3.2.6 Reactions of ions in aqueous solution

State what happens when a transition metal salt dissolves in water. (2 marks)

• Six water molecules act as ligands, each donating a lone pair to form a co-ordinate bond.

• An octahedral metal-aqua ion is formed, for example Fe(NO₃)₂ + 6H₂O → [Fe(H₂O)₆]²⁺ + 2NO₃⁻.

State the formula and colour of the iron(II) and copper(II) aqua ions. (2 marks)

• [Fe(H₂O)₆]²⁺: pale green.

• [Cu(H₂O)₆]²⁺: pale blue.

State the formula and colour of the iron(III) and aluminium aqua ions. (2 marks)

• [Fe(H₂O)₆]³⁺: violet, though it usually appears yellow-brown in solution.

• [Al(H₂O)₆]³⁺: colourless.

Explain why a solution of a metal-aqua ion is acidic. (3 marks)

• The metal ion polarises the water ligands, weakening the O-H bonds.

• A water ligand releases a hydrogen ion, so the complex acts as a Bronsted-Lowry acid.

• [M(H₂O)₆]²⁺ + H₂O ⇌ [M(H₂O)₅(OH)]⁺ + H₃O⁺

Explain why [M(H2O)6]³⁺ is more acidic than [M(H2O)6]²⁺. (4 marks)

• The 3+ ion has a higher charge and a smaller radius, so a higher charge to size ratio.

• It therefore polarises the water ligands more strongly.

• The O-H bonds are weakened further, so hydrogen ions are released more readily.

• A 2+ solution has a pH of about 6, whereas a 3+ solution is about pH 3.

Define a Lewis acid and a Lewis base, and explain how this applies to a complex. (3 marks)

• A Lewis acid is an electron pair acceptor.

• A Lewis base is an electron pair donor.

• In a complex the metal ion is the Lewis acid and each ligand is a Lewis base.

Describe what is seen when NaOH is added dropwise, and then in excess, to [Fe(H2O)6]²⁺. (3 marks)

• A green precipitate of [Fe(H₂O)₄(OH)₂] forms.

• It does not dissolve in excess NaOH.

• The precipitate darkens to brown on standing, as it is oxidised by oxygen from the air to iron(III).

Describe what is seen when NaOH is added dropwise, and then in excess, to [Cu(H2O)6]²⁺. (2 marks)

• A blue precipitate of [Cu(H₂O)₄(OH)₂] forms.

• It does not dissolve in excess NaOH.

Describe what is seen when NaOH is added dropwise, and then in excess, to [Fe(H2O)6]³⁺. (2 marks)

• A brown precipitate of [Fe(H₂O)₃(OH)₃] forms.

• It does not dissolve in excess NaOH.

Describe what is seen when NaOH is added dropwise, and then in excess, to [Al(H2O)6]³⁺. (3 marks)

• A white precipitate of [Al(H₂O)₃(OH)₃] forms.

• It dissolves in excess NaOH to give a colourless solution.

• [Al(H₂O)₃(OH)₃] + OH⁻ → [Al(OH)₄]⁻ + 3H₂O

Explain what amphoteric means and which hydroxide shows it. (3 marks)

• An amphoteric hydroxide dissolves in both acids and bases.

• Aluminium hydroxide is the example required.

• With acid it gives [Al(H₂O)₆]³⁺, and with excess base it gives [Al(OH)₄]⁻.

Explain why the precipitates formed with NaOH and with NH3 are the same. (2 marks)

• Ammonia acts as a base, not as a ligand, in dilute solution.

• It removes hydrogen ions from the water ligands, forming the same neutral hydroxide complex.

Describe what is seen when excess ammonia is added to [Cu(H2O)6]²⁺. (3 marks)

• The blue precipitate first forms as with NaOH.

• It then dissolves to give a deep royal blue solution.

• [Cu(H₂O)₄(OH)₂] + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O + 2OH⁻

State what happens when excess ammonia is added to the Fe²⁺, Fe³⁺ and Al³⁺ precipitates. (2 marks)

• None of them dissolves in excess ammonia; only the copper precipitate does.

• The green, brown and white precipitates therefore remain.

Describe what is seen when sodium carbonate is added to a 2+ metal-aqua ion. (3 marks)

• A precipitate of the metal carbonate forms, with no effervescence.

• [Fe(H₂O)₆]²⁺ + CO₃²⁻ → FeCO₃ + 6H₂O, a green precipitate.

• Copper gives a blue-green precipitate of CuCO₃.

Describe what is seen when sodium carbonate is added to a 3+ metal-aqua ion. (4 marks)

• The hydroxide precipitate forms and effervescence is seen.

• The 3+ ion is acidic enough to react with the carbonate, releasing carbon dioxide.

• 2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[Fe(H₂O)₃(OH)₃] + 3CO₂ + 3H₂O

• The effervescence therefore distinguishes a 3+ ion from a 2+ ion.

Write the equation for [Fe(H2O)6]²⁺ reacting with hydroxide ions. (1 mark)

• [Fe(H₂O)₆]²⁺ + 2OH⁻ → [Fe(H₂O)₄(OH)₂] + 2H₂O

Write the equation for [Fe(H2O)6]³⁺ reacting with hydroxide ions. (1 mark)

• [Fe(H₂O)₆]³⁺ + 3OH⁻ → [Fe(H₂O)₃(OH)₃] + 3H₂O

Write the equation for [Cu(H2O)6]²⁺ reacting with dilute ammonia. (1 mark)

• [Cu(H₂O)₆]²⁺ + 2NH₃ → [Cu(H₂O)₄(OH)₂] + 2NH₄⁺

Write the two equations showing that aluminium hydroxide is amphoteric. (2 marks)

• With acid: [Al(H₂O)₃(OH)₃] + 3H⁺ → [Al(H₂O)₆]³⁺

• With base: [Al(H₂O)₃(OH)₃] + OH⁻ → [Al(OH)₄]⁻ + 3H₂O