reactions in aqueous solutions 3.2.6 Flashcards
What is the structure of metals in a water solution?
the metal will have 6 water molecule ligands co-ordinately bonded by donating a lone pair from the oxygen
What is the colour of [Cu(H2O)6] 2+?
Blue
What is the colour of [Fe(H2O)6] 2+?
Green
What is the colour of [Fe(H2O)6] 3+? What is the real colour and why?
Purple
- the real colour is yellow/brown because small amounts of [Fe(H2O)5(OH)] 2+ are formed which means and H+ is lost making the solution more acidic
What is the colour of [Al(H2O)6] 3+ and why?
Colourless
- it is not a transition metal so doesn’t have an incomplete d orbital therefore no energy gap that can absorb and reflect wavelengths of light
What are the 3 types of reactions metal aqua ions undergo?
1) hydrolysis - loss of an H+ from H2O ligand
2) substitution - replacement of H2O by other ligands
3) redox - metal changes oxidation state
What are the reaction equations for the hydrolysis of M2+ metal aqua ions?
When does the reaction stop for Fe2+ and Cu2+ ions?
1) [M(H2O)6]2+ [M(H2O)5(OH)]1+ + H+
2) [M(H2O)5(OH)]1+ [M(H2O)4(OH)2] + H+
Reaction stops when a neutral complex forms
What are the reaction equations for the hydrolysis of M3+ metal aqua ions?
When does the reaction stop for Fe3+?
When does the reaction stop for Al3+ ions?
1) [M(H2O)6]3+ [M(H2O)5(OH)]2+ + H+
2) [M(H2O)5(OH)]2+ [M(H2O)4(OH)2]1+ + H+
3) [M(H2O)5(OH)]1+ [M(H2O)3(OH)3 + H+
For Fe3+ reaction stops when a neutral complex forms
For Al3+ reaction stops when an [M(OH)6]3- complex forms
What are the products of the hydrolysis of M2+ and M3+ ion complexes called?
Metal (II) hydroxides and metal (III) hydroxides
What does hydrolysis do to the pH of the solution of M2+ or M3+ ion complexes and why?
makes it more acidic due to the increase in H+
What is the pH of a [M(H2O)6]+ solution?
7
What is the pH of a [M(H2O)6]2+ solution?
6
What is the pH of a [M(H2O)6]3+ solution?
3
Why do M3+ ion complexes form more acidic solutions?
- M3+ ions are small and highly charged so have a high charge density
- this means they have greater polarising power which means they are more likely to break the O-H bond in water to release an H+ ion making it more acidic
What are the 3 bases aqua metal ions react with?
1) OH- ions (e.g., NaOH)
2) NH3
3) CO3 2- ions (e.g., Na2CO3)
What will form if a hydrolysis reaction happens with a base?
a neutral, insoluble complex will form as a precipitate
What are the observations and equation for [Fe(H2O)6]2+ when you add a small amount of OH- ions?
green solution to a green precipitate
[Fe(H2O)6]2+ + 2OH- = [Fe(H2O)4(OH)2] + 2H2O
What are the observations and equation for [Fe(H2O)6]2+ when you add excess OH- ions?
no visible change
What are the observations and equation for [Cu(H2O)6]2+ when you add a small amount of OH- ions?
blue solution to a blue precipitate
[Cu(H2O)6]2+ + 2OH- = [Cu(H2O)4(OH)2] + 2H2O
What are the observations and equation for [Cu(H2O)6]2+ when you add excess OH- ions?
no visible change
What are the observations and equation for [Fe(H2O)6]3+ when you add a small amount of OH- ions?
yellow solution to a brown precipitate
[Fe(H2O)6]3+ + 3OH- = [Fe(H2O)3(OH)3] + 3H2O
What are the observations and equation for [Fe(H2O)6]3+ when you add excess OH- ions?
no visible change
What are the observations and equation for [Al(H2O)6]3+ when you add a small amount of OH- ions?
colourless solution to a white precipitate
[Al(H2O)6]3+ + 3OH- = [Al(H2O)3(OH)3] + 3H2O
What are the observations and equation for [Al(H2O)6]3+ when you add excess OH- ions?
the white precipitate will redissolve to a colourless solution
[Al(H2O)3(OH)3] + OH- = [Al(H2O)2(OH)4]1- + H2O
What is the summary for the hydrolysis reactions of aqua metal ions with OH- ions?
- Metal (II) ions form precipitates of Metal (II) hydroxides
- Metal (III) ions form precipitates of Metal (III) hydroxides
- [Al(H2O)3(OH)3] is an anomaly and reacts further with OH- ions
What are the observations and equation for [Fe(H2O)6]2+ when you add a small amount of NH3?
green solution to a green precipitate
[Fe(H2O)6]2+ + 2NH3 = [Fe(H2O)4(OH)2] + 2NH4+
What are the observations and equation for [Fe(H2O)6]2+ when you add excess NH3?
no visible change
What are the observations and equation for [Cu(H2O)6]2+ when you add a small amount of NH3?
Blue solution to a blue precipitate
[Cu(H2O)6]2+ + 2NH3 = [Cu(H2O)4(OH)2] + 2NH4+
What are the observations and equation for [Cu(H2O)6]2+ when you add excess NH3?
blue precipitate dissolves to form a deep blue solution
[Cu(H2O)4)OH)2] + 4NH3 = [Cu(H2O)2(NH3)4]2+ + 2H2O + 2OH- (ligand subsitiution reaction)
What are the observations and equation for [Fe(H2O)6]3+ when you add a small amount of NH3?
yellow solution to a brown precipitate
[Fe(H2O)6]3+ + 3NH3 = [Fe(H2O)3(OH)3] + 3NH4+
What are the observations and equation for [Fe(H2O)6]3+ when you add excess NH3?
no visible change
What are the observations and equation for [Al(H2O)6]3+ when you add a small amount of NH3?
colourless solution to a white precipitate
[Al(H2O)6]3+ + 3NH3 = [Al(H2O)3(OH)3] + 3NH4+
What are the observations and equation for [Al(H2O)6]3+ when you add excess NH3?
no visible change
What is the summary for the hydrolysis reactions of aqua metal ions with NH3?
- Metal (II) ions form precipitates of Metal (II) hydroxides
- Metal (III) ions form precipitates of Metal (III) hydroxides
- Cu2+ is an anomaly and reacts further with NH3 by ligand substitution
What are the observations and equation for [Fe(H2O)6]2+ when you add CO3 2- ions?
green solution to a green precipitate
[Fe(H2O)6]2+ + CO3 2- = FeCO3 + 6H2O
What are the observations and equation for [Cu(H2O)6]2+ when you add CO3 2- ions?
Blue solution to a green/blue precipitate
[Cu(H2O)6]2+ + CO3 2- = CuCO3 + 6H2O
What are the observations and equation for [Fe(H2O)6]3+ when you add CO3 2- ions?
fizzing and a brown precipitate
2[Fe(H2O)6]3+ + 3CO3 2- = 2[Fe(H2O)3(OH)3] + 3H2O + 3CO2
What are the observations and equation for [Al(H2O)6]3+ when you add CO3 2- ions?
fizzing and a white precipitate
2[Al(H2O)6]3+ + 3CO3 2- = 2[Al(H2O)3(OH)3] + 3H2O + 3CO2
What is the summary for the hydrolysis reactions of aqua metal ions with CO3 2- ions?
- Metal (II) ions form precipitates of Metal (II) carbonates
- Metal (III) ions form precipitates of Metal (III) carbonates and bubbles of CO2 gas
- M3+ are acidic enough to react in an acid-base reaction with basic CO3 2- ions but M2+ are not
What metal hydroxide is amphoteric?
[Al(H2O)3(OH)3]
What does amphoteric mean?
it can react as an acid or a base
What are the observations and equation for when you add an alkali to [Al(H2O)3(OH)3]?
colourless solution to a white precipitate
[Al(H2O)3(OH)3] + OH- = [Al(H2O)2(OH)4]1- + H2O
What are the observations and equation for when you add an acid to [Al(H2O)3(OH)3]?
the white precipitate will redissolve to a colourless solution
[Al(H2O)3(OH)3] + 3H+ = [Al(H2O)6]3+
What test tube reactions test for Al3+ ions, Cu2+ ions and Fe2+/Fe3+ ions?
NaOH for Al3+ ions = white precipitate (precipitate dissolves with excess NaOH)
NH3 for Cu2+ ions = blue precipitate (precipitate dissolves and a deep blue solution with excess NH3)
Na2CO3 for Fe2+ ions = a green precipitate forms but no gas
Na2CO3 for Fe3+ ions = a brown precipitate forms but with CO2 gas
