CHMA11H3 Lecture Notes - Lecture 18: Lewis Acids And Bases, Transition Metal, Ion
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16 Feb 2019
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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