Reactions of Ions in Aqueous Solution (AQA A Level Chemistry): Flashcards
Exam code: 7405
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What is a Lewis acid?
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Cards in this collection (15)
What is a Lewis acid?
Lewis acid is a species that can accept a lone pair of electrons to form a dative covalent bond. In a metal-aqua complex, the metal ion acts as the Lewis acid.
When CuSO4 dissolves in water, the Cu2+ ions form the complex ion .......... . The water molecules act as .......... by donating .......... to the metal ion.
When CuSO4 dissolves in water, the Cu2+ ions form the complex ion [Cu(H2O)6]2+. The water molecules act as Lewis bases by donating lone pairs to the metal ion.
True or False?
+3 aqua complex ions are more acidic than +2 aqua complex ions because the +3 ion has a higher charge density.
True.
The higher charge density of a +3 ion polarises the O–H bonds of coordinated water molecules more strongly, increasing the extent of deprotonation and producing a more acidic solution.
What is meant by deprotonation of a metal-aqua complex?
Deprotonation of a metal-aqua complex is the loss of an H+ ion from a coordinated water molecule, producing a hydroxo complex and lowering the overall charge by 1. For example: [Fe(H2O)6]3+ (aq) → [Fe(H2O)5(OH)]2+ (aq) + H+ (aq).
Which type of bond links the water ligands to the central metal ion in a hexaaqua complex?
A dative (coordinate) covalent bond, formed by donation of a lone pair from the oxygen atom of water to the metal ion.
[Fe(H2O)6]3+ has a pKa of .......... . Ethanoic acid has a pKa of 4.8. This means [Fe(H2O)6]3+ is .......... acidic than ethanoic acid.
[Fe(H2O)6]3+ has a pKa of 2.2. Ethanoic acid has a pKa of 4.8. This means [Fe(H2O)6]3+ is more acidic than ethanoic acid.
True or False?
A Lewis base is a species that accepts a lone pair of electrons.
False.
A Lewis base donates a lone pair of electrons. A Lewis acid is the species that accepts the lone pair.
What colour precipitate forms when NaOH (aq) is added to [Fe(H2O)6]2+ (aq)?
A dark green precipitate of Fe(H2O)4(OH)2 (s) (hydrated iron(II) hydroxide).
When excess NH3 (aq) is added to [Cu(H2O)6]2+ (aq), a .......... precipitate first forms, then dissolves in excess NH3 to give a .......... solution containing the complex ion .......... .
When excess NH3 (aq) is added to [Cu(H2O)6]2+ (aq), a pale blue precipitate first forms, then dissolves in excess NH3 to give a deep blue solution containing the complex ion [Cu(NH3)4(H2O)2]2+.
What does amphoteric mean in the context of aluminium hydroxide?
Amphoteric means Al(OH)3 reacts with both acids and bases. It dissolves in dilute HCl to form [Al(H2O)6]3+ and in NaOH (aq) to form [Al(OH)4]-.
True or False?
When Na2CO3 (aq) is added to [Fe(H2O)6]3+ (aq), an iron(III) carbonate precipitate forms.
False.
Fe3+ aqua ions are acidic enough to release CO2 from carbonate ions. The result is a red-brown precipitate of Fe(H2O)3(OH)3 (s) and bubbles of CO2 (g), not a carbonate precipitate.
Al(OH)3(H2O)3 (s) dissolves in excess NaOH (aq) to form the ion .......... . This is an example of Al(OH)3 acting as an .......... .
Al(OH)3(H2O)3 (s) dissolves in excess NaOH (aq) to form the ion [Al(OH)4]-. This is an example of Al(OH)3 acting as an acid.
What colour precipitate forms when NaOH (aq) is added to [Fe(H2O)6]3+ (aq)?
A red-brown precipitate of Fe(H2O)3(OH)3 (s) (hydrated iron(III) hydroxide).
True or False?
Aluminium hydroxide dissolves in both dilute HCl and excess NaOH.
True.
This is because Al(OH)3 is amphoteric. It reacts with acids and with bases.
What is deprotonation?
Deprotonation is the removal of a proton (H+) from a molecule or ion by a base. In transition metal chemistry, bases such as OH- and NH3 deprotonate the water ligands of metal-aqua ions, converting them to hydroxide ligands and forming a precipitate.
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