respectively. The enthalpy change of the preparation reaction was determined as Δ_rH_m⁰=-(9.75±0.27) kJ/mol from the standard molar enthalpies of dissolution. Eventually, the standard molar enthalpy of formation of anhydrous lithium benzoinate was derived to be Δ_fH_m⁰=-(307.82±0.57) kJ/mol by Hess′s law.

Key words: Lithium benzoinate; Adiabatic calorimetry; Low-temperature heat capacity; Solution-reaction calorimeter; Standard molar enthalpy of formation