Post on 28-Jan-2016
Complex Ionsand stuff like that.
A complex ion is the exception to a rule...
It is an ionic compound that has an overall charge.
Complex ions are identified using square brackets. [ ]
Complex Ions
Are soluble.
Usually formed from solutions containing precipitates.
There are 7 you need to memorise.
Complex Ions
Complex Ions• Formed if a precipitate disappears when excess
reagent is added.• Metal cations with several ligands attached.• Ligands have a pair of non-bonding electrons
e.g. H20, NH3, OH-, SCN-
• Usually* the number of ligands is twice the charge on the cation. E.g. Cu2+ forms [Cu(NH3)4]2+
* Except Al3+
Iron Thiocyanate
FeSCN2+
Silver diamine [Ag(NH3)2]+
Aluminium tetrahydroxide
[Al(OH)4]-
Lead tetrahydroxide
[Pb(OH)4]2-
zinc tetrahydroxide
[Zn(OH)4]2-
Zinc tetraamine [Zn(NH3)4]2+
Copper tetraamine
[Cu(NH3)4]2+
Complex ions
[Al(OH)4]-
[Zn(OH)4]2-
[Pb(OH)4]2-
These hydroxide complexes are made by adding excess hydroxide to a hydroxide precipitate.
Making "OH" Complexes
[Cu(NH3)4]2+
[Ag(NH3)2]+
[Zn(NH3)4]2+
These ammonia complexes are made by adding excess (ammonia) ammonium hydroxide to a hydroxide precipitate.
Making NH3 Complexes
Aluminium ions + Sodium hydroxide =
Aluminium hydroxide ppt - Al(OH)3
Adding excess Sodium hydroxide =
Al(OH)3 + OH- --> [Al(OH)4]-
[Al(OH)4]-
Copper ions + Ammonia =
Copper hydroxide ppt - Cu(OH)2
Adding excess Ammonia =
Cu(OH)2 + 2OH- --> [Cu(NH3)4]2+
[Cu(NH3)4]2+When used in small amounts OH- react, when used in large amounts (excess)
NH3 reacts.
add 2 drops of dilute NaOH solution
orange precipitate formsFe3+
add 2 drops KSCN solution
dark red solution confirms Fe3+
New sample
Cloudy (precipitate);Fe(OH)3 is NOT
soluble
Clear solution:Product is
soluble
Observation What it means Species formed
Precipitate is formed (solution becomes
cloudy)
Example: Mixing Iron(III)nitrate solution with sodium hydroxide
solution
A compound has been formed (from ions present) that is
not soluble.
Example: Iron3+ ions form a compound with
OH- ions.
The compound is not charged. Use swap and drop rule to find
formula for compound.
Example:
Fe(OH)3
Solution changes colour and/or
becomes clear.
Example: Iron(III)hydroxide
dissolves with potassium thiocyanate
solution (red)
A complex ion has been formed (from ions present) that is
soluble.
Example: Iron3+ ions form complex ions with
SCN- ions (thiocyanate).
The complex ion is charged.
Learn formulae for 7 complex ions (square
brackets).
Example:
[Fe(SCN)]2+
Observation What it means Species formed
Precipitate is formed (solution becomes
cloudy)
Example: Mixing Iron(III)nitrate solution with sodium hydroxide
solution
A compound has been formed (from ions present) that is
not soluble.
Example: Iron3+ ions form a compound with
OH- ions.
The compound is not charged. Use swap and drop rule to find
formula for compound.
Example:
Fe(OH)3
Solution changes colour and/or
becomes clear.
Example: Iron(III)hydroxide
dissolves with potassium thiocyanate
solution (red)
A complex ion has been formed (from ions present) that is
soluble.
Example: Iron3+ ions form complex ions with
SCN- ions (thiocyanate).
The complex ion is charged.
Learn formulae for 7 complex ions (square
brackets).
Example:
[Fe(SCN)]2+
blue precipitate then deep blue solution Cu2+
blue precipitate formsCu2+
brown precipitate formsAg+
brown precipitate then colourless solution Ag+
add 2 drops, then excessNH3 solution
add 2 drops, then excessNH3 solution
orange precipitate formsFe3+
green precipitate formsFe2+
white precipitate formsAl3+, Pb2+
white precipitate forms and disappears, Zn2+
add dilute H2SO4 solution
colourless solutionAl3+
white precipitatePb2+
add dilute H2SO4 solution
colourless solution
Mg2+
white precipitateBa2+
add 2 drops KSCN
solution
dark red solution confirms Fe3+
precipitate disappears
Al3+, Zn2+, Pb2+
precipitate remains
Mg2+, Ba2+
add 2 drops, thenexcess NH3 solution
white precipitate formsAl3+, Zn2+, Pb2+, Mg2+,
Ba2+
add excessNaOH
solution
add 2 drops of dilute NaOH solution.
no precipitateNH4
+, Na+
add NaOH solution, heat, test gas with
red litmus.
litmus stays red
Na+
litmus goes blue NH4
+
Find the steps in the scheme where the 7
complex ions are formed.
Hint: Look out for “excess” amounts added and the
disappearance of a precipitate.
precipitate disappears
Al3+, Zn2+, Pb2+
add excess NaOH solution
Equations:
Al3+ + 4 OH- → [ ]
Zn2+ + 4 OH- → [ ]
Pb2+ + 4 OH- → [ ]
Al(OH)4 -
Zn(OH)4 2-
Pb(OH)4 2-
Complete the equations
and work out the charge!
White precipitate forms and disappears
Zn2+
add 2 drops, then excess NH3 solution
Equation:
Zn2+ + 4 NH3 → [ ]Zn(NH3)4 2+
Complete the equation and work out the
charge!
Dark red solution confirms
Fe3+
add 2 drops KSCN solution
Equation:
Fe3+ + SCN- → [ ]FeSCN 2+
Complete the equation and work out the
charge!
Blue precipitate, then deep blue solution
Cu2+
add 2 drops, then excess NH3 solution
Equation:
Cu2+ + 4 NH3 → [ ]Cu(NH3)4 2+
Complete the equation and work out the
charge!
Brown precipitate, then colourless solution
Ag+
add 2 drops, then excess NH3 solution
Equation:
Ag+ + 2 NH3 → [ ]Ag(NH3)2 +
Complete the equation and work out the
charge!
Written questions for end of assessment
• You will be given a list of observations• You need to methodically follow the flow charts and
make conclusions and equations as if you were actually carrying it out in real life.
• Eg: Question 1• No ppt with silver nitrate means sulfate or nitrate ion• White precipitate with barium nitrate means it must
be sulfate ion• Ba2+
(aq) + SO42-
(aq) BaSO4(s)