transition elements colours

Question 1

[Cu(H2O)6]2+ (aq)

Answer 1

pale blue solution

Question 2

Cu(OH)2 (s)

Answer 2

• NH3 added drop wise to Cu2+ (aq)
• pale blue precipatate

Question 3

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

Answer 3

• excess NH3 to Cu2+ (aq)
• dark blue

Question 4

CuCl4 2-

Answer 4

• excess HCl to Cu2+ (aq)
• yellow solution

Question 5

Cu+

Answer 5

• reduction of Cu2+ with I-
• white precipitate (CuI)
• brown solution (I2)

Question 6

CoCl4 2-

Answer 6

• formed from CoCl2.6H2O + water and excess HCl
• blue solution

Question 7

[Cr(H2O)6] 3+

Answer 7

• from KCr(SO4)2.12H2O (s)
• pale purple (with heat turns green)
• due to impurities it is common for Cr (III) to appear green in solution

Question 8

Cr(OH)3 (s)

Answer 8

• NH3 dropwise to Cr3+ (aq)
• dark green precipitate

Question 9

[Cr(NH3)6] 3+ (aq)

Answer 9

• excess NH3 to Cr2+
• purple solution

Question 10

CrO4 2-

Answer 10

• oxidation of Cr3+ with hot alkaline H2O2
• yellow solution

Question 11

[Mn(H2O)6]2+

Answer 11

• from MnSO4.4H2O (s)
• pale pink solution

Question 12

Mn(OH)2 (s)

Answer 12

• NaOH dropwise to Mn2+
• pale brown precipitate

Question 13

[Fe(H2O)6]2+

Answer 13

• from FeSO4.7H2O (s)
• pale green

Question 14

Fe(OH)3 (s)

Answer 14

• NaOH dropwise to Fe3+ (aq)
• orange/brown precipitate

Question 15

MnO4-/Fe2+ –> Mn2+/Fe3+

Answer 15

• purple (MnO4-) to pale pink (Mn2+)
• in titrations it is so dilute that it is practically colourless

Question 16

I-/Fe3+ –> I2/Fe2+

Answer 16

orange brown (Fe3+) to brown (I2)

Question 17

Cu2+/Cu

Answer 17

• brown solid (Cu)
• CuSO4 blue solution

Question 18

[Cr(OH)6]3-

Answer 18

• excess OH- to Cr(OH)3
• dark green solution

Question 19

Cr2O7 2-

Answer 19

• from K2Cr2O7 in 0.1mol H2SO4
• orange solution

Question 20

Fe(OH)2

Answer 20

• NaOH dropwise to Fe2+
• dark green precipitate

Question 21

[Fe(H2O)6]3+

Answer 21

• from FeCl3.6H2O
• yellow solution

Question 22

name the 5 transition metal ions and their colours in solution

Answer 22

Cr 3+ = green
-
Fe 2+ = pale green
-
Fe 3+ = yellow
-
Mn 2+ = pale pink

Question 23

what is the pattern for ionic equations of transition metal reactions with OH- ions? use M as for the metal and x for the charge

Answer 23

M X+ + xOH- -> M(OH)x

Question 24

what are the colours of the 5 transition metal hydroxide precipitates?

Answer 24

Cr(OH)3 = grey/green
-
Fe(OH)2 = green
-
Fe(OH)3 = orange/brown
-
Mn(OH)2 = light brown

Question 25

which metal hydroxides change colour on standing with air? give reason why

Answer 25

• Fe(OH)2 turns from green to orange/brown at the surface as Fe2+ oxidises to Fe3+
• Mn(OH)2 darkens on standing with air

Question 26

What happens if excess NaOH is added to Cr(OH)3? give an equation

Answer 26

• Cr(OH)3 precipitate redissolved to form a dark green solution of [Cr(OH)6]3+

-

• Cr(OH)3 (s) + 3OH- (aq) -> [Cr(OH)6]3- (aq)

Question 27

what happens if excess NH3 is added to Cu(OH)2? give full complex equation

Answer 27

• Cu(OH)2 precipitate redissolved to form a dark blue solution of [Cu(NH3)4(H2O)2]2+
• [Cu(H2O)6]2+ (aq) + 4NH3 (aq) -> [Cu(NH3)4(H2O)2]2+ (aq) + 4H2O (l)

Question 28

what happens if excess NH3 is added to Cr(OH)3? give the full complex equation

Answer 28

• Cr(OH)3 precipitate redissolved to form purple solution of [Cr(NH3)6]3+
• [Cr(H2O)6]3+ (aq) + 6NH3 (aq) -> [Cr(NH3)6]3+ (aq) + 6H2O (l)

Question 29

what happens if Cl- ions are added to Cu2+ solution? give full complex equation

Answer 29

• ligand substitution
• colour change from blue to yellow

[Cu(H2O)6]2+ + 4Cl- -> [CuCl4]2- + 6H2O

Question 30

what reagents are used to reduce Fe3+ to Fe2+? what is the colour change

Answer 30

• iodide ions I-
• yellow to pale green

Question 31

what reagents are used to oxidise Cr3+ to Cr2O7 2-? what is the colour change

Answer 31

• warm hydrogen peroxide H2O2/OH-
• green to orange

Question 32

what reagents are used to reduce Cu2+ to Cu+? what is the color change?

Answer 32

• iodide ions I-
• blue solution to off-white precipitate of CuI

Question 33

what happens to Cu+ in solution>

Answer 33

spontaneously disproportionates into Cu2+ and Cu(s)