metal aqua ions Flashcards
(39 cards)
Why do metal aqua ions form?
Metal ions have high charge density. In aqueous solutions, water molecules are attracted very strongly to these ions.
The oxygen atom in a water molecule donates a lone pair of electrons to a vacant orbital in the metal ion, forming a co-ordinate bond.
What are the shape and coordination number of metal aqua ions?
Octrahedral
6
What colour is [Cu(H20)6]2+?
Blue solution
What colour is [Fe(H20)6]2+?
Green
What colour is [Fe(H20)6]3+?
Pale violet/ brown
How is [Fe(H20)6]3+ a place violet/ brown solution?
It will immediately undergo hydrolysis in water to form [Fe(H20)5(OH)]2+ ions which are brown.
What colour is [Al(H20)6]3+?
colourless
What happens in the hydrolysis of metal aqua ions?
The O-H bong in a H2O ligand breaks, releasing a H+ ion and leaving an OH- ligand.
Why are metal aqua ions slightly acidic?
In solution, they loose a H+ from one or more H2O ligands in hydrolysis. The equilibrium of this reaction lies to the left so they are not strong acids.
Which of the possible metal hydroxides formed by hydrolysis of a metal 2+ aqua ion are insoluble?
The neutral complex ions, e,g.
M(H2O)4(OH)2 are
insoluble in water and form as
precipitates. They are known as metal (II) hydroxides.
Which of the possible metal hydroxides formed by hydrolysis of a metal 3+ aqua ion are insoluble?
The neutral complex ions, e.g.
M(H2O)3(OH)3 are
insoluble in water and form as
precipitates. They are known as metal (Ill) hydroxides.
What is the ph of a typical solution of metal 1+ aqua ions?
7 as the metal ion does not have a high enough charge density to weaken the O-H bond causing H+ dissociation.
What is the ph of a typical solution of metal 2+ aqua ions?
6 as the metal ion does not have a very high charge density so does not polarise and weaken the O-H bond enough for many of the H20 ligands to dissociate a H+.
What is the ph of a typical solution of metal 3+ aqua ions?
3 as the metal ion has a high charge density meaning it polarises and weakens the O-H bond in the H2O ligands. This means lots of H+ ions are dissociated but eq still lies to the LHS so only weakly acidic.
What is the ph of a typical solution of metal 4+ aqua ions?
0 as the metal ion has a high charge density meaning it polarises and weakens the O-H bond in the H2O ligands. This means lots of H+ ions are dissociated. ( even more that 3+)
What is the equation for the hydrolysis of a metal aqua ion.
[M(H20)6]2+ <=> [M(H20)5(ОH)]+ + H+
Why does a high charge density on the transition metal mean a weakened O-H bond in the ligand?
The high positive charge pulls electrons towards the metal, subsequently pulling electrons towards O in the O-H bond.
How do metal aqua ions react with bases?
If a base is added to a metal aqua ion, hydrolysis may take place (the base removes the H+ pushing the equilibrium to the right). If this occurs the insoluble neutral complex will be formed.
What are the observations and equations when NaOH is added dropwise to Cu 2+ metal aqua ion?
Blue solution -> Blue ppt
[Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(H₂O)₄(OH)₂ + 2H₂O
What are the observations and equations when NaOH is added dropwise to the Fe 2+ metal aqua ion?
Green solution -> Green ppt (darkens in air) as Fe(H₂O)₄(OH)₂ oxidises to Fe(H₂O)₃(OH)₃ which is brown.
[Fe(H₂O)₆]²⁺ + 2OH⁻ → Fe(H₂O)₄(OH)₂ + 2H₂O
What are the observations and equations when NaOH is added dropwise to Fe 3+ metal aqua ion?
Orange solution -> Brown ppt
[Fe(H₂O)₆]³⁺ + 3OH⁻ → Fe(H₂O)₃(OH)₃ + 3H₂O
What are the observations and equations when NaOH is added dropwise to Al 3+ metal aqua ion?
Colourless -> White ppt
[Al(H₂O)₆]³⁺ + 3OH⁻ → Al(H₂O)₃(OH)₃ + 3H₂O
Which of the metal aqua ions redissolve in excess NaOH? What are the observations and equation?
Al³⁺: White ppt dissolves → Colourless solution
[Al(H₂O)₃(OH)₃] + OH⁻ → [Al(OH)₄]⁻ + 3H₂O
What are the observations and equations when NH₃ is added dropwise to Cu 2+ metal aqua ion?
Blue solution -> Blue ppt
[Cu(H₂O)₆]²⁺ + 2NH₃ → Cu(H₂O)₄(OH)₂ + 2NH₄⁺
