Ions In Aq Solution Flashcards
(36 cards)
we usually think of ions in solutions as free, however because water can act as a…
ligand, TM ions form octahedral complexes with water known as aqua ions
- there is a LP on the O, which can be donated to the TM ion (L acid) in a coordinate bond
colours of aqua ions
Fe2+
Fe3+
Al3+
Cu2+
Fe2+ - green
Fe3+ - violet
Al3+ - colourless
Cu2+ - blue
- in Fe3+ aqua ion solution it normally looks yellowish in actuality
- only solid hydrated salts show true colour
how does the difference in charge affect anything
changes the aqua ions chemistry with carbonates because the increased charge makes the TM ion have more polarising power
polarising power
specific to Al
ability of a cation (+ve ion) to distort the e- density of an anion towards itself
Al has covalent character; strong charge and small ionic radius therefore high charge density. higher effective charge
how do aqua ions have acidity
- the attractive +ve charge of the central TM ion causes a distortion of e- pair in Fe-O
- it strongly attracts it due to the polarising effect
- this causes O to attract e- from OH bond to itself because it becomes delta +ve so more attractive
- this weakens the OH bond to the extent that it breaks the bonds
- so the aqua ions can act as BL acids because they release H+ in solution
what determines acid strength of aqua ions
depends on amount of H+ get released, which depends on how weak the OH bond is, which depends on how strong the polarising effect of the metal ion is
the 3+ ions have higher charge density (charge/size ratio) therefor greater polarising power therefore stronger acids
hydrolysis
the splitting up of water (OH bond breaks due to attractive force of TM ion) is hydrolysis
hydrolysis is what releases the H+, giving aqua ions their acidity
reversible reaction
when will hydrolysis stop
when the +ve charge of the TM ion is balanced by the -ve charge on the Os after they donate H+
so for 2+ aqua ions, they lose 2H+ to form insoluble hydroxides
in solution hydrolysis doesn’t readily happen. when they react with [3] they form [] by []
- NaOH, NH3, Na2CO3
- metal hydroxides
- hydrolysis
what happens to the reversible reaction when aqua ions react with NaOH
- added OH-, eqm shifts to decrease OH-
- so more H+ is released to react together to form water
- so eqm shifts to right
- so hydroxides form
NaOH with Fe2+ aqua ion
[Fe(H2O)6]2+ + 2OH- –> [Fe(H2O)4(OH)2] + 2H2O
green solution to green ppt
no reaction with excess
NaOH with Cu2+ aqua ion
[Cu(H2O)6]2+ + 2OH- –> [Fe(H2O)4(OH)2] + 2H2O
blue solution to blue ppt
no reaction with excess
NaOH with Fe3+ aqua ion
[Fe(H2O)6]3+ + 3OH- –> [Fe(H2O)3(OH)3] + 3H2O
violet solution to brown ppt
no further reaction with excess
NaOH with Al3+ aqua ion
[Al(H2O)6]3+ + 3OH- –> [Al(H2O)3(OH)3] + 3H2O
colourless solution to white ppt
WITH EXCESS
[Al(H2O)3(OH)3] + OH- –> [Al(H2O)2(OH)4]- + H2O
white ppt redissolves
why does the Al3+ hydroxide redissolve with excess OH-
excess OH- causes further hydrolysis of another water molecule in the complex
hydroxide can act as an acid (or a base, amphoteric; the hydroxide can also act as a base and regain H+ to become the aqua ion) so further H+ released as the hydroxide.
reactions with NaOH overall
form metal hydroxide by hydrolysis
excess with Al forms aqua ion again with further hydrolysis
NH3 + Cu2+ aqua ion
2NH3 + [Cu(H2O)6]2+ –> [Cu(H2O)4(OH)2] + 2NH4+
blue solution to blue ppt
reacts with excess causing LIGAND SUBSTITUTION to form a deep blue solution. ppt redissolves
[Cu(H2O)4(OH)2] + 4NH3 –> [Cu(H2O)2(NH3)4]2+ + 2OH- + 2H2O
blue ppt to blue solution; ppt redissolves
why does Cu hydroxide react with excess NH3
why the colour change
- the Cu-O bond is longer and weaker
- so it breaks more easily with excess NH3
- NH3 bond forms instead
- it forms an elongated octahedral complexes. so the complex arranges itself like the h2o on top and bottom (example of geometric TM ion complex isomerism, E/trans)
because the ligand has changed the energy gap between d orbitals changes. so the colour changes. (energy gap is amount of absorption/freq of light absorbed)
NH3 + Fe2+ aqua ion
2NH3 + [Fe(H2O)6]2+ –> [Fe(H2O)4(OH)2] + 2NH4+
green solution to a green ppt
NH3 + Fe3+ aqua ion
3NH3 + [Fe(H2O)6]3+ –> [Fe(H2O)3(OH)3] + 3NH4+
violet solution to brown ppt
NH3 + Al3+ aqua ion
3NH3 + [Fe(H2O)6]3+ –> [Fe(H2O)3(OH)3] + 3NH4+
Colourless Solution to white ppt
reactions with NH3 overall
form metal hydroxides by hydrolysis
in excess, Cu2+ hydroxide reacts by ligand substitution
Na2CO3 + Cu2+ aqua ion
CO3^2- + [Cu(H2O)6]2+ –> CuCO3 + 6H2O
blue solution to blue ppt
Na2CO3 + Fe2+ aqua ion
CO3^2- + [Fe(H2O)6]2+ –> FeCO3 + 6H2O
green solution to green ppt
