6. Reactions of ions in aqueous solution Flashcards
When does complex formation occur?
When a positively charged transition metal ion attracts a ligand with a lone pair of electrons, which forms a coordinate bond with the central ion
What is a Lewis base?
An electron pair donor
What is a Lewis acid?
An electron pair acceptor
Does a ligand act as a Lewis acid or base?
Lewis base
Does a transition metal ion in a complex act as a Lewis acid or base?
Lewis acid
In a complex, what acts as the Lewis acid and what acts as the Lewis base?
- ligand - Lewis base
* transition metal ion - Lewis acid
How do metal ions exist in aqueous solution?
As hexaaquaions
Which metal ions form 2+ hexaaquaions?
Cu2+ and Fe2+
Which metal ions form 3+ hexaaquaions?
Al3+ and Fe3+
Is the formation of hexaaquaions exo or endothermic? Why is this?
Exothermic as bonds are being formed
Is the formation of 2+ or 3+ hexaaquaions more exothermic? Why is this?
Formation of 3+ hexaaquaions because the bonds are stronger
What happens when a solution of hexaaquaions is evaporated?
Water of crystallisation - hexaaquaion remains intact as the coordinate bond between the water molecules and metal ion is so strong that they are not broken when the solution is evaporated
What colour is iron (II) sulphate in solution?
Green
What colour is iron (III) chloride in solution?
Violet
What colour is copper (II) nitrate in solution?
Blue
What colour is aluminium chloride in solution?
Colourless
In a hexaaqua ion, what are the types of bonds that could be broken?
- coordinate
* covalent
What happens if the coordinate bond is broken in a hexaaqua ion?
Ligand substitution - water ligand is replaced by another ligand
What is hydrolysis?
When the O-H in the water molecule of a hexaaqua ion is broken
What effect does water have when it is added to a hexaaqua ion?
It strengthens the metal oxygen bond and the O-H bond weakens. Once the OH bond breaks a H⁺ ion is released from a H₂O ligand
What happens when the OH bond breaks in a hexaaqua ion?
A H⁺ ion is released from a H₂O ligand
What effect does the metal ion in a hexaaqua ion have on electron density in the ligands?
The positive charge pulls electron density away from O atom, and O is electronegative therefore pulls electron density away from H atom
What does the degree of acidity of a solution after hydrolysis depend on?
How many H₃O⁺ are formed
Where does the position of equilibrium for an M²⁺ hexaaquaion lie, and what does this mean for the acidity of the resulting solution?
Lies to left, so not many H₃O⁺ formed, so not very acidic
Where does the position of equilibrium for an M³⁺ hexaaquaion lie, and what does this mean for the acidity of the resulting solution?
Lies further right than M²⁺ ion, so solution contains more H₃O⁺ and so more acidic
How many M²⁺ ions are hydrolysed compared to M³⁺?
- 1 in a million M²⁺
- 1 in a thousand M³⁺
Therefore M³⁺ ions more acidic (but both cases solution is weakly acidic)
What are the factors that affect the degree of acidity of hexaaquaion hydrolysis solutions?
- charge on the metal ion
* size of metal ion
Why does the charge on the metal ion and size affect the degree of acidity of solution after hydrolysis?
Metal centre is more polarising if charge is increased and size decreased as OH bond weakened to greater extent and more H⁺ formed
What can the two factors affecting the acidity of hydrolysis solutions be combined into?
Size to charge ratio (charge density)
What does size to charge ratio measure?
The polarising power of the ion i.e. ability to attract oppositely charged species
What effect does polarising power have on the acidity of the acidity of the hydrolysis solution?
↑ polarising power of metal ion = ↑ attraction for electrons from the O in the H₂O molecule - making OH bond weaker and easier to break - releasing H⁺ ions which form H₃O⁺ with water, increasing acidity
What is the formula for protonated water?
H3O⁺
Why are 3⁺ hexaaqua cations the most acidic in solution?
Greater charge to size ratio than 2+ so have greater polarising power and H₃O⁺ concentration and lower pH
When can further hydrolysis of aquaions take place?
When the H₃O⁺ is removed by the addition of a base
What bases can be used for further hydrolysis of aquaions?
OH⁻, NH₃, CO₃²⁻
What is the product of further hydrolysis?
The insoluble, neutral metal hydroxide
How is the product of further hydrolysis seen?
As a precipitate
What effect does the base have on equilibrium for further hydrolysis of aquaions?
Pushes equilibrium right - base reacts with acid in neutralisation reaction, using acid up and reducing concentration of H₃O⁺
What is the overall equation for the complete hydrolysis of M²⁺ ions?
[M(H₂O)₆]²⁺ + 2H₂O → [M(H₂O)₄(OH)₂] + 2H₃O⁺
What is the overall equation for the complete hydrolysis of M³⁺ ions?
[M(H₂O)₆]³⁺ + 3H₂O → [M(H₂O)₃(OH)₃] + 3H₃O⁺
Does further hydrolysis happen with water? Why?
No - it is not a strong enough base
What reactions show how the hydroxide is precipitated during hydrolysis with sodium hydroxide?
[M(H₂O)₆]²⁺ + 2H₂O → [M(H₂O)₄(OH)₂] + 2H₃O⁺
2H₃O⁺ + 2OH⁻ → 4H₂O
(Then the H₃O⁺ react again with the OH⁻)
Is the reaction of metalaquaions with sodium hydroxide ligand substitution?
No - it is hydrolysis instead
What is the overall general equation for hydrolysis of a 2+ metalaquaion with sodium hydroxide?
[M(H₂O)₆]²⁺ + 2OH⁻ → [M(H₂O)₄(OH)₂] + 2H₂O
How can reactions of metalaquaions with sodium hydroxide be reversed?
By adding a strong acid
Why does adding a strong acid reverse hydrolysis between metalaquaions and sodium hydroxide?
The acid reacts with the OH⁻ to remove it, pushing the equilibrium to the left to replace it
What equation can show why ammonia solution is a weak alkali?
NH3 + H2O → NH4⁺ + OH⁻
What happens when a metalaquaion undergoes hydrolysis with ammonia solution?
The OH⁻ ions (from NH3 + H2O → NH4⁺ + OH⁻) precipitate the metal as its hydroxide in the same way as NaOH
What is the general equation for a M²⁺ ion with NH₃ and the equations that add together to form this?
[M(H₂O)₆]²⁺ + 2OH⁻ → [M(H₂O)₄(OH)₂] + 2H₂O
2NH₃ + 2H₂O ⇋ 2NH₄⁺ + 2OH⁻
overall: [M(H₂O)₆]²⁺ + 2NH₃ → [M(H₂O)₄(OH)₂] + 2NH₄⁺
What is the general equation for a M³⁺ ion with NH₃ and the equations that add together to form this?
[M(H₂O)₆]³⁺ + 3OH⁻ → [M(H₂O)₃(OH)₃] + 3H₂O
3NH₃ + 3H₂O ⇋ 3NH₄⁺ + 3OH⁻
overall: [M(H₂O)₆]³⁺ + 3NH₃ → [M(H₂O)₃(OH)₃] + 3NH₄⁺
For the equation [M(H₂O)₆]²⁺ + 2NH₃ → [(H₂O)₄(OH)₂] + 2NH₄⁺, how has the ammonia been added?
Dropwise
For the equation [M(H₂O)₆]³⁺ + 3NH₃ → [(H₂O)₃(OH)₃] + 3NH₄⁺, how has the ammonia been added?
Dropwise
What happens if excess ammonia is added to the metal salt solution?
Ligand substitution will occur for Cu²⁺
What is the equation for when excess ammonia is added to copper (II) salt solution?
[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
Are carbonate ions weak or strong bases?
Weak
What do carbonate ions react with acids to form?
Carbon dioxide and water
Equation for reaction of carbonate ions with acid?
CO₃²⁻ + 2H₃O⁺ → CO₂ + 3H₂O
What does the reaction of metal aqua ions with carbonates depend on?
The oxidation state of the metal ion
What happens when carbonate ions react with M3+ metalaquaions?
There is sufficient H₃O⁺ to form carbon dioxide and water
What is the general equation the reaction of carbonate ions with M3+ metalaquaions and the equations that add together to form this?
[M(H₂O)₆]³⁺ + 3H₂O ⇋ [M(H₂O)₃(OH)₃] + 3H₃O⁺ (x2)
CO₃²⁻ + 2H₃O⁺ → CO₂ + 3H₂O (x3)
overall: 2[M(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[M(H₂O)₃(OH)₃] + 3CO₂ + 3H₂O
Why can’t M2+ metalaquaions form carbon dioxide and water?
They are very weakly acidic in solution, so not enough acid (H₃O⁺)
What do M2+ metalaquaions form when carbonate ions are added?
Insoluble metal carbonates (MCO₃)
Equation for reaction of M2+ metalaquaions with carbonate ions?
[M(H₂O)₆]²⁺ + CO₃²⁻ → MCO₃ + 6H₂O
Product and observation for reaction of [Cu(H₂O)₆]²⁺ with NaOH or NH₃ dropwise?
[Cu(H₂O)₄(OH)₂] - blue precipitate
Product and observation for reaction of [Cu(H₂O)₆]²⁺ with NaOH in excess?
No change (so [Cu(H₂O)₄(OH)₂] - blue precipitate)
Product and observation for reaction of [Cu(H₂O)₆]²⁺ with NH₃ in excess?
[Cu(NH₃)₄(H₂O)₂]²⁺ - deep blue solution
Product and observation for reaction of [Cu(H₂O)₆]²⁺ with Na₂CO₃?
CuCO₃ - blue-green precipitate
Product and observation for reaction of [Fe(H₂O)₆]²⁺ with NaOH or NH₃ dropwise?
[Fe(H₂O)₄(OH)₂] - green precipitate → if left to stand precipitate turns brown
Product and observation for reaction of [Fe(H₂O)₆]²⁺ with NaOH in excess?
No change (so [Fe(H₂O)₄(OH)₂] - green precipitate that turns brown if left to stand)
Product and observation for reaction of [Fe(H₂O)₆]²⁺ with NH₃ in excess?
No change (so [Fe(H₂O)₄(OH)₂] - green precipitate that turns brown if left to stand)
Product and observation for reaction of [Fe(H₂O)₆]²⁺ with Na₂CO₃?
FeCO₃ - green precipitate
Product and observation for reaction of [Fe(H₂O)₆]³⁺ with NaOH or NH₃ dropwise?
[Fe(H₂O)₃(OH)₃] - brown precipitate
Product and observation for reaction of [Fe(H₂O)₆]³⁺ with NaOH in excess?
No change (so [Fe(H₂O)₃(OH)₃] - brown precipitate)
Product and observation for reaction of [Fe(H₂O)₆]³⁺ with NH₃ in excess?
No change (so [Fe(H₂O)₃(OH)₃] - brown precipitate)
Product and observation for reaction of [Fe(H₂O)₆]³⁺ with Na₂CO₃?
[Fe(H₂O)₃(OH)₃] - brown precipitate and effervescence
Product and observation for reaction of [Al(H₂O)₆]³⁺ with NaOH or NH₃ dropwise?
[Al(H₂O)₃(OH)₃] - white precipitate
Product and observation for reaction of [Al(H₂O)₆]³⁺ with NaOH in excess?
[Al(H₂O)₂(OH)₄] - colourless solution (further hydrolysis)
Product and observation for reaction of [Al(H₂O)₆]³⁺ with NH₃ in excess?
[Al(H₂O)₃(OH)₃] - white precipitate
Product and observation for reaction of [Al(H₂O)₆]³⁺ with Na₂CO₃?
[Al(H₂O)₃(OH)₃] - white precipitate and effervescence
What is the amphoteric character of metal hydroxides?
The ability of metal hydroxides to react with both acids and alkalis
How can the hydrolytic equilibrium reactions we’ve looked at be reversed?
Using a strong acid - the metal hydroxides dissolve to give the metal aqua ion
What happens when strong acid is added to metal hydroxides?
They dissolve to give the metal aqua ion - reversing the hydrolysis
What happens if excess HCl is added to a metal hydroxide solution?
- Ligand substitution occurs with some of the metal hexaaqua ions (complete ligand substitution)
- H₂O ligands are replaced by Cl, changing coordination number, shape, charge so changes colour
When excess HCl is added to metal hydroxide solution, why does the solution change colour?
Some H₂O ligands are changed to Cl, changing coordination number, shape and charge
When metal hydroxides react with acid, what do they act as?
Bases - proton acceptors
What do metal hydroxides form when excess hydroxide is added?
Anionic complexes which are water soluble (further hydrolysis)
Which metals undergo further hydrolysis?
Al
When metal hydroxides react with alkalis, what happens to the H₂O ligands?
They donate a proton (H⁺) and therefore act as Bronsted-Lowry acids
What is the equation for when aluminium hydroxide reacts with excess hydroxide?
[Al(H₂O)₃(OH)₃] + OH⁻ → [Al(H₂O)₂(OH)₄]⁻ + H₂O
How do we know that further hydrolysis occurs when excess hydroxide is added to aluminium hydroxide?
The precipitate formed from the drowse addition dissolves in excess
Why is it difficult to dissolve 2+ ions in alkali?
They are less acidic so would require a very concentrated alkaline solution
What is a common example that shows amphoteric character and why is this?
Aluminium as it easily hydrolyses further
