Benson group increment theory

The Benson method is a method to estimate the enthalpy of formation, of formation and heat capacity of substances in the ideal gas phase. The method was developed by Benson in 1968, later optimized with modern computer programs and further modified for other states of matter.

Principle

The method is based on the assumption that every atom of a molecule contributes a specific contribution to the thermodynamic properties of a molecule. The binding types of all atoms must be known in principle for this spatial geometries ( structural isomers, conformers / rotamers and symmetries ) only lead to smaller features editor at the accumulation of increments (individual posts).

System using the example of methanol

Of methanol consisting of a methyl group and a hydroxy group, the increments are to be used for this purpose refers to the C- (H3) (O) and O (H) (C).

• The calculation of the enthalpy of formation in the gas phase ( 298 K), the two increments and -42.26 -159.33 kJ / mol to be used ( methanol calcd -201.59; . Exp -201.10 kJ / mol).
• To calculate the molar heat capacity Cp, the increments are 25.73 and 18.16 J / mol K used (methanol calc 43.89, exp. 43.89 J / mol K)
• To calculate the entropy of a symmetry contribution must in addition to the increments 127.32 and 121.50 still be considered by -9.13 J / mol K (methanol calcd 239.69, exp. 239.70 J / mol K).

The work of Domalski (1993 ) allowed with other increment rates as well as the calculation of thermodynamic data for the liquid and solid state.

Enthalpy of reaction of methanol

The real benefit lies in being able to calculate these thermodynamic quantities for any molecules. Heats of reaction ( endothermic / exothermic) chemical reactions can be estimated from the differences in the enthalpies of formation exquisite of all reactants, for example, in the oxidation of methanol.

The enthalpy of formation of methanol is -201.6 kJ / mol. During the oxidation of water form ( -241.5 kJ / mol) and formaldehyde ( = -108.6 kJ / mol ), the process in the gas phase is strongly exothermic (calculated as -148 kJ / mol). By allowing the formaldehyde gas in water, so the hydrate forms (calc. = -380.9 kJ / mol). This hydration is slightly exothermic, the equilibrium lies almost completely on the side of the aldehyde hydrate.

Reference data for enthalpies of formation can be found in the CRC Handbook of Chemistry and Physics in Chapter 5.4

Optimizations

The Benson method was further developed in the sequence against the background of powerful computer systems of several physical chemists.