Equilibrium constants from spectrophotometric data: dimer formation in gaseous Br2

Area of Science:

• Chemical Thermodynamics
• Spectroscopy
• Physical Chemistry

Background:

• The dimerization of bromine (2Br 2 right arrow over left arrow Br 4) is a key gas-phase reaction.
• Previous estimates of the equilibrium constant (K) relied on PVT data, lacking precision.
• Accurate K values are crucial for understanding bromine chemistry.

Purpose of the Study:

• To precisely determine the equilibrium constant for bromine dimerization using spectrophotometry.
• To obtain high-precision spectra of bromine monomer and dimer.
• To refine thermodynamic parameters for the dimerization reaction.

Main Methods:

• Employed a classic spectrophotometric method with precise data acquisition.
• Utilized a multiwavelength fitting approach to analyze spectra.
• Incorporated baseline absorption parameters to account for data sensitivity.

Main Results:

• Estimated the equilibrium constant K c = 2.5 ± 0.4 L/mol at 22°C.
• Obtained spectra for bromine monomer (peak 227 nm) and dimer (205 nm) with high precision.
• The new K value is more than double the previous estimate.

Conclusions:

• The revised equilibrium constant provides a more accurate thermodynamic profile for bromine dimerization.
• The study highlights the sensitivity of spectrophotometric analysis to baseline parameters.
• Derived thermodynamic parameters (ΔH° = -9.5 kJ/mol, ΔS° = -51 J mol⁻¹ K⁻¹) are refined based on the new K value.