Chemical Equilibria

How to define the chemical system

It is necessary to write the equilibrium reactions in the following way:

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°

 H2  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.