3.2.6 Reactions of ions in aqueous solution (A-level only)

• • In aqueous solution, the following metal-aqua ions are formed:
  • [M(H2O)6] 2+, limited to M = Fe and Cu
  • [M(H2O)6] 3+, limited to M = Al and Fe
• Explain why the pH of a solution of iron(II) chloride is higher than that of a solution of iron(III) chloride of the same concentration.

  • The Fe2+ ion has a smaller charge to size ratio 1
  • Hence less polarising than Fe3+ or less weakening effect on O-H bonds
• The acidity of [M(H2O)6] 3+ is greater than that of [M(H2O)6] 2+
• Some metal hydroxides show amphoteric character by dissolving in both acids and bases (eg hydroxides of Al3+).
• • Students should be able to

  • describe and explain the simple test-tube reactions of: M2+(aq) ions, limited to M = Fe and Cu, and of M3+(aq) ions, limited to M = Al and Fe, with the bases OH– , NH3 and CO3 2
• • Iron (II) - aqueous ion [Fe(H2O)6]^2+ is a green solution
  • To remember - two E's in green therefore...
• Iron (III) - aqueous ion [Fe(H2O)6]^3+ is a purple solution (yellow brown)
• Copper II ion - [Cu(H2O)6]^2+ is a blue solution
• Aluminium (III) is a colourless solution - [Al(H2O)6]^3+
• Reaction with limited OH- and limited NH3

  • Form the same hydroxides precipitates
  • Form in deprotonation acid-base reactions
  • [Cu(H2O)6 ]2+ (aq) + 2OH- (aq) -> Cu(H2O)4 (OH)2 (s) + 2H2O (l)
  • [Al(H2O)6 ]3+ (aq) + 3OH- (aq)  Al(H2O)3 (OH)3 (s) + 3H2O (l)
• Here the NH3 and OHions are acting as Bronsted-Lowry bases accepting a proton

  • [Fe(H2O)6 ]2+ (aq) + 2NH3 (aq) -> Fe(H2O)4 (OH)2 (s) + 2NH4+ (aq)
  • [Fe(H2O)6 ]3+ (aq) + 3NH3 (aq) -> Fe(H2O)3 (OH)3 (s) + 3NH4+ (aq)
• Reaction with excess OH-

  • ONLY ALUMINIUM HAS FURTHER CHANGE WITH EXCESS OH-

  With excess NaOH the aluminium hydroxide dissolves.

  Al becomes [Al(OH)4 ] - (aq) colourless solution.

  • Al(H2O)3 (OH)3 (s) + OH- (aq ) -> [Al(OH)4 ]- (aq) + 3H2O (l)
• Aluminium hydroxide is classed as amphoteric because it reacts and dissolves in both acids and bases.

  • Al(H2O)3 (OH)3 (s) + 3H+ (aq ) -> [Al(H2O)6 ]3+ (aq)
• Reaction with excess NH3

  • Ligand substitution reaction occurs with Cu and its precipitate dissolves to form a deep blue solution.
  • Cu(OH)2 (H2O)4(s) + 4NH3 (aq) -> [Cu(NH3 )4 (H2O)2 ]2+ (aq) + 2H2O (l) + 2OH- (aq)
  • In this reaction NH3 is acting as a Lewis base donating an electron pair.
• Reactions with carbonate solution

  • The 2+ ions with carbonate solution results in MCO3 ppt being formed
  • Cu blue/green
  • Fe(II) green
  • These are simple substitution reactions CO32- substituting water H2O
• The 3+ ions with carbonate solution form

  • M(OH)3 ppt
  • CO2 gas is evolved
  • Observation: Effervescence
  • Al forms white ppt of Al(OH)3 (H2O)3 + CO2
  • Fe(III) forms brown ppt of Fe(OH)3 (H2O)3 + CO2
• Explain, with the use of an equation, why a solution containing [Al(H2O)6] 3+ has a pH ˂7

  • [Al(H2O)6] 3+ ⇌ [Al(H2O)5(OH)]2+ + H+
  • Al3+ has a small size and high charge OR has a high charge density
  • Weakens the OH bond (in water) releasing H+ ions
• 3+ ions reaction with carbonate solutions

  • Fe3+ - this is giving oxygen (from carbonate and releases CO2) and displacing 3 water molecules, so final product is neutral
  • 2[Fe(H2O)6] 3+ + 3CO3 2– → 2[Fe(OH)3(H2O)3] + 3CO2 + 3H2O
  • Colour: Brown precipitate forms
• To convert Fe(H2O)6 3+ into Fe(H2O)6 2+, suggest a reagant

  • Zn (in acid or CHl or H2SO4)
  • Allow potassium iodide
• Give an equation to show how aluminium-aqua ion can act as a Brønsted–Lowry acid with water.

  • [Al(H2O)6] 3+ + H2O → [Al(H2O)5(OH)]2+ + H3O+
• 2017 Paper Q7.5 = Cu(H2O)6 was added to reagent D to form a yellow-greens solution

  Identify reagant D

  Write an ionic equation for the reaction that occurs [2 marks]

  • CONCENTRATED Hydrochloric acid
  • [Cu(H2O)6]2+ + 4Cl- -> [CuCl4]2- + 6H2O
• Reaction of aqueous Co2+ ion with Concentrated hydrochloric acid

  • Reagent = Concentrated hydrochloric acid
  • Observation = blue precipitate
  • Equation = [Co(H2O)6]2+ + 4Cl- -> [Co(Cl)4]2- + 6H2O