Heat capacity Benson method

Heat Capacity, C° Heat capacity is defined as the amount of energy required to change the temperature of a unit mass or mole one degree typical units are J/kg-K or J/kmol-K. There are many sources of ideal gas heat capacities in the hterature e.g., Daubert et al.,"" Daubert and Danner,JANAF thermochemical tables,TRC thermodynamic tables,and Stull et al. If C" values are not in the preceding sources, there are several estimation techniques that require only the molecular structure. The methods of Thinh et al. and Benson et al. " are the most accurate but are also somewhat complicated to use. The equation of Harrison and Seaton " for C" between 300 and 1500 K is almost as accurate and easy to use ... 

Ideal-gas heat capacities may be estimated from gas-phase heat-capacity measurements, but more often they are calculated from statistical mechanics based on molecular information, usually obtained by spectroscopy [27-29]. The results of such calculations have been tabulated for many molecules [5,11-15,28-30]. Predictive methods also exist based on molecular structure the leading methods are reviewed in The Properties of Gases Liquids [15], Calculations from one of these methods (that of Benson) are available in the NIST Chemistry Webbook [5],... 

The work of Benson and co workers was fundamental for the development of additivity methods for estimation of the heats of formation, heat capacities and entropies of organic compounds in general, not only alkynes. They proposed the two simplest methods atom and bond additivity. These methods are often sufficient for estimation of the thermochemical properties of ideal gases, including also acetylenes. 

This chapter discusses methods of estimating the standard enthalpy of formation, entropy and heat capacity of molecules andfree radicals, using both BENSON s method and those of statistical mechanics. 

These examples only hint at the analysis of heats of formation of organic compounds that is possible. Benson and co-workers summarized the methods and data for calculations for the major functional groups in organic chemistry. ° In addition, the data allow calculation of heat capacities and entropies of these compounds in the same marmer in which heats of formation are determined. Heats of formation are valuable reference points in discussing the stabilities of various isomers or products of reactions, whether they are calculated by bond increments or group increments or are derived as part of a theoretical calculation. 

Berman (1988), in presenting a widely used set of thermodynamic data for metamorphic minerals, not only used a different heat capacity equation (Equation 3.28), but also a somewhat different method for neglecting the properties of the elements. The Berman-Brown method is in fact the same as Benson-Helgeson for enthalpy, but different for Gibbs energy ( 5.7.1, page 144). 

Related Calculations. As an indicates of the degree of accuracy of this method, the observed values forethanol, -propanol,andn-butanolare—2.35 x 10, —2.559 x 10, and—2.83 x 10 J/(kg mol), respectively. Deviations may be as high as 1.25 x 10 J/(kg mol). Benson s method may also be used to estimate ideal-gas heat capacities. 

---