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Questions on reactions of aqua ions with hydroxide, ammonia and carbonate

The reactions of the iron(II), copper(II), iron(III) and aluminium aqua ions with sodium hydroxide, aqueous ammonia and sodium carbonate written as acid–base reactions with state symbols, the colours of the precipitates, aluminium hydroxide as an amphoteric hydroxide, copper(II) hydroxide dissolving in excess ammonia by ligand substitution, the full identification table, chromium(III), manganese(II) and cobalt(II) with hydroxide and ammonia as OCR A and Edexcel set them, and the method for the test-tube practical (AQA required practical 11).

20 original questions · 65 marks · the reactions of aqua ions with hydroxide, ammonia and carbonate notes · Ions in aqueous solution · Flashcards for this unit

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• 6 marks · Outline · RAI-Q17 AQA · CAIE

  Outline a method, using test-tube reactions, to show that a solution contains Cu²⁺(aq) and not Fe²⁺(aq), Fe³⁺(aq) or Al³⁺(aq). Give the expected observations and equations.

  Mark scheme

  Indicative content

  1. put about 1 cm³ of the solution in each of two test tubes and note that it is blue against a white tile
  2. add sodium hydroxide dropwise with shaking: a blue precipitate, which rules out the green, brown and white hydroxides of the other ions
  3. the equation for the first tube is [Cu(H₂O)₆]²⁺(aq) + 2OH⁻(aq) → [Cu(H₂O)₄(OH)₂](s) + 2H₂O(l), with state symbols
  4. to the second tube add aqueous ammonia dropwise and then in excess: a blue precipitate that dissolves to a deep blue solution
  5. the equation for the excess ammonia is [Cu(H₂O)₄(OH)₂](s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH⁻(aq)
  6. excess means adding until the tube is about half full, with shaking, and eye protection is worn

  Levels

  • Level 3 (5–6 marks) the colour of the solution, the blue hydroxide precipitate that rules out the other three ions, the dissolving in excess ammonia to the deep blue complex and both equations with state symbols are all present, with the practical detail of dropwise addition and excess; the observations are matched to each ion excluded
  • Level 2 (3–4 marks) sodium hydroxide and ammonia are used, with the blue precipitate and the deep blue solution in excess ammonia; one equation is missing or unbalanced, or the other three ions are never ruled out
  • Level 1 (1–2 marks) a blue precipitate is reported with no equations, often said to dissolve in excess sodium hydroxide, and nothing shows how Fe²⁺, Fe³⁺ or Al³⁺ would look different

• 4 marks · Describe · RAI-Q1

  Describe what is seen when a few drops of sodium hydroxide solution are added to iron(II) sulfate solution and the tube is then left open to the air, and explain the change on standing.

  Mark scheme

  1. B1 a green precipitate forms from the pale green solution
  2. B1 the precipitate darkens, turning brown at the surface first, on standing in air
  3. B1 oxygen from the air oxidises iron(II) to iron(III)
  4. B1 green [Fe(H₂O)₄(OH)₂] becomes brown [Fe(H₂O)₃(OH)₃], starting where the air reaches it; allow Fe(OH)₂ and Fe(OH)₃

• 4 marks · Give · RAI-Q2

  Give the equation, with state symbols, for the reaction of [Cu(H₂O)₆]²⁺ with a little aqueous ammonia and the observation, and name the type of reaction that occurs with a little ammonia and with excess ammonia.

  Mark scheme

  1. B1 [Cu(H₂O)₆]²⁺(aq) + 2NH₃(aq) → [Cu(H₂O)₄(OH)₂](s) + 2NH₄⁺(aq)
  2. B1 a blue precipitate forms from the blue solution
  3. B1 with a little ammonia: an acid–base reaction, in which ammonia removes protons from the water ligands
  4. B1 with excess ammonia: ligand substitution, in which ammonia replaces ligands round the copper to give a deep blue solution

• 4 marks · Explain · RAI-Q7

  Explain why the blue precipitate formed from [Cu(H₂O)₆]²⁺ and a little ammonia dissolves when excess ammonia is added. Include an equation.

  Mark scheme

  1. B1 in excess, ammonia acts as a ligand rather than as a base, donating the lone pair on its nitrogen
  2. B1 ammonia replaces four of the ligands around copper in a ligand substitution, and the coordination number stays six
  3. B1 [Cu(H₂O)₄(OH)₂](s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH⁻(aq)
  4. B1 the product is a soluble charged complex, seen as a deep blue solution

• 4 marks · Compare · RAI-Q15 AQA · CAIE

  Compare the reactions of aluminium hydroxide with excess sodium hydroxide and with excess aqueous ammonia, and explain the difference.

  Mark scheme

  1. B1 aluminium hydroxide dissolves in excess sodium hydroxide to give a colourless solution
  2. B1 it does not dissolve in excess aqueous ammonia; the white precipitate remains
  3. B1 in excess sodium hydroxide a fourth proton is removed, forming the soluble [Al(H₂O)₂(OH)₄]⁻ ion
  4. B1 ammonia is a weak base, so the hydroxide ion concentration in its solution is too low to remove that proton

• 4 marks · Evaluate · RAI-Q16 AQA · CAIE

  Evaluate a student's plan to identify which of four solutions contains Fe²⁺, Cu²⁺, Fe³⁺ or Al³⁺ by adding a few drops of aqueous ammonia to each and noting only the colour of the precipitate.

  Mark scheme

  1. B1 the plan works: the four precipitates are green, blue, brown and white, all different
  2. B1 the Fe(II) precipitate darkens on standing in air, so it should be observed straight away to avoid confusion with Fe(III)
  3. B1 adding ammonia in excess would confirm copper, whose precipitate dissolves to a deep blue solution
  4. B1 adding sodium hydroxide in excess, or sodium carbonate, would confirm aluminium, by the amphoteric hydroxide or effervescence

• 3 marks · Describe · RAI-Q4 AQA · CAIE

  Describe what is seen when sodium hydroxide solution is added drop by drop, and then in excess, to a solution of aluminium sulfate.

  Mark scheme

  1. B1 the starting solution is colourless
  2. B1 a white precipitate forms with the first drops
  3. B1 the precipitate dissolves in excess to give a colourless solution

• 3 marks · Explain · RAI-Q5 AQA · CAIE

  Explain what is meant by amphoteric, using two equations for aluminium hydroxide to illustrate your answer.

  Mark scheme

  1. B1 an amphoteric substance can react both as an acid and as a base
  2. B1 as an acid: [Al(H₂O)₃(OH)₃](s) + OH⁻(aq) → [Al(H₂O)₂(OH)₄]⁻(aq) + H₂O(l)
  3. B1 as a base: [Al(H₂O)₃(OH)₃](s) + 3H⁺(aq) → [Al(H₂O)₆]³⁺(aq)

• 3 marks · Explain · RAI-Q6

  Explain why aqueous ammonia and sodium hydroxide solution give the same precipitate with a solution of [Fe(H₂O)₆]³⁺.

  Mark scheme

  1. B1 both ammonia and hydroxide ions act as Brønsted–Lowry bases
  2. B1 each removes three protons from the water ligands of [Fe(H₂O)₆]³⁺, leaving the neutral complex [Fe(H₂O)₃(OH)₃]
  3. B1 the neutral complex is insoluble, so the same brown precipitate forms with either reagent

• 3 marks · Identify · RAI-Q9 AQA

  Identify the metal-aqua ion in a solution that gives a brown precipitate with sodium hydroxide, a brown precipitate that does not dissolve in excess ammonia, and effervescence with sodium carbonate. Give a reason from the observations for each part of your answer.

  Mark scheme

  1. B1 the ion is [Fe(H₂O)₆]³⁺
  2. B1 the brown hydroxide precipitate identifies iron(III)
  3. B1 effervescence with carbonate shows a 3+ aqua ion, acidic enough to release carbon dioxide

• 3 marks · Calculate · RAI-Q10

  Calculate the volume of 0.400 mol dm⁻³ sodium hydroxide solution needed to precipitate all of the iron in 25.0 cm³ of a 0.100 mol dm⁻³ solution of [Fe(H₂O)₆]³⁺.

  Mark scheme

  1. M1 moles of [Fe(H₂O)₆]³⁺ = 0.100 × 25.0 ÷ 1000 = 2.50 × 10⁻³ mol
  2. M1 three OH⁻ are needed per aqua ion, so moles of OH⁻ = 7.50 × 10⁻³ mol
  3. A1 volume = 7.50 × 10⁻³ ÷ 0.400 = 0.01875 dm³, so 18.8 cm³ to three significant figures

• 3 marks · Calculate · RAI-Q11 AQA

  Calculate the volume of carbon dioxide, at room temperature and pressure, released when excess sodium carbonate is added to 50.0 cm³ of a 0.200 mol dm⁻³ solution of [Al(H₂O)₆]³⁺.

  Mark scheme

  1. M1 moles of [Al(H₂O)₆]³⁺ = 0.200 × 50.0 ÷ 1000 = 1.00 × 10⁻² mol
  2. M1 the equation gives 3CO₂ for every 2 aqua ions, so moles of CO₂ = 1.50 × 10⁻² mol
  3. A1 volume = 1.50 × 10⁻² × 24.0 = 0.360 dm³, to three significant figures

• 3 marks · Calculate · RAI-Q12 AQA · CAIE

  Calculate the mass of aluminium hydroxide, written as Al(OH)₃, precipitated when ammonia is added to 20.0 cm³ of a 0.150 mol dm⁻³ solution of [Al(H₂O)₆]³⁺. Ar: Al 27.0, O 16.0, H 1.0.

  Mark scheme

  1. M1 moles of Al = 0.150 × 20.0 ÷ 1000 = 3.00 × 10⁻³ mol, the same as moles of Al(OH)₃
  2. M1 Mr of Al(OH)₃ = 27.0 + 3 × (16.0 + 1.0) = 78.0
  3. A1 mass = 3.00 × 10⁻³ × 78.0 = 0.234 g, to three significant figures

• 3 marks · Suggest · RAI-Q13

  Suggest what is seen when sodium hydroxide solution is added drop by drop, and then in excess, to zinc sulfate solution, given that zinc hydroxide is amphoteric and [Zn(H₂O)₆]²⁺ is colourless. Write an equation for the reaction in excess.

  Mark scheme

  1. B1 a white precipitate of [Zn(H₂O)₄(OH)₂] forms from the colourless solution
  2. B1 the precipitate dissolves in excess sodium hydroxide to a colourless solution
  3. B1 [Zn(H₂O)₄(OH)₂](s) + 2OH⁻(aq) → [Zn(OH)₄]²⁻(aq) + 4H₂O(l), or an equivalent balanced equation

• 3 marks · Explain · RAI-Q19 OCR A · Edexcel · CAIE

  Explain, with an equation, why the grey-green precipitate formed from [Cr(H₂O)₆]³⁺ and a few drops of sodium hydroxide dissolves when excess sodium hydroxide is added.

  Mark scheme

  1. B1 chromium(III) hydroxide is amphoteric, so it reacts with excess hydroxide acting as an acid
  2. B1 hydroxide ions remove three more protons from the water ligands, giving a soluble negative complex ion, seen as a dark green solution
  3. B1 [Cr(H₂O)₃(OH)₃](s) + 3OH⁻(aq) → [Cr(OH)₆]³⁻(aq) + 3H₂O(l)

• 3 marks · Suggest · RAI-Q20 Edexcel

  Suggest the identity of the aqua ion in a pink solution that gives a blue precipitate with a little aqueous ammonia, the precipitate dissolving in excess ammonia to a pale straw-coloured solution that darkens on standing in air. Write an equation, with state symbols, for the reaction in excess ammonia.

  Mark scheme

  1. B1 the ion is [Co(H₂O)₆]²⁺: a pink solution giving a blue hydroxide that dissolves in excess ammonia points to cobalt(II), not copper(II), whose solution is blue
  2. B1 [Co(H₂O)₄(OH)₂] + 6NH₃ → [Co(NH₃)₆]²⁺ + 4H₂O + 2OH⁻, with all six ligands replaced by ammonia
  3. B1 state symbols (s), (aq), (aq), (l) and (aq), with the equation balanced for atoms and charge

• 3 marks · Justify · RAI-Q22 AQA · CAIE

  Justify the instruction to add sodium hydroxide solution until it is in excess, not just a few drops, when deciding whether a colourless solution contains Al³⁺ ions or Mg²⁺ ions.

  Mark scheme

  1. B1 a few drops give a white precipitate with either ion, aluminium hydroxide or magnesium hydroxide, so that observation alone cannot tell them apart
  2. B1 aluminium hydroxide is amphoteric and dissolves in excess hydroxide to a colourless solution: [Al(H₂O)₃(OH)₃](s) + OH⁻(aq) → [Al(H₂O)₂(OH)₄]⁻(aq) + H₂O(l)
  3. B1 magnesium hydroxide is a basic hydroxide with no acidic character, so its white precipitate stays in excess; the difference appears only once excess has been added

• 2 marks · Complete · RAI-Q3

  Complete the equation, with state symbols, for the reaction of [Fe(H₂O)₆]²⁺ with aqueous ammonia: [Fe(H₂O)₆]²⁺(aq) + 2NH₃(aq) →

  Mark scheme

  1. B1 the products are [Fe(H₂O)₄(OH)₂] and 2NH₄⁺
  2. B1 state symbols (s) for the hydroxide and (aq) for the ammonium ion

• 2 marks · Suggest · RAI-Q14 AQA · CAIE

  Suggest why a student who pours sodium hydroxide quickly into a solution of aluminium sulfate may record that no precipitate forms.

  Mark scheme

  1. B1 where the alkali enters, the hydroxide is locally in excess
  2. B1 any white aluminium hydroxide that forms there redissolves at once, so the precipitate is never seen

• 2 marks · Give · RAI-Q21 OCR A

  Give the equation, with state symbols, for the reaction of [Mn(H₂O)₆]²⁺ with a little aqueous ammonia, and give the observation.

  Mark scheme

  1. B1 [Mn(H₂O)₆]²⁺(aq) + 2NH₃(aq) → [Mn(H₂O)₄(OH)₂](s) + 2NH₄⁺(aq)
  2. B1 a light brown precipitate forms, which darkens on standing in air and does not dissolve in excess

Practise reactions of aqua ions with hydroxide, ammonia and carbonate one question at a time