Significant discrepancies between van't Hoff and calorimetric enthalpies

Area of Science:

• Physical Chemistry
• Chemical Thermodynamics

Background:

• The van't Hoff equation is commonly used to analyze the temperature dependence of equilibrium constants (K).
• It assumes a linear relationship between ln K and 1/T, with slope -delta HvH/R.
• This linear approximation is frequently applied in chemical equilibrium studies.

Purpose of the Study:

• To investigate the validity of the linear van't Hoff approximation.
• To determine if the apparent enthalpy (delta HvH) is constant with temperature.
• To compare apparent enthalpy with calorimetrically measured enthalpy.

Main Methods:

• Analysis of equilibrium constant (K) data across a range of temperatures (T).
• Calculation of the van't Hoff apparent enthalpy (delta HvH) from ln K vs. 1/T plots.
• Comparison of delta HvH with true enthalpy values derived from calorimetric measurements.

Main Results:

• The assumption of a linear ln K vs. 1/T plot is often invalid.
• The van't Hoff apparent enthalpy (delta HvH) is temperature-dependent in many cases.
• Significant discrepancies exist between delta HvH and true enthalpy, as well as in heat capacity changes (delta Cp).

Conclusions:

• The traditional application of the van't Hoff equation can lead to inaccurate enthalpy values.
• Temperature-dependent enthalpy and heat capacity changes must be considered for accurate thermodynamic analysis.
• Re-evaluation of equilibrium constant temperature variation studies is warranted.