Chemistry Reference
In-Depth Information
Table 7.2 Acid particles in several substances
Substance
Acid-particles
Spectator ions
H
3
O
+
(aq) ions
Cl
(aq) ions
Hydrochloric acid (aq)
H
3
O
+
(aq) ions
SO
4
2
(aq) ions, HSO
4
(aq) ions
Sulfuric acid (aq)
Pure sulfuric acid (l)
H
2
SO
4
molecules
-
HSO
4
ions
Na
+
ions
Sodium hydrogen sulfate (s)
H
2
PO
4
ions
Na
+
ions
Sodium dihydrogen phosphate(s)
aqueous solutions and should not be called “false.” However, that problem needs to
be discussed with the students in chemistry lessons.
According to the Brønsted definition, it has to be emphasized that the term
“acid” does not mean a substance, but an acid particle; some examples show this
(see Table
7.2
). The H
3
O
+
(aq) ions are written with the intention to show particles
that can donate one proton and leave H
2
O molecules; sometimes the symbol H
+
(aq)
ion is taken because it is shorter to write.
For alkaline solutions, these two levels can be distinguished by using different
terms. One can speak of sodium hydroxide solution or sodium hydroxide (solid)
with the meaning of a substance. The term “base” instead should be used for
particles that can accept protons: OH
ions or NH
2
ions or NH
3
molecules. For
acids, one has to deduce the meaning of the term “acid” as a substance or as a
particle from its context. Particles cannot generally be categorized into acids and
bases. Depending on the reaction partner, some particles react as acids as well as
bases: H
2
OorNH
3
molecules, OH
or HSO
4
ions. These are also called
amphiprotic. For acid-base reactions, it is useful to symbolize protolysis and
acid-base pairs.
Isomerism.
Substances with the same molecular formula, but different molecular
structures, are called isomers. There are two main forms of isomerism: constitu-
tional and stereoisomerism. The first one can be subdivided into positional isomer-
ism,
constitutional
isomerism) and geometric isomerism (
cis
-
trans
isomerism), optical as well as
conformational
tautomerism, valence, as well as functional
isomerism (
¼
stereo isomerism) (see Fig.
7.2
).
Stereoisomers, whose molecules contain several asymmetrical carbon atoms and
that are no optical isomers are called diastereoisomers (for example,
(rotational)
isomerism (
¼
a
-
D
-glucose
and
b
-
D
-glucose).
7.2 Teaching Processes: From Everyday to Expert
and Symbol Language
“Every lesson has to start with the child's experience.” (Dewey) “New experiences
that children gain during lessons are being organized with the help of pre-existing
concepts.” (Ausubel)
These famous statements are nowadays being confirmed empirically by educa-
tional psychologists who support constructivism. Individuals build their own
