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A2 · Practice questions · Homogeneous and heterogeneous catalysts

Same phase or surface?

Six original Cambridge-style questions on homogeneous and heterogeneous catalysts.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

New route, lower barrier, regenerated.

01
Recall
[3 marks]

Write two equations to show how Fe2+ ions catalyse the reaction between S2O82− and I−.

  • S2O82− + 2Fe2+ → 2SO42− + 2Fe3+. ✓
  • 2Fe3+ + 2I− → 2Fe2+ + I2. ✓
  • Fe2+ is regenerated; the steps add up to S2O82− + 2I− → 2SO42− + I2. ✓
02
Explain
[2 marks]

Explain why the Fe-catalysed route is faster than the direct reaction.

  • The direct reaction needs two negative ions to collide, so repulsion gives a high activation energy. ✓
  • Each catalysed step is between oppositely charged ions, so each has a lower activation energy. ✓
03
Recall
[3 marks]

Sketch and label the energy profile for an exothermic reaction with and without a homogeneous catalyst.

  • Uncatalysed: one large hump from reactants down to lower-energy products. ✓
  • Catalysed: two smaller humps, with the intermediate in the dip between them. ✓
  • Reactant and product levels are the same for both, so ΔH is unchanged. ✓
04
Explain
[3 marks]

Describe how the catalyst in a catalytic converter removes CO and NO from exhaust gases.

  • Adsorption: CO and NO bond to Pt/Rh active sites, weakening their bonds and holding them close together. ✓
  • Reaction: 2CO + 2NO → 2CO2 + N2 on the surface. ✓
  • Desorption: CO2 and N2 leave, freeing the sites for more molecules. ✓
05
Explain
[2 marks]

Silver adsorbs gases weakly and tungsten adsorbs them very strongly. Explain why neither is a good catalyst.

  • Silver: too few reactant molecules are held long enough, so bonds are not weakened. ✓
  • Tungsten: products bind so strongly that they do not desorb, so active sites stay blocked. ✓
06
Recall
[2 marks]

Write equations showing how NO2 catalyses the oxidation of SO2 in the atmosphere.

  • SO2 + NO2 → SO3 + NO. ✓
  • NO + ½O2 → NO2 (NO2 regenerated). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.