Edexcel A Level Chemistry: Transition metal colours

[Cu(H₂O)₆]²⁺

Pale blue Solution

[Fe(H₂O)₆]²⁺

Pale green solution

[Fe(H₂O)₆]³⁺

Yellow Solution

[Mn(H₂O)₆]²⁺

Pale Pink Solution

[Cr(H₂O)₆]³⁺

Green Solution

Orange to yellow

both +6 oxidation states of chromium

CrO4 2- - yellow

Cr2O7 2- - orange

Cu(OH)₂

Pale Blue precipitate

Fe(OH)₂

green precipitate, turning brown on exposure to air

Fe(OH)₃

red-brown precipitate

Cr(OH)₃

Green PPT

[Cr(OH)₆]³⁻

Dark green solution

Cr₂O₇²⁻

Orange solution

[Cu(NH₃)₄(H₂O)₂]²⁺

Dark blue solution

[CuCl₄]²⁻

Yellow solution

[Cr(NH₃)₆]³⁺

Violet solution

Purple solution → grey green precipitate → violet solution

CrO₄²⁻

Yellow solution

Permanganate ion

MnO₄⁻, oxidising agent, purple in solution, reduces from +7 to Mn 2+ in colourless solution

Dichromate ion

Cr₂O₇²⁻

Chromate ion

CrO₄²⁻

[Co(H₂O)₆]²⁺

Pink solution

[CoCl4]2-

Blue solution

[Co(H₂O)₄(OH)₂]

Blue precipitate ANHYDROUS

pink HYDRATED.

Co(H2O)6 2+ → [Co(NH₃)₆]²⁺

pink solution → brown solution which darkens on standing

[Co(NH₃)₆]³⁺

A deep red-brown solution

Observation adding some OH⁻ to Chromium (III) ions

Green solution → green precipitate

Observation adding XS OH⁻ to Chromium (III) ions

Green solution → green precipitate → dark green solution

What happens when Fe(OH)₂ is left exposed to air

Fe(OH)₂ is oxidised to Fe(OH)₃

stability constant, Kstab

The equilibrium constant for the formation of a complex ion in a solvent from its constituent ions

Large Kstab means

stable complex

Small Kstab means

unstable complex

Observation adding XS OH⁻ to Iron (II) ions

green precipitate remains, no further change

Observation adding some OH⁻ to Iron (III) ions

yellow solution → red-brown precipitate

Observation adding XS OH⁻ to Iron (III) ions

red-brown precipitate remains, no further change

Observation adding some OH⁻ to cobalt (II) ions

pink solution → blue precipitate

Observation adding XS OH⁻ to cobalt (II) ions

blue precipitate remains, no further change

Observation adding some OH⁻ to copper (II) ions

blue solution → blue precipitate

Observation adding XS OH⁻ to copper (II) ions

Blue precipitate remains, no further change (NON AMPHOTERIC)

Observation adding some OH⁻ to group II ions

white precipitate forms

Observation adding XS OH⁻ to group II ions

white precipitate remains, no further change

Observation adding some OH⁻ to group I ions

no visible change

Observation adding XS OH⁻ to group I ions

no visible change

Observation adding some NH₃ to Chromium (III) ions

green solution → green precipitate

Observation adding XS NH₃ to Chromium (III) ions

green precipitate slowly dissolves forming a violet solution

Observation adding XS NH₃ to Iron (II) ions

pale green solution → pale green precipitate (FeOH2H2O4), further oxidation to red-brown precipitate (FeOH3), precipitate darkens

Observation adding some NH₃ to Iron (III) ions

yellow solution → red-brown precipitate of [Fe(H2O)3(OH)3]

NH3 acts as base, not as ligand here

Observation adding some NH₃ to Iron (II) ions

green solution → green precipitate

Observation adding XS NH₃ to Iron (III) ions

red-brown precipitate remains, no further change

Observation adding some NH₃ to cobalt (II) ions

pink solution (hexaaquaion) → blue precipitate (CoNH36)

Observation adding XS NH₃ to cobalt (II) ions

Pink solution (Co2+) → green precipitate (Co OH2) → brown solution, which darkens to a red-brown solution on standing (oxidation of Co2+ to Co3+)

Observation adding some NH₃ to copper (II) ions

Blue solution → blue precipitate

Observation adding XS NH₃ to copper (II) ions

Blue precipitate → deep blue solution

Ligand

particle with lone pair of electrons that bonds to metals by a co-ordinate bond

Complex

metal ion with co-ordinately bonded ligands

Co-ordinate number

number of co-ordinate bonds from ligands to metal ion

Coordinate (dative) bond

A covalent bond in which both electrons come from the same atom

Monodentate ligands

A ligand which can donate one pair of electrons to a central metal ion

Monodentate ligand examples

NH₃, OH⁻, Cl⁻

Bidentate ligand

A ligand which has 2 pairs of electrons which can be donated to the same central metal ion

Bidentate ligand examples

Diaminoethane, Ethanedioate

Polydentate ligands

a ligand that is attached to a central metal ion by bonds from two or more donor atoms

EDTA⁴⁻

Hexadentate ligand