Chemistry Reference
In-Depth Information
This general use of the term “valence” is useless today and therefore expendable.
The term “ion” and the
charge number
of the ion can be used for ionic compounds:
starting from Al
3+
ions, F
ions, or O
2
ions we can describe corresponding
symbols like {(Al
3+
)
1
(F
)
3
} or {(Al
3+
)
2
(O
2
)
3
} and shorten these symbols - if
necessary - to AlF
3
or Al
2
O
3
.
Nonmetal atoms can be described in their molecules with a special
bonding
number
: C atoms have the bonding number 4, O atoms have the bonding number
2 and H atoms 1. Molecular building sets are based on this idea; molecular
structures can even be predicted using bonding numbers. The terms “charge
number” and “bonding number” should be used until the nucleus-shell model will
show further details of ionic and molecular structures.
Energy.
Three terms have to be distinguished: energy, enthalpy, and free
enthalpy. The scientific term “energy” is used for constant volumes, the term
“enthalpy” for constant pressure conditions. So “enthalpy” should be introduced
at school since measurements of volumes with a syringe for example normally run
under constant pressure. Usually the thermodynamical distinction is not that strict
in lessons at school, the terms “reaction heat,” “thermal energy,” and “amount of
energy” are used parallelly. The free reaction enthalpy
G
is being discussed in the
context of “spontaneous” chemical reactions. It depends on both - the reaction
enthalpy,
D
D
H
, as well as reaction entropy,
D
S
, and the Gibbs-Helmholtz equation
shows the relation:
S
.
School chemistry often tries to set chemical processes apart from physical
processes. But since enthalpies are being measured for melting, evaporation,
or dissolution processes, which cannot be distinguished from reaction enthalpies,
it does not make sense to make the differentiation between chemical and physical
processes: every change of substance accompanied by energy transfer has to be
called a “chemical reaction.”
Different symbols are common for the qualitative specification of the energy
transfer. Terms like exothermic and endothermic or
D
G
¼ D
H
T
D
D
H
<
0 are possible:
hydrogen (g)
þ
oxygen (g)
!
water (l)
þ
thermal energy
(7.1)
hydrogen (g)
þ
oxygen (g)
!
water (l)
;
exothermic (or
D
H
<
0
Þ
(7.2)
If the thermal energy is put into one line with the substances connected by a “+”
[see (
7.1
)], students might think that the thermal energy is involved in the reaction
as some kind of “heat substance,” as it was believed centuries ago. Therefore, it is
advantageous to separate the substances from the energy transfer with a semicolon
[see (
7.2
)].
If quantitative measured energies are discussed, the values may be different. The
formation enthalpy of 1 mol of particles should be written as kJ/mol [see (
7.3
)], but
generally reaction enthalpy is indicated by the unit kJ only because the enthalpy is
connected to the number of moles as given by the reaction equation [see (
7.4
)]:
