Maxwell-Boltzmann distribution and catalystsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A chemist sketches Maxwell–Boltzmann distributions of molecular energies for the same sample of gas at 300 K and at 310 K. She notes that the mean kinetic energy of the molecules rises by only about 3% over this range, yet the rate of a reaction in the gas roughly doubles.

   (a)

   Compared with the distribution at 300 K, what does the distribution at 310 K show?

   [1 mark]

   • AThe peak is lower and moves to a higher energy, and the total area is unchanged
   • BThe peak is higher and moves to a higher energy, and the total area increases
   • CThe peak is lower and moves to a lower energy, and the total area is unchanged
   • DThe peak stays at the same energy but the total area increases

   (b)

   What does the area under a Maxwell–Boltzmann curve represent?

   [1 mark]

   • AThe activation energy of the reaction
   • BThe mean energy of the molecules
   • CThe total number of molecules in the sample
   • DThe rate of the reaction

   (c)

   Explain, using the Maxwell–Boltzmann distribution, why a rise in temperature of only 10 K can double the rate even though the mean kinetic energy rises very little.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hydrogen peroxide solution decomposes slowly at room temperature to form water and oxygen. When a small amount of solid manganese(IV) oxide is added, oxygen is released rapidly, and the manganese(IV) oxide can be recovered unchanged at the end.

   (a)

   How does manganese(IV) oxide increase the rate of decomposition?

   [1 mark]

   • AIt increases the mean kinetic energy of the molecules
   • BIt provides an alternative route with a lower activation energy
   • CIt increases the enthalpy change of the reaction
   • DIt is used up, supplying extra reactant particles

   (b)

   Which quantity is the same with and without the catalyst?

   [1 mark]

   • AThe activation energy
   • BThe number of steps in the mechanism
   • CThe pathway of the reaction
   • DThe enthalpy change of the reaction

   (c)

   Describe how the reaction profile for the catalysed route differs from the profile for the uncatalysed route, given that the catalyst forms an intermediate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ethanol can be made industrially by the catalysed hydration of ethene with steam, using phosphoric acid on a silica support at about 570 K. A laboratory alternative is to react bromoethane with aqueous sodium hydroxide, which gives sodium bromide as the only other product. A company wants to know which route is more sustainable.

   (a)

   Explain, in terms of activation energy, how a catalyst allows a process to be carried out at a lower temperature, and why this makes the process more sustainable.

   [3 marks]

   (b)

   Calculate the atom economy for the bromoethane route (C₂H₅Br + NaOH → C₂H₅OH + NaBr) and for the hydration route (C₂H₄ + H₂O → C₂H₅OH), taking the desired product as ethanol. Use Aᵣ values: H 1.0, C 12.0, O 16.0, Na 23.0, Br 79.9. State which route has the higher atom economy.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A reaction has an enthalpy change of −90 kJ mol⁻¹ and an uncatalysed activation energy of 150 kJ mol⁻¹. With a solid catalyst the reaction takes place in two steps through an intermediate bonded to the catalyst surface. Taking the energy of the reactants as zero, the first peak is at +80 kJ mol⁻¹, the intermediate is at +30 kJ mol⁻¹ and the second peak is at +60 kJ mol⁻¹.

   (a)

   Explain, with reference to Maxwell–Boltzmann distributions, how an increase in temperature and the addition of the catalyst each increase the rate of this reaction.

   [6 marks]

   (b)

   Describe the reaction profiles for the uncatalysed and catalysed routes, giving the energy at each peak and at the intermediate, and calculate the activation energy of each step of the catalysed route. State which step is rate-determining and by how much the highest activation energy has been reduced.

   [6 marks]

   Total for question 4: 12 marks

End of questions