CatalystsAQA A-Level Chemistry: Flashcards

Exam questions on Catalysts

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.
   Write the balanced equation for the decomposition of hydrogen peroxide and explain why manganese(IV) oxide does not appear in it.2 marks
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.
   Explain why the reaction is faster when the catalyst is used, even though the temperature is unchanged.2 marks
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.
   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