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 forms

Cu2+

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

solutiondark 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)