3.2.6 Reactions of ions in aqueous solutions Flashcards
What happens when a transition metal compound dissolves in water?
Water molecules form coordinate covalent bonds with the metal ion.
Creating metal-aqua complex ions.
Give an example of a hydrated ion for iron(II).
[Fe(H₂O)₆]²⁺
Why are metal-aqua ion solutions acidic?
They undergo hydrolysis with water, releasing H₃O⁺ ions.
The water acts as a base and gains protons from the metal-aqua ion.
Write the hydrolysis equation for a 2+ ion like Fe²⁺.
[Fe(H₂O)₆]²⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]⁺ + H₃O⁺
Why are 3+ metal-aqua ions more acidic than 2+ ions?
The 3+ metal ions have a smaller ionic radius, while possesing a higher charge
This polarises the water ligands more strongly
Weakening the O-H bonds of H₂O so H⁺ ions can dissociate more easily.
Write the hydrolysis equation for Fe³⁺.
[Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺
What three bases do metal-aqua ions form precipitates with?
(that you need to know)
NaOH, NH₃ and Na₂CO₃
What happens when OH⁻ is added to metal-aqua ions?
An insoluble metal hydroxide precipitate forms
What is the final product of Iron 3+ ion hydrolysis with OH⁻?
Fe(H₂O)₃(OH)₃(s)
Give an equation for the reaction of ammonia dissolved in water.
NH₃ (aq) + H2O(l) ⇌ NH4+(aq) + OH⁻(aq)
Colour and formulae of Cu²⁺ with OH⁻ or NH₃?
Blue precipitate of Cu(H₂O)₄(OH)₂
Colour and formulae of Cu²⁺ with excess NH₃?
Deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺
Give an equation to show the reaction of excees ammonia with the Cu(H₂O)₄(OH)₂ precipitate.
Cu(H₂O)₄(OH)₂ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2OH⁻ + 2H2O
What is the colour and formulae of Fe²⁺ precipitate with OH⁻ or NH₃?
Green precipitate
Fe(H₂O)₄(OH)₂
BUT goes brown standing in air
oxidation to Fe³⁺
What is the colour and formulae of Fe³⁺ with OH⁻ or NH₃?
Brown precipitate
Fe(H₂O)₃(OH)₃
What is the colour and formulae of Al³⁺ with OH⁻ or NH₃ ?
White precipitate
Al(H₂O)₃(OH)₃
BUT precipitate dissolves with excess OH⁻ to give a colourless solution of [Al(H₂O)₂(OH)₄]⁻
Why does sodium carbonate form carbonates with M²⁺ ions but not M³⁺?
M³⁺ ions are stronger acids so there is a higher [H₃O⁺] in solution.
[M(H₂O)₆]³⁺+ 3H₂O ⇌ M(H₂O)₃(OH)₃(s) + 3H₃O⁺
The carbonate reacts with H₃O⁺ to form CO₂ and H₂O.
This removes H₃O⁺ & shifts the equilibrium to the right
So the hydrolysis is favoured M(H₂O)₃(OH)₃, and more is produced.
What are the colours and formulae of Cu²⁺, Fe²⁺, Fe³⁺, and Al³⁺ with Na₂CO₃?
- Cu²⁺: Green-blue precipitate of CuCO₃
- Fe²⁺: Green precipitate of FeCO₃
- Fe³⁺: Brown precipitate of Fe(H₂O)₃(OH)₃ & bubbles of CO₂
- Al³⁺: White precipitate of Al(H₂O)₃(OH)₃ & bubbles of CO₂
What does it mean if aluminium hydroxide (Al(H2O)3(OH)3) is amphoteric?
It can react with both acids and bases.
* With acid - forms [Al(H₂O)₆]³⁺
- With base - forms [Al(H₂O)₂(OH)₄]⁻
