Transition Metal Complex Reactions and Colours

Question 1

Vanadium (V) colour.

Answer 1

Yellow

Question 2

Vanadium (IV) colour.

Answer 2

Blue

Question 3

Vanadium (III) colour.

Answer 3

Green

Question 4

Vanadium (II) colour.

Answer 4

Violet

Question 5

Dichromate colour.

Answer 5

Orange

Question 6

Chromium (III) colour.

Answer 6

Green

Question 7

Chromium (II) colour.

Answer 7

Blue

Question 8

[Cu(H2O)6]2+ + 2OH- observation and reaction.

Answer 8

[Cu(OH)2(H2O)4] + 2H2O, light blue solution to light blue precipitate.

Question 9

[Fe(H2O)6]2+ + 2OH- observation and reaction.

Answer 9

[Fe(OH)2(H2O)4] + 2H2O, green solution to green precipitate.

Question 10

[Fe(H2O)6]3+ + 3OH- observation and reaction.

Answer 10

[Fe(OH)3(H2O)3] + 3H2O, yellow solution to brown precipitate.

Question 11

[Co(H2O)6]2+ + 2OH- observation and reaction.

Answer 11

[Co(OH)2(H2O)4] + 2H2O, pink solution to blue precipitate.

Question 12

[Cr(H2O)6]3+ + 3OH- observation and reaction.

Answer 12

[Cr(OH)3(H2O)3] + 3H2O, green solution to green precipitate.

Question 13

[Cr(OH)3(H2O)3] + 3OH- observation and reaction.

Answer 13

[Cr(OH)6]3- +3H2O, green precipitate redissolves to green solution.

Question 14

Dichromate ion colour (Cr2O7 2-).

Answer 14

Orange

Question 15

Which metal ion can be used to reduce dichromate to Cr 3+ but not Cr 2+ making it useful in titrations?

Answer 15

Fe 2+

Question 16

What happens when zinc reduces dichromate?

Answer 16

Reduces to Cr 3+ then to Cr 2+ resulting in two colour changes.

Question 17

Ammonia acts as a what when reacting with H2O ligands?

Answer 17

Base like OH-

Question 18

[Cu(OH)2(H2O)4] + 4NH3 observation and reaction.

Answer 18

[Cu(NH3)4(H2O)2]2+ + 2OH- + 2H2O, light blue precipitate to royal/dark blue solution.

Question 19

[Cr(OH)3(H2O)3] + 6NH3 observation and reaction.

Answer 19

[Cr(NH3)6]3+ + 3OH- + 3H2O, green precipitate to purple solution.

Question 20

What colour is [CuCl4]2-?

Answer 20

Lime green solution.

Question 21

[Co(OH)2(H2O)4] + 6NH3 observation and reaction.

Answer 21

[Co(NH3)6]2+ +3OH- +3H2O, blue precipitate to light brown solution.

Question 22

[Co(H2O)6]2+ + 6NH3 observation and reaction.

Answer 22

[Co(NH3)6]2+ +6H20, pink solution to light brown solution.

Question 23

What colour is [CoCl4]2-?

Answer 23

Dark blue solution.

Question 24

First step in reacting Cr3+ with H2O2.

Answer 24

Add excess NaOH to [Cr(H2O)6]3+ to make [Cr(OH)6]3-

Question 25

Overall reaction when reacting Cr3+ with H2O2.

Answer 25

2[Cr(OH)6]3- + 3H2O2 —> 2CrO4 2- +2OH- +8H2O

Question 26

How can we convert CrO4 2- back to dichromate?

Answer 26

Add H2SO4

Question 27

What colour is Cr6+?

Answer 27

Yellow