CHMA11H3 Lecture Notes - Lecture 18: Lewis Acids And Bases, Transition Metal, Ion

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CHMA11 Lecture 18: Qualitative Chemical Analysis and Common Ion Equilibria
- Qualitative Chemical Analysis: using differences in compounds solubility to separate
ions via selective precipitation
o Selective precipitation: systematic way to determine which metal ions are
present in an unknown solution
o Qualitative indicates that this process involves qualities or properties of
compounds
It involves finding the kinds of ions found in the solution
Different from quantitative analysis methods
These methods are more concerned with determining the
quantity of substances in a mixture
o Basic theory of qualitative analysis
Samples containing mixture of metal cations is subjected to the addition
of several precipitating agents
At each point a precipitating agent is added, specific metal cations will
precipitate out of the mixture and can be separated out as solids
The remaining aqueous mixture is then subjected to the next
precipitating agent so the process can continue
Key is that after the addition of each of the precipitating agents,
the remaining mixture is quickly put into a centrifuge device
o This is done to separate all of the precipitated solid from
the remaining liquid
o The liquid is then subjected to further precipitation while
the solid can be set aside for further analysis
We continue this process until we are left with a solution containing only
the desired ions we want
- Complex Ion Equilibria
o Phenomenon that involves transition metal ions in solution
Remember transition metals tend to be very good at accepting electrons
This makes them good Lewis acids
In aqueous solutions, water will tend to act as electron donors
This makes the water act as Lewis base
The electrons are added to the transition metal ions to hydrate
them
o Complex Ion: ion that contains a central metal ion bound to one or more ligands
Ligand: neutral molecule or ion that acts as a Lewis base with the central
metal ion
The addition of a stronger Lewis base will result in the displacement of
the original Lewis base that is bound to the complex ion
Example scenario
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