2.6 Aqueous Ions

• What do ligands act as?

  Electron pair donors (Lewis base)
• What do metal ions act as?

  Electron pair acceptor (Lewis acid)
• What are metal-aqua ions?


  Metal ions that exist as hexaaqua ions in solution

  [M(H2O)6]2+ or [M(H2O)6]3+
• What type of reaction is breaking a coordinate bond?

  Ligand substitution
• What type of reaction is breaking a O-H bond in water? Give an equation

  Hydrolysis

  [M(H2O)6]3+ + H2O -> [M(HSO)5(OH)]2+ + H3O+
• What does the degree of acidity depend on?

  How many H3O+ are formed
• Why are 3+ ions more acidic than 2+ ions?

  -3+ have a greater charge density

  -metal ion is more polarising so there is a greater attraction for the electrons from the O in the water molecule.

  -this makes the O-H bond weaker and easier to break

  -breaking the O-H bond releases a H+ ion which forms H3O+ with water, increasing acidity
• Which metal ion is the most acidic and why?

  Al3+ because it is the smallest ion and therefore has the greatest charge density
• How does complete hydrolysis occur?

  By removing the H3O+ with the addition of a base as it shifts equilibria to the right because OH- reacts with the acid
• Give an equation to show copper solution reacting with NaOH dropwise and in excess

  [Cu(H2O)6]2+ + 2OH- ⇋ [Cu(H2O)4(OH)2] + 2H2O
• What observations would you see for this reaction?

  Blue solution to blue precipitate
• Give an equation to show iron(II) solution reacting with NaOH dropwise and in excess

  [Fe(H2O)6]2+ + 2OH- ⇋ [Fe(H2O)4(OH)2] + 2H2O
• What observations would you see?

  Green solution to green precipitate
• Give an equation to show iron(III) solution reacting with NaOH dropwise and in excess


  [Fe(H2O)6]3+ + 3OH- ⇋ [Fe(H2O)3(OH)3] + 3H2O
• What observation would you see?

  Purple solution to brown precipitate
• Give an equation to show aluminium solution reacting with NaOH in dropwise


  [Al(H2O)6]3+ + 3OH- ⇋ [Al(H2O)3(OH)3] + 3H2O
• What observation would you see?

  Colourless solution to white precipitate
• Give an equation to show aluminium solution reacting with NaOH from dropwise to excess


  [Al(H2O)6]3+ + 3OH- ⇋ [Al(H2O)3(OH)3] + 3H2O

  [Al(H2O)3(OH)3] + OH— ⇋ [Al(H2O)2(OH)4]- + H2O
• What observation would you see?

  White precipitate to colourless solution when NaOH is in exces
• Why does aluminium react with excess OH- but the other metals do not?

  Aluminium can act as a lewis acid due to its amphoteric character
• How can you reverse hydrolysis?

  Adding a strong acid because the acid reacts with the OH- to remove it, pushing equilibrium to the left to replace it. As a result the hexaaqua ion is reformed
• Why is ammonia solution a weak alkali? Give an equation.

  It is an alkali due to the OH-. Equilibria in this reaction lies to the left therefore not a lot of OH- is formed.

  NH3 + H2O ⇋ NH4+ + OH-
• Give an equation to show copper solution reacting with NH3 dropwise

  [Cu(H2O)6]2+ + 2NH3 ⇋ [Cu(H2O)4(OH)2] + 2NH4+
• What observations would you see?

  Blue solution to blue precipitate
• Give an equation to show copper solution reacting with NH3 in excess


  [Cu(H2O)6]2+ + 4NH3 ⇋ [Cu(H2O)2(NH3)4]2+ + 4H2O
• What observation would you see?

  Blue solution to deep blue solution
• What is this reaction an example of?

  Ligand substitution
• Give an equation to show copper solution reacting with NH3 dropwise until in excess

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

  [Cu(H2O)4(OH)2] + 4NH3 ⇋ [Cu(H2O)2(NH3)4]2+ + 2H2O + 2OH-
• What observation would you see?

  Blue precipitate dissolves and deep blue solution is formed
• Give an equation to show iron(II) solution reacting with NH3 dropwise and in excess

  [Fe(H2O)6]2+ + 2NH3 ⇋ [Fe(H2O)4(OH)2] + 2NH4+
• What observation would you see?

  Green solution to green precipitate
• Give an equation to show iron(III) solution reacting with NH3 dropwise and in excess


  [Fe(H2O)6]3+ + 3NH3 ⇋ [Fe(H2O)3(OH)3] + 3NH4+
• What observation would you see?

  Purple solution to brown precipitate
• Give an equation to show aluminium solution reacting with NH3 dropwise and in excess

  [Al(H2O)6]3+ + 3NH3 ⇋ [Al(H2O)3(OH)3] + 3NH4+
• Why does no further hydrolysis occur?

  Ammonia is not a strong enough base as equilibria lies to left so not enough OH- in solution
• What is the difference between an alkali and a base?

  A base is a something that react with an acid to form water and a salt. An alkali is any base that is soluble in water.
• How do carbonate ions react with water? Give an equation.

  Carbonate ions are weak bases and therefore react with acids to form carbon dioxide and water

  CO32- + H3O+ -> CO2 + 3H2O
• What does the reaction of metal aqua ions and carbonate depend on?

  The oxidation state of the metal ion because with M3+ ions there is sufficient H3O+ to form carbon dioxide and water
• Give an equation to show copper solution reacting with sodium carbonate


  [Cu(H2O)6]2+ + CO32- -> CuCO3 + 6H2O
• What observation would you see?

  Blue solution to a green-blue precipitate