CatalystsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Ammonia is made industrially by the Haber process, in which nitrogen and hydrogen gases pass over a solid iron catalyst: N₂(g) + 3H₂(g) → 2NH₃(g).

   (a)

   Which statement correctly describes how the iron catalyst works?

   [1 mark]

   • AIt makes the reaction more exothermic, so the products are at a lower energy
   • BIt provides an alternative reaction route with a lower activation energy and is chemically unchanged at the end of the reaction
   • CIt gives the gas molecules more energy so more of them can react
   • DIt lowers the activation energy of the original route

   (b)

   The iron catalyst is a solid and the reactants are gases. This type of catalyst is described as

   [1 mark]

   • Aheterogeneous, because it is in a different phase from the reactants
   • Bhomogeneous, because it is in the same phase as the reactants
   • Cautocatalytic, because one of the products speeds up the reaction
   • Dan inhibitor, because it is in a different phase

   (c)

   Describe how a solid catalyst such as iron provides a surface for the reaction of gaseous reactants.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A reaction A → B has an enthalpy change of −60 kJ mol⁻¹ and, uncatalysed, an activation energy of 120 kJ mol⁻¹. When a solid catalyst is used the activation energy falls to 70 kJ mol⁻¹.

   (a)

   What is the activation energy for the reverse reaction, B → A, without the catalyst?

   [1 mark]

   • A60 kJ mol⁻¹
   • B120 kJ mol⁻¹
   • C70 kJ mol⁻¹
   • D180 kJ mol⁻¹

   (b)

   What is the enthalpy change for A → B in the presence of the catalyst?

   [1 mark]

   • A−110 kJ mol⁻¹
   • B−10 kJ mol⁻¹
   • C−60 kJ mol⁻¹, as the catalyst does not change the energy of the reactants or products
   • D+10 kJ mol⁻¹

   (c)

   Describe how the reaction profile for the catalysed reaction differs from the uncatalysed reaction, and state what is the same in the two.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the Contact process, sulfur dioxide and oxygen gases pass over a solid vanadium(V) oxide catalyst at about 700 K: 2SO₂(g) + O₂(g) → 2SO₃(g), ΔH = −196 kJ mol⁻¹. In a simplified model the uncatalysed reaction has an activation energy of 250 kJ mol⁻¹ and the catalysed reaction has an activation energy of 100 kJ mol⁻¹.

   (a)

   Explain how a solid catalyst speeds up the reaction between two gases, and why the catalyst is used as small pellets or spread over a support.

   [3 marks]

   (b)

   Calculate the activation energy of the reverse reaction, with and without the catalyst, and state how the catalyst changes the enthalpy change.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser company runs a gas-phase process over a solid iron catalyst at about 700 K and high pressure. Without the catalyst, a temperature well above 1000 K would be needed to reach a reasonable rate.

   (a)

   Explain the economic benefits of using the catalyst in this industrial process.

   [6 marks]

   (b)

   The iron catalyst is used as small porous pieces. Explain how a solid catalyst increases the rate of reaction of gaseous nitrogen and hydrogen, and why small porous pieces are used.

   [6 marks]

   Total for question 4: 12 marks

End of questions