Chemistry Reference
In-Depth Information
Absorbed frequencies can be displayed by comparing and calculating the signals of
the unchanged reference beam, characteristic absorption peaks form in the
IR spectrum. HCl molecules in carbon tetrachloride solution, for example, have
apeakat2,800cm
1
(2,800 vibrations per cm). The corresponding frequency
(vibrations per second) of the IR radiation has been absorbed by themolecules. Certain
groups of atoms independently absorb IR radiation in the same wave length range, the
functional groups show characteristic absorption bands: the molecular structure of
a new substance can be derived from those absorption bands in the IR spectrum.
Computers are today able to print three-dimensional pictures of molecular structures,
which serves as a template for building molecular models with the help of molecular
kits - a molecular model can be build by these and additional information.
Chemical symbols
: The combination of nonmetal atoms can lead to molecules,
which exist in a linear or bent structures, nonmetal atoms build rings, tetrahedral,
octahedral, or any other shape. Combinations of nonmetal atoms can also form
infinite structures like giant or honeycomb structures (see Fig.
10.25
).
Molecular and giant structures can be described with structural symbols as well
as with stoichiometric symbols [
9
]. Since the structural symbol is often very
complicated, one can switch to the stoichiometric symbol, if there is only one
possible structure. But as soon as there are isomeric structures, the structural
symbol must be used.
The molecular symbol always states the composition of the whole molecule and
the arrangement of all atoms in the molecule, like C
6
H
6
for the benzene molecule -
shortening this symbol to C
1
H
1
or CH is not usual. In most cases, half-structural
symbols are sufficient: the structure of the ethanol molecule is then stated with the
symbol C
2
H
5
OH or CH
3
CH
2
OH, the structure of the acetic acid molecule with
CH
3
COOH (see Fig.
10.26
).
10.5.6 Reactions of Nonmetals: Regrouping of Atoms
in Molecules
White phosphorus reacts with oxygen to form white crystals of solid phosphorus
pentoxide. All three substances are built of molecules, the solid substances phos-
phorus and phosphorus pentoxide have a molecule lattice. The reaction symbol for
the regrouping of molecules can be derived from the corresponding models of the
molecular structures (see Fig.
10.27
).
The gases hydrogen and oxygen react to form water vapor that condenses at room
temperature to liquid water. If a hydrogen-oxygen mixture is being ignited in a
eudiometer tube at room temperature, it can be observed that the mixture completely
reacts to water, when the volume ratio is 2:1. If this experiment is run in a heated
eudiometer tube at temperatures
t
100
C, two volume parts of hydrogen react with
one volume part of oxygen to produce two volume parts of water vapor.
This phenomenon can be explained with the regrouping of H
2
and O
2
molecules to
H
2
O molecules and illustrated with a model drawing (see Fig.
10.28
). The basis of
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