Question 1

Why do some metal ions give acidic solutions in water?

Accepted Answer

Hydrated metal ions like [Fe(H₂O)₆]³⁺ are acidic because the high charge density on the metal ion polarises the O-H bonds in the coordinated water molecules, making it easier for a proton to be released. The equilibrium [M(H₂O)₆]ⁿ⁺ + H₂O ⇌ [M(H₂O)₅(OH)]⁽ⁿ⁻¹⁾⁺ + H₃O⁺ shifts to the right for ions with high charge and small size, such as Fe³⁺ and Al³⁺, resulting in a low pH.

Question 2

How do you remember which metal hydroxides are amphoteric?

Accepted Answer

A common mnemonic is 'Zany Alchemists' for Zinc and Aluminium. Both Zn²⁺ and Al³⁺ form hydroxides that dissolve in excess NaOH to give colourless hydroxo complexes: [Zn(OH)₄]²⁻ and [Al(OH)₄]⁻. Lead(II) hydroxide is also amphoteric but less commonly tested. Remember that transition metals like Cu²⁺ and Fe³⁺ do not dissolve in excess NaOH.

Question 3

What is the difference between adding NaOH and adding NH₃ to a solution of Cu²⁺?

Accepted Answer

Adding NaOH to [Cu(H₂O)₆]²⁺ gives a pale blue precipitate of Cu(OH)₂, which does not dissolve in excess NaOH. Adding a small amount of NH₃ also gives the same precipitate because NH₃ acts as a base, deprotonating the aqua ion. However, with excess NH₃, the precipitate dissolves to form a deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺ via ligand substitution. So the key difference is that excess NH₃ leads to a soluble complex, while excess NaOH does not.

Question 4

Why does [Co(H₂O)₆]²⁺ change from pink to blue when concentrated HCl is added?

Accepted Answer

Concentrated HCl provides a high concentration of chloride ions, which act as ligands and substitute water molecules. The reaction is [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O. The pink octahedral complex is converted to a blue tetrahedral complex. The colour change is due to the different splitting of d-orbitals in octahedral vs tetrahedral geometry, leading to absorption of different wavelengths of light.

Question 5

How do you write balanced equations for the reaction of Al³⁺ with NaOH?

Accepted Answer

First, the hexaaqua ion reacts with hydroxide to form the precipitate: [Al(H₂O)₆]³⁺ + 3OH⁻ → Al(OH)₃(H₂O)₃ + 3H₂O. Often simplified as Al³⁺ + 3OH⁻ → Al(OH)₃. Then, with excess NaOH, the amphoteric hydroxide dissolves: Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻. In full, Al(OH)₃(H₂O)₃ + OH⁻ → [Al(OH)₄]⁻ + 3H₂O. Always include state symbols: (aq) for ions, (s) for precipitate.

Question 6

What is the trend in acidity of hexaaqua ions across the periodic table?

Accepted Answer

Acidity increases with increasing charge density of the metal ion. For ions of the same charge, acidity increases as the ionic radius decreases (e.g., Mg²⁺ < Ca²⁺). For ions of similar size, higher charge gives greater acidity (e.g., Na⁺ < Mg²⁺ < Al³⁺). Transition metal ions with high oxidation states, like Fe³⁺, are quite acidic (pH around 2-3). Group 1 and 2 ions with low charge density give neutral or slightly acidic solutions.

Reactions of ions in aqueous solution (A-level only)

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Related Topics in AQA A-Level Chemistry

Acids and bases (A-level only)

Alcohols

Aldehydes and ketones (A-level only)

Alkanes