How to define the chemical system
It is necessary to write the equilibrium reactions in the following way:
- only chemical components to the left of the
symbol, and
- only one product (e.g. a complex or a solid) to the right of the
symbol.
For example, for an aqueous system with inorganic carbon, the following species might be formed:
CO2(g), H2CO3(aq),
HCO3− and CO32−.
One of these species must be chosen as a component (the formation of
C(s), CH4(g), etc, is neglected here).
Water and H+ should always be chosen as components.
If HCO3− is chosen as component,
the following stoichiometric coefficients and equilibrium constants are obtained:
Reaction |
Stoichiometry |
logK° |
|
H2O |
H+ |
HCO3− |
|
H2O − H+
OH− |
1 | |
−1 | |
0 | |
−14.00 |
HCO3− − H2O + H+
CO2(g) |
−1 | |
1 | |
1 | |
7.83 |
HCO3− + H+
H2CO3(aq) |
0 | |
1 | |
1 | |
6.35 |
HCO3− − H+
CO32− |
0 | |
−1 | |
1 | |
−10.33 |
|
It is then very important not to mix things up with reactions like:
CO32− + 2 H+
H2CO3(aq)
which contains two products: if HCO3−
is the component, then both CO32− and
H2CO3(aq) are products!
Complexes are soluble products of reactions between a metal ion and ligands,
for example Cu(CO3)22−.
Originally they were called complex ions, but complex is also
used for neutral aqueous species such as Fe(OH)3,
CuCO3, etc.