colours for transition elements

Question 1

ligand substitution reactions for adding NH₃ to Cu²⁺ (in water)

first precipitation reaction, then ligand substitution with excess NH₃

Answer 1

• [Cu (H₂O)₆ ] ₍ₐ₎+ 2NH₃ ₍ₐ₎ –> [Cu (OH)₂ (H₂O)₄ ] ₍ₛ₎ +2NH₄⁺₍ₐ₎
• pale blue solution to pale blue precipitate (initially)
• [Cu (H₂O)₆ ] ₍ₐ₎ + 4NH₃ ₍ₐ₎ –> [Cu (NH₃)₄ (H₂O)₂ ] ²⁺₍ₐ₎ + 4H₂O₍ₗ₎
• pale blue solution to dark blue solution

Question 2

ligand substitution reactions for adding HCl to Cu²⁺ (in water)

Answer 2

• [Cu (H₂O)₆ ] + 4Cl⁻ –> [CuCl₄]²⁻ + 6H₂O
• pale blue solution to yellow solution

Question 3

ligand substitution reactions for adding with NH₃ with Cr³⁺

first precipitation reaction, then ligand substitution with excess NH₃

Answer 3

• [Cr (H₂O)₆ ] ³⁺ + 3NH₃ –> [Cr (H₂O)₃ (OH)₃ ] + 3NH₄⁺
• violet solution to grey precipitate
• [Cr (H₂O)₆ ] ³⁺ + 6NH₃ –> [Cr (NH₃) ₆ ] ³⁺ + 6H₂O
• violet solution to purple solution

Question 4

precipitation reaction with NaOH/NH₃
Cu²⁺

Answer 4

• Cu²⁺ + 2OH⁻ –> Cu (OH)₂
• blue solution to blue precipitate
• Cu also reacts with excess NH₃ (ligand substitution reaction)

Question 5

precipitation reaction with NaOH/NH₃
Fe²⁺

Answer 5

• Fe²⁺ + 2OH⁻ –> Fe (OH) ₂
• pale green solution to green precipitate

Question 6

precipitation reaction with NaOH/NH₃
Fe³⁺

Answer 6

• Fe³⁺ + 3OH⁻ –> Fe (OH)₃
• yellow solution to orange precipitate

Question 7

precipitation reaction with NaOH/NH₃
Mn²⁺

Answer 7

• Mn²⁺ + 2OH⁻ –> Mn (OH) ₂
• pale pink solution to pale brown solution

Question 8

precipitation reaction with NaOH/NH₃
Cr³⁺

Answer 8

• Cr³⁺ + 3OH⁻ –> Cr (OH)₃
• violet solution to grey precipitate
• Cr reacts with excess NH₃ in a ligan substitution reaction

Question 9

oxidation of Fe²⁺/Fe³⁺ and reduction of MnO₄⁻/Mn²⁺
form overall equation and state colour changes

Answer 9

• 5Fe²⁺ + MnO₄⁻ + 8H⁺ –> Mn²⁺ + 5Fe³⁺ + 4H₂O
• purple solution to colourless solution (Mn)

Question 10

oxidation of I⁻/I₂ and reduction of Fe³⁺/Fe²⁺
form overall equation and state colour changes

Answer 10

• 2Fe³⁺ + 2I⁻ –> 2Fe²⁺ + I₂
• orange solution to pale green solution (Fe) and I₂ is brown

Question 11

oxidation of Zn/Zn²⁺ and reduction of Cr₂O₇²⁻/Cr³⁺
form overall equation and state colour changes
explain what happens when excess Zn added

Answer 11

• 3Zn + Cr₂O₇²⁻ + 14H⁺ –> 3Zn²⁺ + 2Cr³⁺ + 7H₂O
• orange solution to green solution (Cr)
• when excess Zn is added, Cr³⁺ ions are reduced further to Cr²⁺
  Zn + 2Cr³⁺ –> Zn²⁺ + 2Cr²⁺
  green solution to blue solution (Cr)

Question 12

write an equation for the reaction of Cu²⁺ with I⁻
state colour changes associated with this reaction

Answer 12

• 2Cu²⁺ + 4I⁻ –> 2CuI + I₂
• pale blue solution to white precipitate (Cu) and I₂ is brown