Significant discrepancies between van't Hoff and calorimetric enthalpies. III

Area of Science:

• Chemical Thermodynamics
• Physical Chemistry
• Solution Chemistry

Background:

• The enthalpy change (DeltaH) is crucial for understanding the temperature dependence of equilibrium constants in reversible reactions.
• It is commonly assumed that enthalpy is independent of temperature, but this assumption may not hold true for many reactions in solution.
• Discrepancies often exist between calculated enthalpy values (DeltaH(vH)) and those obtained through direct calorimetry (DeltaH(cal)).

Purpose of the Study:

• To investigate the temperature dependence of enthalpy changes for reversible reactions in solution.
• To compare calculated enthalpy values (DeltaH(vH)) with experimentally determined values (DeltaH(cal)).
• To identify reactions where DeltaH(vH) and DeltaH(cal) show agreement, despite temperature variations.

Main Methods:

• Studied reversible reactions in solution.
• Utilized isothermal titration calorimetry to measure enthalpy changes.
• Investigated specific reactions: heptylamine with heptanoic acid in dodecane, and alpha-cyclodextrin with sodium heptanoate in water.

Main Results:

• Enthalpy (DeltaH(vH)) is seldom independent of temperature for reversible reactions in solution.
• Temperature-dependent DeltaH(vH) values often differ significantly from direct calorimetric measurements (DeltaH(cal)).
• The study examined two specific systems to find agreement between these enthalpy values.

Conclusions:

• The assumption of temperature-independent enthalpy is often invalid for solution-phase reactions.
• Accurate thermodynamic analysis requires considering the temperature dependence of enthalpy.
• Further research is needed to identify conditions or reaction types where calculated and calorimetric enthalpies align.