We know a lot about different kinds of ionic compounds and how they will behave in aqueous
solution.
Some are water soluble, and will dissolve in water, while some are water insoluble,
and will remain solid in water, and can also form as a precipitate if the individual ions
involved find each other in solution.
But we also want to learn about complex ions.
These are large ions in which a number of groups called ligands will coordinate to a
central atom to form a complex that has an overall positive or negative charge.
For example, silver chloride is not water soluble, because the lattice energy keeping
these ions bound together in the solid phase is greater than the energy of solvation that
will result if the ions break apart.
However, silver chloride is somewhat soluble in ammonia, because these substances can form
a complex ion.
In ammonia, silver chloride will dissociate and two ammonia molecules can coordinate to
a silver one ion to form this complex ion and the chloride ion.
Because the main chemistry that is occurring is that the silver ion and the ammonia molecules
form the complex ion, we can cancel out the chloride ions from both sides, as this will
be a spectator ion, which leaves us with the following net ionic equation, which exclusively
shows the formation of the ion.
This doesn’t happen only in ammonia, this can happen in water as well.
If aluminum nitrate is dissolved in water, it will dissociate to form aluminum three
ions and nitrate ions, but six water molecules can coordinate to each aluminum ion to form
the following complex ion.
In both of these cases, we simply need to make sure each ion is balanced.
If there are two ammonias in this complex ion, two ammonias must be involved on the
reactant side.
For this one, if six water molecules are needed to form the complex ion, there must be six
waters on the reactant side.
Once again, it is not the dissociation of the ionic compound that is the main chemistry
occurring, as this is technically just a phase change.
It is the formation of the complex ion that we really want to be discussing.
For this reason, we can highlight this chemistry in the following equation, and notice that
nitrate is a spectator ion this time.
We can cancel that out, which leaves us with the following net ionic equation.
Just as with other types of equations we have tried to balance, it is important to see what
chemistry is really occurring, whether it is electron transfer, proton transfer, or
complex ion formation, and be able to take an overall equation and cancel out any spectator
ions to give a net ionic equation that exclusively describes the chemistry that is going on in
solution.
Let’s check comprehension.