reactions of ions in aqueous solution (A2 inorganic chemistry) Flashcards
what is a metal-aqua complex?
formed when a transition metal compound is added to water, 6 water molecules will form coordinate bonds with the central metal ion
e.g. [Fe(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺, [Fe(H₂O)₆]³⁺, [Al(H₂O)₆]³⁺
explain why metal-aqua ions are acidic when in solution
an H⁺ ion is released from one of the water molecules that has formed a coordinate bond with the central metal ion, to form a hydroxonium ion which makes the solution acidic
this reaction is hydrolysis
e.g. [Fe(H₂O)₆]²⁺ (aq) + H₂O (l) ⇌ [Fe(H₂O)₅(OH)]⁺ (aq) + H₃O⁺ (aq)
this reaction also changes the overall charge of the metal-aqua complex
why are more positively charged metal-aqua complexes more acidic in solution
similarly sized ions with higher charge density mean they are more polarising
attract electrons in the O-H bond in the water ligands more strongly
this weakens the bond which makes it more likely that the H⁺ ion will be released
therefore dissociate more readily
further hydrolysis of metal-aqua ions
we can use OH⁻ ions to hydrolyse metal-aqua complexes further to form insoluble metal hydroxides
when we hydrolyse a metal-aqua ion, an equilibrium is established
adding OH⁻ ions will lower the concentration of H₃O⁺, causing equilibrium to shift to the right to replace the removed H₃O⁺
adding more OH⁻ causes further hydrolysis to take place in this new equilibrium, as more H₃O⁺ ions are removed and equilibrium shifts right once again
this process keeps happening until a neutral, solid complex is formed that forms a precipitate in solution
[X(H₂O)₆]³⁺ (aq) + H₂O (l) ⇌ [X(H₂O)₅(OH)]²⁺ (aq) + H₃O⁺ (aq)
[X(H₂O)₅(OH)]²⁺ (aq) + H₂O (l) ⇌ [X(H₂O)₄(OH)₂]⁺ (aq) + H₃O⁺ (aq)
[X(H₂O)₄(OH)₂]⁺ (aq) + H₂O (l) ⇌ X(H₂O)₃(OH)₃ (s) + H₃O⁺ (aq)
amphoteric metal hydroxides
e.g. the formation of solid and insoluble aluminium hydroxide can be reversed and will dissolve in both excess acid and base
Al(H₂O)₃(OH)₃ will act as a Brønsted-Lowry base when we add an acid by accepting H⁺ ions to form H₃O⁺ in solution
Al(H₂O)₃(OH)₃ (s) + 3H₃O⁺ (aq) → [Al(H₂O)₆]³⁺ (aq) + 3H₂O (aq)
Al(H₂O)₃(OH)₃ will act as a Brønsted-Lowry acid when we add a base, as it will donate H⁺ ions to react with the OH⁻
Al(H₂O)₃(OH)₃ (s) + OH⁻ (aq) → [Al(H₂O)₂(OH)₄] (aq) + H₂O (aq)
adding ammonia to metal-aqua ions
when we add small quantities of ammonia to metal-aqua ions, we produce the same insoluble metal hydroxides as we do when we add a base
this is because, when in solution, ammonia exists in this equilibrium: NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
however this is a weak base with equilibrium lying relatively far to the left
when excess ammonia is added, we get partial ligand substitution with some metal hydroxides, where some ligands are displaced to form a charged, soluble complex
e.g. Cu(OH)₂(H₂O)₄ (aq) + 4NH₃ (aq) → [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 2OH⁻ (aq) + 2H₂O (l)
metal-aqua ions with carbonates
reacting 2+ metal-aqua ions with carbonate produces insoluble metal carbonates with a complete substitution of the water ligands
[X(H₂O)₆]²⁺ (aq) + CO₃²⁻ (aq) ⇌ XCO₃ (s) + 6H₂O (l)
when we add carbonates to 3+ complex ions, the carbonate ions will react with free hydroxonium ions instead of displacing the water ligands, as 3+ metal-aqua ions are more acidic than 2+ metal-aqua ions so there are more H₃O⁺ ions in solution
this reaction produces a metal hydroxide, and equilibrium shifts right as H₃O⁺ ions are removed
2[X(H₂O)₆]³⁺ (aq) + 3CO₃²⁻ (aq) ⇌ 2X(OH)₃(H₂O)₃ (s) + 3CO₂ (g) + 3H₂O (l)
observe bubbles of CO₂ gas with this reaction, which can identify the charge of our complex ion
test tube reactions with sodium hydroxide
Fe²⁺ - forms iron(II) hydroxide Fe(OH)₂(H₂O)₄, produces dirty green precipitate
Cu²⁺ - forms copper hydroxide Cu(OH)₂(H₂O)₄, produces pale blue precipitate
Fe³⁺ - forms iron(III) hydroxide Fe(OH)₃(H₂O)₃, produces orange precipitate
Al³⁺ - forms aluminium hydroxide Al(OH)₃(H₂O)₃, produces white precipitate
Al³⁺ (NaOH in excess) - precipitate dissolves as aluminium hydroxide is amphoteric, forms colourless solution of [Al(OH)₄(H₂O)₂]⁻
test tube reactions with ammonia
should be conducted in a fume cupboard
Fe²⁺ - forms iron(II) hydroxide Fe(OH)₂(H₂O)₄, produces dirty green precipitate
Cu²⁺ - forms copper hydroxide Cu(OH)₂(H₂O)₄, produces pale blue precipitate
Fe³⁺ - forms iron(III) hydroxide Fe(OH)₃(H₂O)₃, produces orange precipitate
Al³⁺ - forms aluminium hydroxide Al(OH)₃(H₂O)₃, produces white precipitate
Cu²⁺ (NH₃ in excess) - precipitate dissolves as there is partial ligand substitution, forms dark blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺
test tube reactions with sodium carbonate (Na₂CO₃)
Fe²⁺ - forms iron carbonate FeCO₃, produces green precipitate
Cu²⁺ - forms copper carbonate CuCO₃, produces green-blue precipitate
Fe³⁺ - forms iron(III) hydroxide Fe(OH)₃(H₂O)₃, produces brown precipitate and bubbles of CO₂ gas
Al³⁺ - forms aluminium hydroxide Al(OH)₃(H₂O)₃, produces white precipitate and bubbles of CO₂ gas
Fe²⁺ oxidises readily with air to form Fe³⁺, so solutions need to be made fresh
