Reactions of Ions in Aqueous Solution

Question 1

What type of ion is [M(H₂O)₆]³⁺ and why is its solution more acidic than [M(H₂O)₆]²⁺? (3)

Answer 1

• A hexaaqua complex ion
• 3+ ion has higher charge density → polarises O-H bonds more
• H⁺ is released more easily → solution more acidic

Question 2

Describe the reaction of [Fe(H₂O)₆]²⁺ with OH⁻. Give the equation and colour change. (2)

Answer 2

• [Fe(H₂O)₆]²⁺ + 2OH⁻ → [Fe(H₂O)₄(OH)₂] (s) + 2H₂O
• Pale green solution → green precipitate

Question 3

What happens when [Al(H₂O)₆]³⁺ reacts with OH⁻? Include the equation and colour. (2)

Answer 3

• [Al(H₂O)₆]³⁺ + 3OH⁻ → [Al(H₂O)₃(OH)₃] (s) + 3H₂O
• Colourless solution → white precipitate

Question 4

Why is the Al³⁺ precipitate amphoteric? What happens in excess NaOH? (3)

Answer 4

• It reacts with both acids and bases
• In excess OH⁻:
  [Al(H₂O)₃(OH)₃] + OH⁻ → [Al(OH)₄]⁻ + 3H₂O
• White ppt dissolves → colourless solution

Question 5

Describe the reaction of [Cu(H₂O)₆]²⁺ with NH₃ (small vs excess). Include colour changes. (4)

Answer 5

Small amount:

• [Cu(H₂O)₆]²⁺ + 2NH₃ → [Cu(H₂O)₄(OH)₂] (s) + 2NH₄⁺
• Pale blue → blue precipitate

Excess:

• [Cu(H₂O)₄(OH)₂] + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2OH⁻
• Blue ppt → deep blue solution

Question 6

Write the equation and colour change when [Fe(H₂O)₆]³⁺ reacts with CO₃²⁻. (2)

Answer 6

• 2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Fe(OH)₃(s) + 3CO₂ + 3H₂O
• Yellow solution → brown precipitate + effervescence

Question 7

What is observed when [Cu(H₂O)₆]²⁺ reacts with CO₃²⁻? Give the equation. (3)

Answer 7

• [Cu(H₂O)₆]²⁺ + CO₃²⁻ → CuCO₃ (s) + 6H₂O
• Blue solution → green-blue precipitate
• No CO₂ gas evolved (no hydrolysis)

Question 8

What is the difference in behaviour of M²⁺ and M³⁺ ions with CO₃²⁻? (3)

Answer 8

• M²⁺ forms metal carbonates (e.g. CuCO₃)
• M³⁺ ions hydrolyse water → form OH⁻ and release CO₂
• → 3+ ions are acidic, 2+ ions are not

Question 9

Write an ionic equation for hydrolysis of [Fe(H₂O)₆]³⁺. (2)

Answer 9

• [Fe(H₂O)₆]³⁺ ⇌ [Fe(H₂O)₅(OH)]²⁺ + H⁺
• → acidic solution

Question 10

Describe the full ligand substitution when [Co(H₂O)₆]²⁺ reacts with excess NH₃. (2)

Answer 10

• [Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O
• Pink → straw-brown solution

Question 11

What would you observe when [Al(H₂O)₆]³⁺ reacts with CO₃²⁻? Include equation. (2)

Answer 11

• 2[Al(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Al(OH)₃(s) + 3CO₂ + 3H₂O
• Colourless solution → white precipitate + effervescence

Question 12

What is the colour change when [Fe(H₂O)₆]³⁺ is mixed with OH⁻?

Answer 12

• Yellow solution → brown precipitate (Fe(OH)₃)

Question 13

How does NH₃ act in aqueous ion reactions? (2)

Answer 13

• As a base: accepts H⁺ → forms OH⁻ → precipitate forms
• As a ligand (in excess): substitutes water → forms soluble complex