Metal-aqua ions and their acidityAQA A-Level Chemistry: Subtopic test

1. 1
   A student measures the pH of two aqueous solutions, each of concentration 0.1 mol dm⁻³. Iron(II) nitrate solution has a pH of 3.7 and iron(III) nitrate solution has a pH of 1.9.

   (a)

   Which species is present in freshly made iron(III) nitrate solution?

   [1 mark]

   • A[Fe(H₂O)₆]²⁺
   • B[Fe(H₂O)₆]³⁺
   • C[Fe(OH)₆]³⁻
   • D[Fe(H₂O)₃]³⁺

   (b)

   Which equation shows why the iron(III) nitrate solution is acidic?

   [1 mark]

   • A[Fe(H₂O)₆]³⁺ → [Fe(H₂O)₅(OH)]³⁺ + H⁺
   • B[Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + OH⁻
   • C[Fe(H₂O)₆]³⁺ + OH⁻ → [Fe(H₂O)₆(OH)]²⁺
   • D[Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺

   (c)

   Calculate how many times greater the concentration of H⁺ ions is in the iron(III) nitrate solution than in the iron(II) nitrate solution.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Aluminium chloride and copper(II) sulfate are each dissolved in water to make solutions of the same concentration. The aluminium chloride solution has a lower pH than the copper(II) sulfate solution. The ionic radius of Al³⁺ is 0.053 nm and that of Cu²⁺ is 0.073 nm.

   (a)

   What is the charge/size ratio of the Al³⁺ ion, in e nm⁻¹ to two significant figures?

   [1 mark]

   • A0.018
   • B27
   • C57
   • D0.16

   (b)

   Which species is formed when [Cu(H₂O)₆]²⁺ acts as a Brønsted–Lowry acid in water?

   [1 mark]

   • A[Cu(H₂O)₅(OH)]⁺
   • B[Cu(H₂O)₅(OH)]²⁺
   • C[Cu(H₂O)₆(OH)]⁺
   • D[Cu(OH)₆]⁴⁻

   (c)

   Use the data to explain why the aluminium chloride solution has a lower pH than the copper(II) sulfate solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Crevice corrosion occurs where stagnant water is trapped against steel. Iron dissolves to form iron(II) ions, which are oxidised by dissolved oxygen to iron(III) ions. Engineers find that the liquid trapped in the crevice becomes much more acidic than the surrounding water.

   (a)

   Write an equation to show why a solution containing [Fe(H₂O)₆]³⁺ is acidic, and state the role of the aqua ion in this reaction.

   [3 marks]

   (b)

   Explain, in terms of the bonding in the aqua ion, how [Fe(H₂O)₆]³⁺ releases H⁺ ions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student tests 0.1 mol dm⁻³ solutions of copper(II) sulfate, iron(II) sulfate, aluminium sulfate and iron(III) sulfate with a pH meter. All four solutions are acidic. The two solutions containing M³⁺ ions have a lower pH than the two containing M²⁺ ions.

   (a)

   Explain these observations. Include an equation for one of the M²⁺ ions.

   [6 marks]

   (b)

   The ionic radius of Fe²⁺ is 0.078 nm and that of Fe³⁺ is 0.065 nm. Use these data to justify why iron(III) sulfate solution has a lower pH than iron(II) sulfate solution of the same concentration.

   [6 marks]

   Total for question 4: 12 marks

End of questions