chapter one the estimation of physical properties

THE ESTIMATION OF PHYSICAL PROPERTIES
1.6 CHAPTER ONE
consisting of small spherically-symmetric molecules. Typical corresponding-states
correlations express a desired dimensionless property as a function of P r , T r , and
the chosen third parameter.
Unfortunately, the properties of strongly polar molecules are often not satisfactorily
represented by the two- or three-constant correlations which do so well for
nonpolar molecules. An additional parameter based on the dipole moment has often
been suggested but with limited success, since polar molecules are not easily characterized
by using only the dipole moment and critical constants. As a result, although
good correlations exist for properties of nonpolar fluids, similar correlations
for polar fluids are often not available or else show restricted reliability.
Structure and Bonding
All macroscopic properties are related to molecular structure and the bonds between
atoms, which determine the magnitude and predominant type of the intermolecular
forces. For example, structure and bonding determine the energy storage capacity
of a molecule and thus the molecule’s heat capacity.
This concept suggests that a macroscopic property can be calculated from group
contributions. The relevant characteristics of structure are related to the atoms,
atomic groups, bond type, etc.; to them we assign weighting factors and then determine
the property, usually by an algebraic operation that sums the contributions
from the molecule’s parts. Sometimes the calculated sum of the contributions is not
for the property itself but instead is for a correction to the property as calculated
by some simplified theory or empirical rule. For example, the methods of Lydersen
and of others for estimating T c start with the loose rule that the ratio of the normal
boiling temperature to the critical temperature is about 2:3. Additive structural increments
based on bond types are then used to obtain empirical corrections to that
ratio.
Some of the better correlations of ideal-gas heat capacities employ theoretical
values of C p (which are intimately related to structure) to obtain a polynomial
expressing C p as a function of temperature; the constants in the polynomial are
determined by contributions from the constituent atoms, atomic groups, and types
of bonds.
1-4 ORGANIZATION OF THE BOOK
Reliable experimental data are always to be preferred over results obtained by
estimation methods. A variety of tabulated data banks is now available although
many of these banks are proprietary. A good example of a readily accessible data
bank is provided by DIPPR, published by the American Institute of Chemical Engineers.
A limited data bank is given at the end of this book. But all too often
reliable data are not available.
The property data bank in Appendix A contains only substances with an evaluated
experimental critical temperature. The contents of Appendix A were taken
either from the tabulations of the Thermodynamics Research Center (TRC), College
Station, TX, USA, or from other reliable sources as listed in Appendix A. Substances
are tabulated in alphabetical-formula order. IUPAC names are listed, with
some common names added, and Chemical Abstracts Registry numbers are indicated.
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