not exist as a classical “Newtonian” reality. On the other hand, chemical structural
theory is ufeasible without the introduction of relatively drastic approximations
used from physics: molecule possesses the “chemical structure” only within the
assumption of so called Born-Oppenheimer approximation (Max Born, 1882–
1970, Robert Oppenheimer, 1904–1967). This heuristic axiom starts with the fact
that the movements of electrons are so much faster than the movements of atomic
nuclei, that we could build the model of molecule in which atomic nuclei are
immovable. This conceptualism of entities in chemistry is not limited to atoms and
molecules. Soon we will demonstrate that electrons can also be “physical” and
“chemical”.
The conclusion that follows is important: the same category, for instance the
structure, or electrons, could be considered within different epistemologies
depending on the level of complexity in which the entities in consideration appear.
Emergent properties on chemical level of complexity can be analysed in the best
way by using chemical concepts, which cannot be completely reduced to physical
concepts. As it has been analysed in the first chapter, every complexity level can be
well represented exclusively by its own epistemology.
LeBel’s key experiment was the observation of the declining of the plane of the
polarized light in left of right side, what depends whether the solution contains
“left-handed” [l] or “right-handed” [d] molecules. Physical parameter, the declination of the polarized light (optical rotation) LeBel has associated to molecular
structure. This important step in chemistry has triggered the establishment of new
branch of chemistry, the physical chemistry (or, chemical physics). The scope of
this field of science is the investigation of relationships between measured physical
parameters such as melting points, boiling points, refractive index, density, and
chemical structures of molecules.
The basic task of physical chemistry has been ambitious: predicting physical
parameters of particular substance starting from the chemical structural theory.
Some attempts of such predictions of properties of chemical elements can be found
already in the Mendeleev periodic law.
François-Marie Raoult (1830–1901) was professor at the University of
Grenoble who has decided to correlate melting and boiling points with chemical
compositions of compounds. All chemists know for the Raoult’s law about the
partial pressures in mixtures of liquids: partial pressure of vapour of every component of the ideal mixture of liquids is equal to the vapour pressure of the pure
component multiplied with the molar fraction of this liquid. Without entering in the
details, we can simply say that, by using this law, it is possible to calculate the
relative molecular mass from the measured depression of melting point of different
mixtures. The relative molecular mass is related to the molecular structure, its
composition.
Physical chemistry has, more or less successfully, found not only a series of
correlations of physical measurements with molecular structures, but also the laws
of the rates of chemical reactions. Van’t Hoff has systematically measured the rates
of chemical reactions establishing in this way chemical kinetics as a new branch of
chemistry. Van’t Hoff has also defined a notion of molecularity of reactions, i.e.
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11 Models and Reality
theory is ufeasible without the introduction of relatively drastic approximations
used from physics: molecule possesses the “chemical structure” only within the
assumption of so called Born-Oppenheimer approximation (Max Born, 1882–
1970, Robert Oppenheimer, 1904–1967). This heuristic axiom starts with the fact
that the movements of electrons are so much faster than the movements of atomic
nuclei, that we could build the model of molecule in which atomic nuclei are
immovable. This conceptualism of entities in chemistry is not limited to atoms and
molecules. Soon we will demonstrate that electrons can also be “physical” and
“chemical”.
The conclusion that follows is important: the same category, for instance the
structure, or electrons, could be considered within different epistemologies
depending on the level of complexity in which the entities in consideration appear.
Emergent properties on chemical level of complexity can be analysed in the best
way by using chemical concepts, which cannot be completely reduced to physical
concepts. As it has been analysed in the first chapter, every complexity level can be
well represented exclusively by its own epistemology.
LeBel’s key experiment was the observation of the declining of the plane of the
polarized light in left of right side, what depends whether the solution contains
“left-handed” [l] or “right-handed” [d] molecules. Physical parameter, the declination of the polarized light (optical rotation) LeBel has associated to molecular
structure. This important step in chemistry has triggered the establishment of new
branch of chemistry, the physical chemistry (or, chemical physics). The scope of
this field of science is the investigation of relationships between measured physical
parameters such as melting points, boiling points, refractive index, density, and
chemical structures of molecules.
The basic task of physical chemistry has been ambitious: predicting physical
parameters of particular substance starting from the chemical structural theory.
Some attempts of such predictions of properties of chemical elements can be found
already in the Mendeleev periodic law.
François-Marie Raoult (1830–1901) was professor at the University of
Grenoble who has decided to correlate melting and boiling points with chemical
compositions of compounds. All chemists know for the Raoult’s law about the
partial pressures in mixtures of liquids: partial pressure of vapour of every component of the ideal mixture of liquids is equal to the vapour pressure of the pure
component multiplied with the molar fraction of this liquid. Without entering in the
details, we can simply say that, by using this law, it is possible to calculate the
relative molecular mass from the measured depression of melting point of different
mixtures. The relative molecular mass is related to the molecular structure, its
composition.
Physical chemistry has, more or less successfully, found not only a series of
correlations of physical measurements with molecular structures, but also the laws
of the rates of chemical reactions. Van’t Hoff has systematically measured the rates
of chemical reactions establishing in this way chemical kinetics as a new branch of
chemistry. Van’t Hoff has also defined a notion of molecularity of reactions, i.e.
108
11 Models and Reality