24.1 acid base chemistry of aqueous transition metal ions
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How are metal aqua complex ions formed
. When metals dissolve in water, the water molecules form coordinate bonds with the central transition metal ions Eg 6 water molecules form co ordinate bonds with the metal ion by donating a non bonding pair of electrons from their oxygen
What are the 4 metal complex ions we need to know
Fe2+ and Cu2+ as the 2+ ions so [Fe(H2O)6]2+ and [Cu(H2O)6]2+ Al3+ and Fe3+ as the 3+ ions So [Al(H2O)6]3+ and [Fe(H2O)6]3+
What 3 bases do we need to know the reactions of metal ions with
OH- NH3 CO3 2-
What is the copper metal aqua ion What colour is it
[Cu(H2O)6 ]2+ Each H2O ligand donates a pair of electrons from its oxygen to the copper ion , forming 6 co ordinate bonds It is blue
What is the iron (II) metal aqua ion What about iron (III) metal ion What is the aluminium metal aqua ion
[Fe(H2O)6]2+ It is green [Fe(H2O)6]3+ It is pale violet when it is not oxidised, but turns brown when it is [Al(H2O)6]3+ It is colourless The charge of the overall complex is the same as the transition metal ion because the H2O ligand is neutral.
Do these 4 metal aqua complexes form acidic or basic solution
They all form acidic solutions, however aluminium is ampholytic so can also form basic solutions
What is the hydrolysis reaction of the metal aqua 2+ ions
Any solution that contains metal aqua 2+ ions There is a reaction between the metal aqua ion and the water, and this reaction is called hydrolysis The metal aqua 2+ ions release H+ ions, so an acidic solution is formed But there is only a slight dissociation so the solution is only a weak acid
What happens when the complex [Fe(H2O)6]2+ reacts with water in a hydrolysis reaction This can also occur with [Cu(H2O)6]2+
It will dissociate in water to form [Fe(H2O)6] 2+(aq) + H2O(l) —> [Fe(OH)(H2O)5]+(aq) + H+ The complex has an overall charge of + instead or 2+ because the OH- ion makes it that This can keep repeating, eg [Fe(OH)(H2O)5]+ —> Fe(OH)2(H2O)4(s) + H+ Fe(OH)2(H2O)4(s) —> [Fe(OH)3(H2O)3]- (aq) + H+ Eventually it stops when [Fe(OH)5(H2O)]3-(aq) —> [Fe(OH)6]4- + H+
How do the metal 3+ion complexes have hydrolysis reactions Eg [Fe(H2O)6]3+ or [Al(H2O)6]3+
[Fe(H2O)6]3+ —> [Fe(H2O)5(OH)]2+ + H+ then [Fe(H2O)5(OH)]2+ —> [Fe(H2O)4(OH)2]+ + H+ then [Fe(H2O)4(OH)2]+ —> Fe(H2O)3(OH)3(s) + H+
How do metal aqua 3+ ions react in water compared to 2+ ions Give an example, eg of [Al(H2O)6]3+
They react in the same way but form more acidic solutions [Al(H2O)6]3+ + H2O(l) —> [Al(OH)(H2O)5]2+(aq) + H3O+ And H3O+ is formed because inside the brackets there are 11 H and 6 O as one H2O is turned into OH so there is a spare H which is H+ ion in the water
Why are metal aqua 3+ solutions more acidic than 2
. Metal 3+ ions are very small and have a big charge so they’ve got a high charge density The metal 2+ ions have a much lower charge density • This makes the 3+ ions much more polarising than the 2+ ions • So they attract electrons from the oxygen atoms of the coordinatly bonded water molecules more strongly • Weakening the O-H bond, so it is more likely a hydrogen will be released when the bond breaks More hydrogen ions means more acidic solution, so metal 3+ ions are more acidic than metal 2+ ions
Describe what happens when limited OH- ions are added the a 2+ metal ion complex [Fe(H2O)6] 2+ What is the colour change
[Fe(H2O)6]2+(aq) + 2OH-(aq) —> Fe(H2O)4(OH)2 + 2H2O(l) Remember that the number of OH- ions added are the same as the charge of the metal ion Colour changes from green solution to green precipitate
Describe what happens when limited OH- ions are added the a 2+ metal ion complex [Cu(H2O)6]2+ What is the colour change
[Cu(H2O)6]2+ (aq) + 2OH-(aq) —> [Cu(H2O)4(OH)2] (aq) + 2H2O(l) Colour goes from blue solution to blue precipitate
describe what happens when reacting a metal 3+ ion eg Fe3+ with limited OH- ions What is the colour change
[Fe(H2O)6] 3+(aq) + 3OH-(aq) —> [Fe(H2O)3(OH)3] (s) + 3H2O Remember that the number of OH- ions you add is the same as the metal ion charge Colour goes from a violet solution to a BROWN precipitate
Describe what happens when a metal ion 3+ complex , eg aluminium is reacted with limited OH- ions from NaOH What are the colour changes
[Al(H2O)6]3+(aq) + 3OH-(aq) —> Al(H2O)3(OH)3(s) + 3H2O(l) Goes from colourless solution to a white precipitate
What happens when limited ammonia (NH3) is added to metal aqua ion complexes What type of reaction is it
This is an acid base reaction Water molecules can be replaced by the neutral ligand ammonia, so coordination number won’t change The ammonia molecule gains a proton so acts as a base However it is basic so has the same effect as OH- ions at removing protons
Describe the reaction for when limited ammonia (NH3) is added to the metal ion complex [Cu(H2O)6]2+ (NH3) acts as OH- What is the colour change
[Cu(H2O)6]2+(aq) + 2NH3(aq) —> Cu(H2O)4(OH)2+ 2NH4+ They have the same colour changes as the OH- ions So from blue solution to a blue precipitate
Describe the reaction for when limited ammonia (NH3) is added to the metal ion complex [Fe(H2O)6]2+ What is the colour change
[Fe(H2O)6]2+ (aq) + 2NH3(aq) —> Fe(H2O)4(OH)2(s) + 2NH4+(aq) The colour changes from a green solution to a green precipitate
Describe the reaction for when limited ammonia (NH3) is added to the metal ion complex [Fe(H2O)6]3+
[Fe(H2O)6]3+(aq) + 3NH3(aq) —> Fe(H2O)3(OH)3(s) + 3NH4+(Aq) Colour changes from a purple solution to a brown precipitate
Describe the reaction for when limited. ammonia (NH3) is added to the metal ion complex [Al(H2O)6]3+
[Al(H2O)6]3+ (aq) + 3NH3(aq) —-> Al(H2O)3(OH)3 (s) + 3NH4+ Colour changes from colourless to white precipitate
What happens when excess OH- ions are reacted with a metal 2+ ion complex [Fe(H2O)6]2+
It is the same reaction as before, but with the product you add one more OH- ion to it Eg [Fe(H2O)6]2+(aq) + 2OH-(aq) –> Fe(H2O)4(OH)2(s) + 2H2O Then Fe(H2O)4(OH)2 (s) + OH-(aq) —> [Fe(OH)3]- + 4H2O So it kicks off the H2Os
What happens when excess OH- ions are reacted with a metal 3+ ion complex Eg [Al(H2O)6]3+ What is the colour change
An extra OH- is added onto the original product so Al(H2O)3(OH)3 + OH- —> [Al(OH)4]- + 3H2O And it turns from a white precipitate to a colourless solution
Aluminium aqua ion is amphoteric What does this mean
It reacts with both acids and bases, unlike the other ones we have studied which just react with bases
Describe the reaction between Al(H2O)3(OH)3 and an acid
It is acting as a base to accept H+ ions Al(H2O)3(OH)3 + 3H+ —> [Al(H2O)6]3+ Here, it forms its aqua ion complex again So colour changes from white precipitate to colourless solution