Colours Of Inorganic Ions And Complexes

Question 1

[Cu(H2O)6] 2+

Answer 1

Pale blue

From CuSO4*5H2O

Question 2

Cu(OH)2 (s)

Answer 2

Pale blue precipitate

NH3 dropwise to Cu2+

Question 3

[Cu(NH3)4 (H2O)2 ]2+

Answer 3

Dark blue

Excess NH3 to Cu 2+

Question 4

CuCl4 2-

Answer 4

Yellow

Excess HCl to Cu 2+

Question 5

Cu+

Answer 5

White precipitate (CuI)
Brown (I2)

Reduction of Cu 2+ to I-

Question 6

Cu 2+ / Cu

Answer 6

Brown solid (Cu)
Blue solution (CuSO4)

Disproportionation of Cu+

Question 7

CoCl4 2-

Answer 7

Blue

From CoCl2 * 6H2O plus water and excess HCl

Question 8

[Cr(H2O)6] 3+

Answer 8

Pale purple
Heat ——-> green
Due to impurities, it is common for Cr(III) to appear green in solution

From KCr(SO4)2 * 12H2O

Question 9

Cr(OH)3 (s)

Answer 9

Dark green precipitate

NH3 dropwise to Cr 3+

Question 10

[Cr(NH3)6] 3+

Answer 10

Purple

Excess NH3 to Cr 2+

Question 11

[Cr(OH)6] 3-

Answer 11

Dark green

Excess OH- to Cr(OH)3

Question 12

Cr2O7 2-

Answer 12

Orange

From K2Cr2O7 in 0.1 moldm-3 H2SO4

Question 13

CrO4 2-

Answer 13

Yellow

Oxidation of Cr 2+ with hot alkaline H2O2

Question 14

[Mn(H2O)6] 2+

Answer 14

Pale pink

From MnSO4 * 4H2O

Question 15

Mn(OH)2 (s)

Answer 15

Pale brown precipitate

NaOH dropwise to Mn 2+

Question 16

[Fe(H2O)6] 2+

Answer 16

Pale green

From FeSO4 * 7H2O

Question 17

Fe(OH)2

Answer 17

Dark green precipitate

NaOH dropwise to Fe 2+

Question 18

[Fe(H2O6)] 3+

Answer 18

Yellow

From FeCl3 * 6H2O

Question 19

Fe(OH3) (s)

Answer 19

Orange-brown precipitate

NaOH dropwise to Fe3+

Question 20

MnO4 - / Fe2+ ————> Mn2+/Fe3+

Answer 20

Purple (MnO4 -) to pale pink (Mn2+)

In titrations so dilute that it is practically colourless

Question 21

I-/Fe 3+ ——-> I2 / Fe2+

Answer 21

Orange brown (Fe 3+) to brown (I2)