Chemistry Reference
In-Depth Information
H
H
C
C
H
H
C
C
C
C
H
H
Fig. 5.2
Kekul
´
's hypothesis for the structure of the benzene molecule and formulae of some
substituted derivatives [
4
]
Since Roentgen's discovery of X-rays in 1895, many scientists worked fever-
ishly on the hypothesis of X-ray diffraction. Laue finally proved it in 1912 and
commented on his crucial thoughts: “The discovery of the X-ray interference
distinguishes the value of the scientific hypothesis” [
5
]. Laue arrived at his famous
hypothesis because every day he saw in his institute the crystal lattice models of
many crystals which were used in Munich by Sohnke and von Groth - and believed
in these structures. So he got the idea to take natural crystals like copper sulfate or
zinc sulfide as diffraction grids to realize interferences of X-rays by passing them
through these crystals (see Fig.
5.3
).
Many scientists have sent X-rays through crystals long before, but their
observations were confined to the directly passing ray, which did not show anything
more than the weakening by the crystal; they did not see the much weaker diffracted
rays. The hypothesis of taking crystals as diffraction grids gave the idea to search its
surroundings - and they have been found. In 1914 Bragg and his son published the
structure of sodium chloride in England, they formulated the Bragg equation as the
famous reflection requirement for X-rays and opened the great field of X-ray
crystallography.
Data collection:
The most important function of the experiment is the validation
of hypotheses. Another is to run tests to describe substances and to compare a
special substance with other matter. Traditionally density, melting, and boiling
temperature are measured to gain a first identification of the substance. Additionally
water solubility, solubility in other solvents, refraction index, viscosity, optical
activity, thermal conductivity, electrical conductivity, pH-dependency, redox
potential, equilibrium and stability constant, spectroscopic behavior, etc. can be
tested and the measured values tabulated.
Accordingly chemical processes can be described by determination of mass
and volume ratio of reactions, by stoichiometry of the converted amounts of subs-
tances, by the number of reacting atoms, ions and molecules, through structural
formulae and chemical equations. Furthermore solubility product, equilibrium
