Explain heterogeneous catalyst surface action
Describe adsorption, bond weakening, reaction at a surface and product desorption with Cambridge examples.
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
Where do reactants adsorb in heterogeneous catalysis?
What you will learn
- Describe adsorption of reactants on a solid catalyst
- Explain bond weakening and reaction at the surface
- Describe product desorption and use Haber or exhaust examples
A solid surface brings reactants together
In heterogeneous catalysis, reactant molecules adsorb onto active sites on a solid surface. Adsorption can bring molecules close together and weaken bonds, making reaction through an alternative pathway easier.
Products then leave the surface by desorption, freeing sites for more reactants. If products bind too strongly, active sites can become blocked; the syllabus focus is adsorption, bond weakening and desorption.
Use the examples named in the syllabus
Iron is used in the Haber process. Palladium, platinum and rhodium are used in catalytic removal of oxides of nitrogen from engine exhaust gases.
Keep the surface mechanism sequence clear: adsorption, bond weakening/reaction at active sites, then product desorption. The catalyst is regenerated and the overall activation barrier is lower.
Pause and explain
What happens after products form at a catalytic surface?
Worked example
Describe how a solid catalyst can speed a reaction between gas molecules.
Show the worked solution
- Name adsorption onto active sites.
- Explain contact and bond weakening at the surface.
- Explain product desorption and site reuse.
Answer Gas reactants adsorb onto active sites, are brought into contact and have bonds weakened so they react more readily. Products desorb, leaving sites available for further reactants.
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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.
Describe the role of iron in the Haber process using the heterogeneous surface mechanism.
Give me a hint
Use the sequence of surface steps without claiming iron changes equilibrium.
Compare my reasoning
- Nitrogen and hydrogen adsorb onto iron sites.
- Adsorption weakens bonds and supports reaction.
- Ammonia desorbs and sites are reused.
N₂ and H₂ adsorb on the iron surface; adsorption brings them together and weakens bonds so they react more readily. NH₃ desorbs, freeing sites for more reactants.
Look for these in your work
- Haber example named
- Adsorption and bond weakening included
- Desorption and regeneration included
Name the Cambridge syllabus metals used in catalytic removal of nitrogen oxides from car exhaust gases.
Give me a hint
Recall the stated heterogeneous catalyst examples.
Compare my reasoning
- Identify the three listed transition metals.
- State their exhaust-gas context.
- Keep the role as catalyst.
Palladium, platinum and rhodium are used in catalytic removal of oxides of nitrogen from car exhaust gases.
Look for these in your work
- Metals listed
- Application named
- Catalytic role clear
A student explains heterogeneous catalysis as reactants dissolving into the catalyst and never leaving. Correct the sequence.
Give me a hint
Distinguish surface adsorption from product desorption.
Compare my reasoning
- Reactants adsorb at surface sites.
- Bonds weaken and reaction occurs at the surface.
- Products desorb and release sites.
Reactants adsorb onto active surface sites, react through a lower-energy route as bonds weaken, and products desorb. The catalyst surface is then available again.
Look for these in your work
- Surface adsorption stated
- Bond weakening included
- Products leave and sites regenerate
Common mistakes
- Describing absorption into the bulk instead of surface adsorption
- Omitting bond weakening
- Forgetting product desorption
- Saying the catalyst is permanently consumed
What can you explain now?
Where do reactants adsorb in heterogeneous catalysis?
Compare with the explanation
At active sites on the solid surface
The catalytic reaction occurs at the surface.
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Make it stick
How do adsorption, bond weakening and desorption make a solid catalyst reusable?
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.