Explain homogeneous catalyst cycles
Describe how a homogeneous catalyst participates in one step and is regenerated in a later step.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 26. Rate laws are empirical unless an elementary step is explicitly stated. Written practice is self-checked
Before you begin
- Use AS collision theory, concentration, graph gradients, and balanced reaction equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must happen to a homogeneous catalyst after it participates in one step?
What you will learn
- Describe a catalyst used in one step and regenerated in another
- Show how an alternative sequence lowers the reaction barrier
- Use NOx/SO₂ or Fe²⁺/Fe³⁺ examples from the syllabus
A homogeneous catalyst changes form during the cycle
A homogeneous catalyst is in the same phase as the reactants. It reacts in one step to form an intermediate or changed catalyst species, then is regenerated in a later step.
Adding the catalytic steps gives the same overall reaction because the catalyst cancels. The alternate sequence provides a pathway with a lower activation energy.
Apply the idea to syllabus examples
In atmospheric oxidation of sulfur dioxide, NO₂ reacts with SO₂ to form NO and SO₃; NO is then oxidised by oxygen to regenerate NO₂. The NO₂/NO species cycle while SO₂ is oxidised overall.
Fe²⁺/Fe³⁺ ions can catalyse the I⁻/S₂O₈²⁻ reaction through an alternative sequence. State the change in oxidation state and regeneration rather than inventing an unprovided detailed rate law.
Pause and explain
In the NO₂/NO cycle, why is NO₂ a catalyst?
Worked example
Show how NO₂ can catalyse oxidation of SO₂ while being regenerated.
Show the worked solution
- Write NO₂ + SO₂ → NO + SO₃.
- Write NO + ½O₂ → NO₂.
- Add the steps and cancel NO₂ and NO.
Answer Adding the steps gives SO₂ + ½O₂ → SO₃. NO₂ is used in the first step and regenerated in the second, so it acts as a catalyst.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use the two steps NO₂ + SO₂ → NO + SO₃ and NO + ½O₂ → NO₂ to identify the catalyst and overall reaction.
Give me a hint
Add both equations and cancel species on opposite sides.
Compare my reasoning
- NO₂ is consumed in step 1 and formed in step 2.
- NO is formed then consumed.
- Cancel both to obtain the net oxidation.
NO₂ is the catalyst and NO is an intermediate. The overall reaction is SO₂ + ½O₂ → SO₃.
Look for these in your work
- Catalyst tracked
- Intermediate identified
- Net equation balanced
Fe²⁺/Fe³⁺ ions catalyse I⁻ oxidation by S₂O₈²⁻. Explain what must happen to the iron oxidation state during the catalytic sequence.
Give me a hint
A catalyst ion changes in one step and must be restored later.
Compare my reasoning
- One iron oxidation state participates in an electron-transfer step.
- The other state is formed.
- A later step regenerates the starting state.
Fe²⁺ and Fe³⁺ interconvert in the alternative reaction steps, and the iron catalyst is regenerated overall while iodide is oxidised by peroxodisulfate.
Look for these in your work
- Redox-state change stated
- Step sequence described
- Catalyst regeneration stated
A learner adds the catalytic steps but leaves NO₂ on the product side of the overall equation. Explain how to correct the sum.
Give me a hint
The same catalyst amount is consumed and regenerated.
Compare my reasoning
- Place all reactants and products from both steps together.
- Cancel equal NO₂ on opposite sides.
- Check the remaining equation and atoms.
Cancel NO₂ because it is used in one step and regenerated in the other. The net equation is SO₂ + ½O₂ → SO₃.
Look for these in your work
- Catalyst cancellation applied
- Net equation simplified
- Overall reaction retained
Common mistakes
- Leaving catalyst species in the net equation
- Calling a catalyst a product because it appears in a step
- Describing homogeneous catalysis as surface adsorption
- Inventing detailed steps beyond the supplied mechanism
What can you explain now?
What must happen to a homogeneous catalyst after it participates in one step?
Compare with the explanation
It is regenerated in a later step
Regeneration means it is not consumed overall.
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Make it stick
How does a homogeneous catalyst change form through a cycle and return unchanged in the net equation?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.