3.2.6
Reactions of Ions in Solution Flashcards
Active recall practice for metal hydroxide precipitates, carbonate reactions, qualitative analysis, and redox processes.
Metal Hydroxide Precipitates: read all 7 questions and answers
- Describe the appearance and formula of the precipitate formed when NaOH is added dropwise to a solution of Fe²⁺(aq).A green precipitate of Fe(H₂O)₄(OH)₂(s) is formed, which slowly turns brown at the surface on standing as it is oxidised to iron(III) hydroxide.
- Which metal hydroxide precipitate is amphoteric, and what is observed when excess NaOH is added to it?Aluminium hydroxide, Al(H₂O)₃(OH)₃(s); it dissolves in excess NaOH to form a colourless solution containing [Al(OH)₄]⁻(aq).
- What is observed when excess ammonia is added to a copper(II) hydroxide precipitate?The pale blue precipitate of Cu(H₂O)₄(OH)₂(s) dissolves to form a deep blue solution containing [Cu(NH₃)₄(H₂O)₂]²⁺(aq).
- Write the equation for the reaction of [Al(H₂O)₆]³⁺ with dilute sodium hydroxide to form a precipitate.[Al(H₂O)₆]³⁺(aq) + 3OH⁻(aq) → Al(H₂O)₃(OH)₃(s) + 3H₂O(l).
- Compare the reaction of [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺ with sodium carbonate solution.Fe²⁺ forms a green precipitate of FeCO₃(s) with no gas; Fe³⁺ forms a brown precipitate of Fe(H₂O)₃(OH)₃(s) and carbon dioxide gas (effervescence).
- Why do 3+ metal ions form carbon dioxide gas with sodium carbonate, whereas 2+ metal ions do not?3+ metal ions have a higher charge density and are more polarizing, making the hexaaqua ion strongly acidic; they react with carbonate ions via an acid-base reaction, releasing CO₂.
- Write the balanced equation for the reaction of [Al(H₂O)₆]³⁺ with carbonate ions.2[Al(H₂O)₆]³⁺(aq) + 3CO₃²⁻(aq) → 2Al(H₂O)₃(OH)₃(s) + 3CO₂(g) + 3H₂O(l).
Qualitative Tests for Ions: read all 6 questions and answers
- Describe the chemical test for sulfate ions, including the observations and any necessary precautions.Add dilute hydrochloric acid followed by barium chloride solution; a white precipitate of BaSO₄(s) forms. The acid removes any carbonate impurities.
- Why must sulfuric acid NOT be used to acidify a barium chloride test for sulfate ions?Sulfuric acid contains sulfate ions, which would react with barium chloride to form a false positive white precipitate of barium sulfate.
- State the test and observation for carbonate ions in solution.Add a dilute strong acid (such as HCl); bubbles of carbon dioxide gas are produced, which turn limewater cloudy.
- How does nitric acid prevent false positives when testing for halide ions with silver nitrate?It reacts with and removes carbonate or sulfite ions, which would otherwise form competing white precipitates of silver carbonate or silver sulfite.
- Describe the test and observations for ammonium (NH₄⁺) ions in solution.Add warm sodium hydroxide solution; ammonia gas is produced, which has a pungent smell and turns damp red litmus paper blue.
- Which halide ion forms a silver precipitate that is insoluble in dilute ammonia but dissolves in concentrated ammonia?Bromide ion (Br⁻), forming a cream precipitate of silver bromide (AgBr).
Redox & Colour Changes: read all 5 questions and answers
- Describe the colour change when aqueous Fe²⁺ is titrated against acidified potassium manganate(VII).The purple manganate(VII) solution is reduced to colourless Mn²⁺, so the end point is reached when the solution in the flask turns a permanent pale pink.
- Write the half-equation for the reduction of dichromate(VI) ions in acidic solution, and state the colour change.Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O; the colour changes from orange to green.
- How can Fe³⁺(aq) ions be reduced to Fe²⁺(aq) ions in the laboratory, and what is the observation?Add zinc metal and dilute hydrochloric acid; the yellow-brown solution of Fe³⁺ is reduced to a pale green solution of Fe²⁺.
- Write the balanced overall equation for the redox reaction between Fe²⁺ and acidified MnO₄⁻ ions.MnO₄⁻(aq) + 5Fe²⁺(aq) + 8H⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l).
- What is the colour change when a solution of Fe²⁺(aq) is oxidised by oxygen on standing in air?The pale green solution turns yellow-brown due to the formation of Fe³⁺(aq) ions.