1. Complex Ions
and stuff like that.

2. Complex Ions 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. [ ]

3. Complex Ions Are soluble.
Usually formed from solutions containing precipitates.
There are 7 you need to memorise.

4. 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+
Al3+ forms tetra….
H/O and expt complex ions practical and theory
EXPT 2 full unknowns writ and hand in steps, observations, name formula equations for ppts and balanced equations for complex ions formed – mark and return
AS 2.1 takes 1.5 hours
* Except Al3+

5. Complex ions 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+
Learn all these, you are likely to be tested on those in red
Fe3+ as SCN is 1-
A precipitate of AgCl(s) (left) readily dissolves in an aqueous solution of NH3 because of the formation of the complex ion [Ag(NH3)2]+ (right). The moleuclar view of the final solution shows [Ag(NH3)2]+ and Cl- ions and NH3 molecules.

7. Making "OH" Complexes
[Al(OH)4]-
[Zn(OH)4]2-
[Pb(OH)4]2-
These hydroxide complexes are made by adding excess hydroxide to a hydroxide precipitate.

8. Making NH3 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.

9. [Al(OH)4]- Aluminium ions + Sodium hydroxide =
Aluminium hydroxide ppt - Al(OH)3
Adding excess Sodium hydroxide =
Al(OH)3 + OH- --> [Al(OH)4]-

10. [Cu(NH3)4]2+ Copper ions + Ammonia = Copper hydroxide ppt - Cu(OH)2
When used in small amounts OH- react, when used in large amounts (excess) NH3 reacts.
Copper ions + Ammonia =
Copper hydroxide ppt - Cu(OH)2
Adding excess Ammonia =
Cu(OH) OH- --> [Cu(NH3)4]2+

11. Cloudy (precipitate); add 2 drops KSCN solution
add 2 drops of dilute NaOH solution
Cloudy (precipitate);
Fe(OH)3 is NOT soluble
orange precipitate forms Fe3+
New sample
add 2 drops KSCN solution
dark red solution confirms
Fe3+
Clear solution:
Product is soluble

12. 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+

13. 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+

14. 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+
precipitate remains Mg2+, Ba2+
add 2 drops, then excess NH3 solution
white precipitate forms Al3+, Zn2+, Pb2+, Mg2+, Ba2+
add excess NaOH solution
add 2 drops of dilute NaOH solution.
no precipitate NH4+, Na+
add NaOH solution, heat, test gas with red litmus.
litmus stays red Na+
litmus goes blue NH4+
blue precipitate then deep blue solution Cu2+
blue precipitate forms Cu2+
brown precipitate forms Ag+
brown precipitate then colourless solution Ag+
add 2 drops, then excess NH3 solution
orange precipitate forms Fe3+
green precipitate forms Fe2+
white precipitate forms Al3+, Pb2+
white precipitate forms and disappears, Zn2+
add dilute H2SO4 solution
colourless solution Al3+
white precipitate Pb2+
add dilute H2SO4 solution
colourless solution Mg2+
white precipitate Ba2+
add 2 drops KSCN solution
dark red solution confirms Fe3+

15. Equations: Al3+ + 4 OH- → [ ] Zn2+ + 4 OH- → [ ] Al(OH)4 -
precipitate disappears Al3+, Zn2+, Pb2+
add excess NaOH solution
Equations:
Al OH- → [ ]
Zn OH- → [ ]
Pb OH- → [ ]
Complete the equations and work out the charge!
Al(OH)4 -
Zn(OH)4 2-
Pb(OH)4 2-

16. Equation: Zn2+ + 4 NH3 → [ ] Zn(NH3)4 2+
White precipitate forms and disappears Zn2+
add 2 drops, then excess NH3 solution
Complete the equation and work out the charge!
Equation:
Zn NH3 → [ ]
Zn(NH3)4 2+

17. Equation: Fe3+ + SCN- → [ ] FeSCN 2+ Dark red solution confirms Fe3+
add 2 drops KSCN solution
Dark red solution confirms Fe3+
Complete the equation and work out the charge!
Equation:
Fe3+ + SCN- → [ ]
FeSCN 2+

18. Equation: Cu2+ + 4 NH3 → [ ] Cu(NH3)4 2+
Blue precipitate, then deep blue solution Cu2+
add 2 drops, then excess NH3 solution
Complete the equation and work out the charge!
Equation:
Cu NH3 → [ ]
Cu(NH3)4 2+

19. Equation: Ag+ + 2 NH3 → [ ] Ag(NH3)2 +
Brown precipitate, then colourless solution Ag+
add 2 drops, then excess NH3 solution
Complete the equation and work out the charge!
Equation:
Ag+ + 2 NH3 → [ ]
Ag(NH3)2 +

20. Testing for Ions
Copy the grid.
CO32-
OH-
SO42-
NO3-
Mg2+
Na2+
Zn2+
Ba2+
1. Add reagent to a sample of the solution
2. Observe carefully
No precipitate
Precipitate forms and remains if excess reagent is added
Precipitate forms and dissolves with excess reagent (complex ion)
3. Test a fresh sample with a different reagent
4. By a process of elimination you can identify the ion

21. 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)