Transition metals as catalystsAQA A-Level Chemistry: Subtopic test

1. 1
   An ammonia plant makes ammonia by the Haber process. A gaseous mixture of nitrogen and hydrogen at 450 °C and 200 atm is passed over a bed of solid iron. The plant manager reports that, whenever the feed gas is contaminated with traces of sulfur compounds, the output of ammonia falls.

   (a)

   Which statement correctly describes the iron used in this process?

   [1 mark]

   • AIt is a homogeneous catalyst because iron is a transition metal
   • BIt is a heterogeneous catalyst because it is in a different phase from the reactants
   • CIt is a homogeneous catalyst because it takes part in the reaction
   • DIt is an autocatalyst because ammonia speeds up the reaction

   (b)

   What is the most likely reason for the fall in ammonia output when sulfur compounds are present?

   [1 mark]

   • AThe sulfur compounds raise the activation energy of the gas-phase reaction
   • BThe sulfur compounds shift the equilibrium towards the reactants
   • CThe sulfur compounds bond to the iron surface and block active sites
   • DThe sulfur compounds dissolve in the iron and make it homogeneous

   (c)

   Explain why catalyst poisoning has a cost implication for the plant.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sulfuric acid is manufactured by the Contact process. Sulfur dioxide and oxygen are passed at 450 °C over vanadium(V) oxide, V₂O₅, which is spread thinly on the surface of inert silica pellets. The reaction is: SO₂(g) + ½O₂(g) ⇌ SO₃(g).

   (a)

   What are the oxidation states of vanadium in V₂O₅ and in V₂O₄?

   [1 mark]

   • A+5 and +3
   • B+4 and +5
   • C+3 and +4
   • D+5 and +4

   (b)

   Why is the V₂O₅ spread on silica pellets rather than used as a solid lump?

   [1 mark]

   • AIt gives a large surface area using a small mass of the expensive catalyst
   • BIt lowers the activation energy more than a lump of V₂O₅ would
   • CIt makes the catalyst homogeneous with the gases
   • DIt moves the equilibrium position towards SO₃

   (c)

   Write two equations to show how V₂O₅ acts as a catalyst in this reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In aqueous solution, iodide ions react with peroxodisulfate ions: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq). A technician finds that the reaction is very slow when the two solutions are mixed, but speeds up greatly when a few drops of iron(II) sulfate solution are added.

   (a)

   Explain, with reference to the ions involved, why the uncatalysed reaction is slow.

   [3 marks]

   (b)

   Write two equations to show how Fe²⁺ ions catalyse the reaction, and explain why Fe²⁺ is an effective catalyst for it.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student heats a flask of acidified ethanedioic acid solution to about 60 °C and adds acidified potassium manganate(VII) solution from a burette. The first few drops lose their purple colour very slowly. After a short time each further drop is decolourised almost instantly. The overall equation is: 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂.

   (a)

   Explain these observations. Include equations for the catalytic steps.

   [6 marks]

   (b)

   Explain how the properties of transition metals make them effective catalysts. Refer to an example of a heterogeneous catalyst and an example of a homogeneous catalyst.

   [6 marks]

   Total for question 4: 12 marks

End of questions