Calorimetric study of the thermodynamics of iron (III) complexation with nicotinic acid in aqueous ethanol

The changes in enthalpy of Fe(III) complex formation with nicotinic acid (Delta H-r), the dissolution enthalpies of nicotinic acid in aqueous ethanol (mole fraction of the organic component varied in the range 0 < X-EtOH < 0.7 mol fractions) were determined by calorimetry at the temperature 298.15 +/- 0.01 K and I = 0.25 mol center dot L-1 (NaClO4). The changes in enthalpy of transfer of nicotinic acid in a mixed solvent Delta H-tr(0)(HNic) were calculated. An increase in the value of Delta H-tr(0)(HNic) was observed. The changes in the enthalpy of transfer of the complexation reaction from water to a mixed solvent Delta(tr)Hr re calculated. The significant increase in the exothermicity of the reaction is observed when a small amount of ethanol is added to water. With a further increase in the concentration of ethanol in the solution, a decrease in the heat effect is observed up to the transition to the endothermic region at X-EtOH of more than 0.5 mol. fr. The experimental data are considered using an approach based on the thermodynamic characteristics of the solvation of each participant in the processes under study.The analysis of the contributions of reactants enthalpy of reaction into formation [FeHNic](3+) showed that the change in enthalpy of the reaction, when changing the solvent composition, is caused by the changes in solvation state of nicotinic acid at X-EtOH from 0 to 0.1 mol. fr. and desolvation of the reaction product in more concentrated solutions (X-EtOH > 0.1 mol. fr.).

Automated summary

The changes in enthalpy of Fe(III) complex formation with nicotinic acid and the dissolution enthalpies of nicotinic acid in aqueous ethanol were measured. The results showed an increase in exothermicity of the reaction when ethanol was added, but a decrease in heat effect with further increasing ethanol concentration. Analysis of the solvation thermodynamic characteristics revealed that the change in solvent composition caused the changes in enthalpy of the reaction.