CatalystsAQA A-Level Chemistry: Subtopic test

1. 1
   In a school laboratory a student adds a spatula of black manganese(IV) oxide powder to 25 cm³ of hydrogen peroxide solution at room temperature. Oxygen is evolved rapidly, whereas without the powder it forms extremely slowly. At the end of the experiment the student filters off the powder, washes and dries it, and finds that its mass is unchanged and that it is still black manganese(IV) oxide.

   (a)

   Which statement correctly defines a catalyst?

   [1 mark]

   • AA substance that increases the rate of a reaction and is used up in forming the products
   • BA substance that increases the rate of a reaction without being changed in chemical composition or amount
   • CA substance that increases the rate of a reaction by raising the temperature of the reacting mixture
   • DA substance that increases the rate of a reaction by increasing the yield of products at equilibrium

   (b)

   What can be concluded from the observation that the mass of recovered manganese(IV) oxide is unchanged?

   [1 mark]

   • AThe manganese(IV) oxide takes no part at all in the reaction
   • BThe manganese(IV) oxide has lowered the enthalpy change of the reaction
   • CThe manganese(IV) oxide is not used up, so its amount is unchanged
   • DThe manganese(IV) oxide has been converted into a different compound and back again

   (c)

   Write the balanced equation for the decomposition of hydrogen peroxide and explain why manganese(IV) oxide does not appear in it.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The gas-phase reaction between two reactants is exothermic, with an enthalpy change of −45 kJ mol⁻¹. Without a catalyst the activation energy is 120 kJ mol⁻¹. When a solid catalyst is used, the reaction follows an alternative route with an activation energy of 70 kJ mol⁻¹. The temperature is the same in both cases.

   (a)

   What is the enthalpy change for the reaction when the catalyst is used?

   [1 mark]

   • A−95 kJ mol⁻¹
   • B+5 kJ mol⁻¹
   • C−70 kJ mol⁻¹
   • D−45 kJ mol⁻¹

   (b)

   By how much does the catalyst lower the activation energy?

   [1 mark]

   • A50 kJ mol⁻¹
   • B70 kJ mol⁻¹
   • C190 kJ mol⁻¹
   • D45 kJ mol⁻¹

   (c)

   Explain why the reaction is faster when the catalyst is used, even though the temperature is unchanged.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a car's catalytic converter, the exhaust gases nitrogen monoxide and carbon monoxide react at about 600 K: 2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g). Without a catalyst the reaction is extremely slow, but platinum and rhodium on a ceramic support allow it to occur in the short time the gases spend in the converter.

   (a)

   Describe the shape of the Maxwell–Boltzmann distribution of molecular energies for the exhaust gases at 600 K, and state what the area under the curve represents.

   [3 marks]

   (b)

   Explain, with reference to the Maxwell–Boltzmann distribution, how the catalyst increases the rate of the reaction at 600 K without any change in temperature.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical company makes a gaseous product using a solid catalyst at 750 K. Its research team has developed a new catalyst that provides an alternative reaction route with a lower activation energy. The company wants to know whether the new catalyst would allow it to operate at a lower temperature while keeping the same rate of reaction.

   (a)

   Explain, with reference to the Maxwell–Boltzmann distribution, how the new catalyst increases the rate of reaction at 750 K.

   [6 marks]

   (b)

   Evaluate whether the new catalyst would allow the company to lower the temperature and keep the same rate of reaction.

   [6 marks]

   Total for question 4: 12 marks

End of questions