3.2.6 Reactions of ions in aqueous solution Flashcards
What is a metal aqua-ion
A transition metal dissolved in water
How do transition metal compounds dissolve in water?
- The water molecules form co-ordinate bonds with the metal ions
- This forms a metal aqua-complex ion
- In general six water molecules form co-ordinate bonds with each metal ion
- The water molecules do this by donating a non-bonding pair of electrons from thier oxygen
- The overall charge will be the same as the transition metal as water ligands are neutral
What happens in a solution containing metal-aqua 2+ ions
A hydrolysis or acidity reaction takes place - e.g:
Cu(H2O)62+(aq) + H2O(l) <=> [Cu(OH)(H2O)5]+(aq) + H3O+(aq)
The metal-aqua 2+ ions release H+ ions, so an acidic solution is formed. There’s only slight dissociation tough, so the solution is weakly acidic
What happens in a solution containing metal aqua 3+ ions
They dissociate more than metal-aqua 2+ ions, and so form more acidic solutions - e.g:
Fe(H2O)63+(aq) + H2O(l) <=> [Fe(OH)(H2O)5]2+(aq) + H3O+(l)
Why do 3+ metal-aqua ions form more acidic solutions than 2+ metal-aqua ions?
- Metal 3+ ions are pretty small but have a big charge - so they’ve got a higher charge density (charge/size ratio). The metal 2+ ions have a much lower charge density
- This makes the 3+ ions much more polarising than the 2+ ions. More polarising power means that they attract electrons from the oxygen atoms of the co-ordinated water molecules more strongly, weakening the O-H bond
- So it’s more likely that a hydrogen ion will be released. And more hydrogen ions means a more acidic solution
What happens when you hydrolyse metal-aqua ions further by adding OH- ions
An insoluble metal hydroxide precipitate is formed.
Why and how do metal-aqua 3+ ions produce a metal hydroxide precipitate when OH- ions are added?
1 - In water, metal-aqua 3+ ions form the equilibrium:
M(H2O)63+(aq) + H2O(l) <=> [M(OH)(H2O)5]2+(aq) + H3O+(aq)
If you add OH- ions to the equilibrium, H3O+ ions are removed - the equilibrium moves right
2 - Now another equilibruim is set up in the solution:
[M(OH)(H2O)5]2+(aq) + H2O(l) <=> [M(OH)2(H2O)4]+(aq) + H3O+(aq)
Again the OH- ions remove H3O+ ions from the solution, shifting the equilibrium right
3 - This happens one more time - now you’re left with an insoluble uncharged metal hydroxide:
[M(OH)2(H2O)4]+(aq) + H2O(l) <=> [M(OH)3(H2O)3](s) + H3O+(aq)
How does metal-aqua 2+ ions produce a metal hydroxide precipitate when OH- ions are added?
Explain how it’s different to metal-aqua 3+ ions
The same process as metal-aqua 3+ ions, just two steps instead of three to make the metal hydroxide becomes uncharged and insoluble.
1:
M(H2O)62+(aq) + H2O(l) <=> [M(OH)(H2O)5]+(aq) + H3O+(aq)
2:
[M(OH)(H2O)5]+(aq) + H2O(l) <=> [M(OH)2(H2O)4](s) + H3O+(aq)
Explain how metal hydroxides can be amphoteric
- All metal hydroxides will dissolve in acid. They act as bronstead-lowry bases and accept H+ ions, reveresing the hydrolysis reaction that formed the metal hydroxide.
- Some metal hydroxides are amphoteric - they can act as both acids and bases, meaning they will dissolve in an excess of base aswell as in acids
- Aluminium hydroxide is amphoteric. In the presences of a base, e.g. NaOH, it acts as a Bronstead-Lowry acid and donates H+ ions to the OH- ions, forming a soluble compound. It also acts as a Bronstead-Lowry base in the presence of acid and accepts H+ ions from the H3O+ ions in solution.
With acid:
Al(OH)3(H2O)3(s) + 3H3O+(aq) -> [Al(H2O)6]3+(aq) + 3H2O(aq)
With base:
Al(OH)3(H2O)3(s) + OH-(aq) -> [Al(OH)4]-(aq) + 3H2O(l)
Why does adding ammonia solution form a precipitate when added to a metal-aqua ion?
- Normally adding hydroxide ion to a solution is done with a strong base like NaOH solution, however ammonia solution can be used too
- When ammonia dissolves in water this equilibrium occurs:
NH3(aq) + H2O(l) <=> NH4+(aq) + OH-(aq)
- Because hydroxide ions are formed, adding a small amount of ammonia solution gives the same result as NaOH
Why does adding excess ammonia cause the metal hydroxide precipitate of an metal-aqua ion dissolve?
Sometimes, such as with Cu(OH)2(H2O)4 a further rection happens if you add excess ammonia solution - The H2O and OH- ligands are displaced by the NH3 ligands. This forms a charged complex which is soluble in water, so the precipitate dissolves.
Cu(OH)2(H2O)4(s) + 4NH3(aq) <=> [Cu(NH3)4(H2O)2]2+(aq) + 2OH-(aq) + 2H2O(l)
Give the equation to show how M 2+ ions react with sodium carbonate to form insoluble metal carbonates
[M(H2O)6]2+(aq) + CO32-(aq) <=> MCO3(s) + 6H2O(l)
What happens when metal 3+ ions react with sodium carbonate?
Metal 3+ ions are stronger acids so there is a higher conc. of H3O+ ions in solution. Rather than displacing water from the metal ions, the carbonate ions react with the H3O+.
CO32-(aq) + 2H3O+(aq) <=> CO2(g) + 3H2O(l)
This removes them from solution and shifting the equilibria to the right. So the precipitate that froms is M(OH)3(H2O)3, rather than M2(CO3)3
Outline the test tube reaction method used to identify metal ions
- Measure out sample of the unknown metal ion solution into three seperate test tubes
- To the first tube, add NaOH solution dropwise, record any observed changes, add more NaOH so it’s in excess, record changes again
- To the second tube add ammonia solution dropwise, record any observed changes, add excess ammonia, record changes again
- Add sodium carbonate solution to the third test tube dropwise, record any changes
Explain what happens when you add NaOH to an unknown metal-aqua ion in a test tube reaction?
All four metal-aqua ions will form precipitates with sodium hydroxide, but only the aluminium hydroxide precipitate will dissolve in as excess of NaOH, due to it being amphoteric
