Reactions of Inorganic Compounds in Aqueous Solutions
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How do the water molecules bind to the metal ion in an octahedral arrangement?
They act as ligands, they each use one of the lone pairs of electrons to form a co-ordinate bond with the metal ion.
Compare the acidity of an M^3+ and an M^2+ metal aqua ion.
M3+ is more acidic than M^2+ as it has a higher charge density and is smaller so it is more polarising and this makes proton dissociation more likely. The O-H bond weakens in the water in the aqua ion. H+ is released so the solution becomes more acidic.
Give the reaction of [Fe(H2O)6]^2+ reacting with water.
[Fe(H2O)6]^3+(aq) + H2O (l) (reversible) [Fe(H2O)5(OH)]2+(aq) + H3O+ (aq)
What is a hydrolysis reaction in this situation?
Where the O-H bonds of water are broken and new species are formed.
What is a Bronsted Lowry Acid?
A substance that donates a proton.
What is a Bronsted Lowry Base?
A substance that accepts a proton.
Why can’t the Bronsted Lowry theory be used in every case?
It has difficulty with acids that don’t contain hydrogen e.g. AlCl3.
What colour is [Fe(H2O)]^2+ ?
Pale green
What colour is [Fe(H2O)]^3+ ?
Pale brown
How can you distinguish between [Fe(H2O)]^3+ and [Fe(H2O)]^2+?
Add dilute alkali which precipitates the hydroxides whose colours are more obviously different.
What is meant by amphoteric?
A substance that can act as both an acid and a base.
What is an example of an amphoteric substance?
Aluminium hydroxide
Equation for Aluminium Hydroxide reacting with an acid?
Al(H2O)3(OH)3 + 3HCl -> Al(H2O)6^3+ + 3Cl^-
Equation for Aluminium Hydroxide reacting with a base?
Al(H2O)3(OH)3 + OH- -> [Al(OH)4]- + 3H2O
What is a Lewis Acid?
Electron Acceptor
What is a Lewis Base?
Electron Donor
Why does the O-H bond weaken?
When the metal ion accepts electrons (Lewis Acid), this withdraws electrons away from the O-H bond and this weakens the bond.
Give the equation for M^3+ reacting with an OH-.
[M(H2O)6]^3+ + OH- -> [M(H2O)5(OH)]^2+ + H2O
What are the four major metal aqua ions needed to be known for the A Level Spec?
Fe(III) Fe(II) Cu(II) Aluminium
How can you distinguish between the four major metal aqua ions?
They form different coloured precipitates
What precipitate does Al^3+ form?
White precipitate
What precipitate does Fe^2+ form?
Green precipitate
What precipitate does Cu^2+ form?
Blue precipitate
What precipitate does Fe^3+ form?
Purple precipitate