Chemistry Reference
In-Depth Information
discussed and challenged with the help of display models. Despite initial difficulties
with scientific models, the learner is open to give up his alternative conception for a
new mental model, when concrete models of the structure of matter are conse-
quently used. Advanced mental models can be developed afterwards.
Reduction of complex contexts
. The reduction of difficult contexts is a funda-
mental teacher's task: wherever possible complex contexts have to be reduced
without losing the essentials. If the definition of element and compound is being
reduced to the mental model of elements being built of atoms and compounds being
built of molecules - as it happened in the 1950s - this reduction has to be avoided.
If the chemical bonding of atoms in molecules is being reduced to the mental model
of atomic valences and one introduces the bonding number 4 for the C atom or the
bonding number 1 for the H atom, the reduction is reasonable and easily applicable
to the methane molecule CH
4
, and other hydrocarbon molecules. This conception
is acceptable - it can be extended in lessons on the nucleus-shell model later on
to form the mental model of binding electron pairs between the atoms in molecules.
Generalization of facts
. Beginning with the use of models, students are often
able to generalize. If the CH
4
-tetrahedron is being introduced as a model of the
methane molecule and the extension to the ethane molecule is demonstrated, then
students are able to deduce all homologues of the alkane series by generalization.
They develop a spatial ability for the structure of molecules and are more and more
able to understand usual structural formulae or even half-structural symbols.
Illustration of reactions
. If a chemical reaction is to be interpreted, structural
models of the substances before and after the reaction can be demonstrated to the
learner (see Chap. 7). Before the reaction to prepare and illustrate ester formation,
for example, the structure of the involved acetic acid molecule and alcohol mole-
cule has to be demonstrated with the molecular model kit. The elimination of
a water molecule and the reaction equation using half-structural formulae can be
written and illustrated with structural models:
CH
3
COOH
þ
C
2
H
5
OH
!
CH
3
COOC
2
H
5
þ
H
2
O
No expert would describe the ester formation with empirical formulae only:
C
2
H
4
O
2
þ
C
2
H
6
O
1
!
C
4
H
8
O
2
þ
H
2
O
It is easy for reactions of organic chemistry: molecular structures before and after
the reaction have to be taught and half-structural symbols have to be derived -
otherwise empirical formulae of the above kind have to be memorized. In inorganic
chemistry it is often thought that one can use symbols like Na, Zn or Al for metals and
NaCl, ZnS or Al
2
O
3
for the corresponding salts. Without illustrating the structure of
metals and salts by sphere packings and crystal structure models, however, learners
will develop misconceptions of molecules: NaCl molecules or ZnS molecules. The
empirical formulae lack any structural information and therefore these formulae are
not helpful to understand substances and reactions: mental models of the structure of
metals and salts have to be demonstrated (see [
16
-
19
,
23
] and also Chaps. 7 and 10).
