transition metals colours

K₂Cr₂O₇ potassium dichromate (VI)

bright orange

CoCl ₂ cobalt (II) chloride

pink-purple

NiSO₄ nickel (II) sulfate

green

CuSO4·5H2O hydrated copper (II) sulfate

blue

iron (II) and iron (III) in solution

+2: pale green

+3: yellow

Cr (III) and Cr (VI)

+3: green

+6: yellow or orange

[CoCl₄)^2-

[CuCl₄]^2-

cis [Co(NH₃)₄Cl₂]⁺

violet

trans [Co(NH₃)₄Cl₂]⁺

green

[Cu(H2O)6]2+

pale blue

what happens when XS NH3(aq) is added dropwise to a solution containing [Cu(H₂O)₆]²⁺

• a pale blue ppt of Cu(OH)2 forming in the first stage of the rxn

• the Cu(OH)2 ppt dissolving in XS NH3 to form a dark blue solution

what happens when an XS of conc HCl is added to a solution containing [Cu(H₂O)₆]²⁺

pale blue solution changes to form a yellow solution

• if water is added to the yellow solution, a blue solution will be formed, although more dilute and paler in colour than the original blue solution

• if you take care making observations, you will see an intermediate green solution (the result of the two solutions mixing)

what happens when KCr(SO₄)₂ • 12(H₂O) (aka chrome alum) is dissolved in water

the complex ion [Cr(H2O)6]3+ (a pale purple solution) is formed

what happens when you add XS NH3(aq) dropwise to [Cr(H₂O)₆]³⁺

1. initially a grey-green ppt of Cr(OH)3 is formed

2. the Cr(OH)3 ppt dissolves in XS NH3 to form the complex ion [Cr(NH₃)₆]³⁺

[Cr(H₂O)₆]³⁺(aq) (violet) + 6NH₃(aq) → [Cr(NH₃)₆]³⁺(aq) (purple) + 6H₂O(l)

colours of Cu2+(aq), Fe2+(aq), Fe3+(aq), and Mn2+(aq)

Cu2+(aq): blue solution

Fe2+(aq): pale green solution

Fe3+(aq): yellow/light brown (or purple in some cases)

Mn2+(aq): [Mn(H₂O)₆]²⁺ is pale pink

colours of Cu2+(aq), Fe2+(aq), Fe3+(aq), and Mn2+(aq) when + NaOH(aq)

Cu2+(aq): pale blue ppt

Fe2+(aq): green ppt

Fe3+(aq): orange-brown ppt

Mn2+(aq): pale brown ppt

table for rxns of the aqueous transition metals w NaOH(aq)

table for rxns of Cr3+(aq) ions w NaOH(aq)

Cu(OH)₂(s)

blue ppt which dissolves in XS NH3 to form a deep blue solution [Cu(NH₃)₄(H₂O)₂]²⁺(aq)

Cr(OH)3(s)

green ppt which dissolves in XS NH3 to form a purple solution [Cr(NH₃)₆]³⁺

how do Fe2+, Fe3+, and Mn2+ react w XS NH3(aq)

in the same way they react w NaOH(aq)

form ppt of Fe(OH)2(s), Fe(OH)3(s), and Mn(OH)2(s)

there is no further rxn w NH3(aq) to these ppts don’t dissolve

summary of rxns of aqua ions

[M(H2O)6]2+(aq) summary

[M(H2O)6]3+(aq) summary

colour of MnO4- and Mn2+

purple to colourless

redox rxn between Fe2+(aq) and MnO4- (manganate VII ions)

• Fe2+ is oxidised to Fe3+

• MnO4- is reduced to Mn2+

• the solution containing MnO4- ions is purple and is decolourised by Fe2+(aq) to form a colourless solution containing Mn2+(aq)

reduction of Fe3+ to Fe2+ by I-(aq)

• orange-brown Fe3+(aq) are reduced to pale green Fe2+(aq)

• but this is obscured by the oxidation of the I- to form I2(aq) which has a brown colour

colours of Cr₂O₇^2-, Cr³⁺, and Cr²⁺

Cr₂O₇^2- orange

Cr³⁺ green

Cr²⁺ pale blue