Classify homogeneous and heterogeneous catalysts
Distinguish catalyst types by comparing the phase of catalyst and reactants.
Section 26 · Reaction kinetics
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
A gas-phase catalyst and gas-phase reactants are in what catalyst class?
What you will learn
- Define homogeneous catalyst
- Define heterogeneous catalyst
- Classify a catalyst system from its phases
Compare physical phases
A homogeneous catalyst is in the same phase as the reactants. A heterogeneous catalyst is in a different phase from the reactants, often a solid catalyst with gaseous or liquid reactants.
Classification depends on the actual reaction conditions. The same substance can be in different roles in different systems; identify phases rather than relying only on a familiar example.
Catalysts provide an alternative pathway
A catalyst increases reaction rate by providing an alternative mechanism with a lower activation energy and is regenerated overall. It does not change the equilibrium position or equilibrium constant.
For Topic 26, use the phase comparison to classify the catalyst, then describe the mode of action for the specified homogeneous or heterogeneous example.
Pause and explain
Why is solid iron with N₂(g) and H₂(g) heterogeneous?
Worked example
Classify solid iron in a reaction mixture of nitrogen and hydrogen gases.
Show the worked solution
- Identify the phase of iron.
- Identify the phases of N₂ and H₂.
- Compare them.
Answer Iron is solid and the reactants are gases, so the iron catalyst is heterogeneous.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
A dissolved Fe³⁺ ion catalyses a reaction between dissolved iodide and peroxodisulfate ions. Classify the catalyst.
Give me a hint
All listed species are aqueous.
Compare my reasoning
- Fe³⁺ is aqueous.
- The reactants are aqueous ions.
- Compare their phases.
The catalyst and reactants are in the same aqueous phase, so it is homogeneous.
Look for these in your work
- Catalyst phase identified
- Reactant phases identified
- Classification correct
A solid platinum surface catalyses a gas-phase reaction. State the catalyst class.
Give me a hint
Compare solid catalyst with gaseous reactants.
Compare my reasoning
- Platinum is solid.
- Reactants are gases.
- The phases differ.
It is a heterogeneous catalyst.
Look for these in your work
- Phases compared
- Different phase identified
- Class named
A learner calls a catalyst homogeneous because it is mixed evenly through the vessel. Explain the correct criterion.
Give me a hint
Homogeneous and heterogeneous refer to phase, not visual mixing.
Compare my reasoning
- Identify the catalyst phase.
- Identify reactant phases.
- Compare phase identity.
The classification depends on whether catalyst and reactants occupy the same phase, not whether they look evenly mixed. Same phase is homogeneous; different phases are heterogeneous.
Look for these in your work
- Visual mixing rejected
- Phase criterion stated
- Both definitions applied
Common mistakes
- Classifying from the chemical formula rather than phase
- Calling a solid catalyst homogeneous with gases
- Saying a catalyst changes equilibrium yield
- Forgetting that a catalyst is regenerated overall
What can you explain now?
A gas-phase catalyst and gas-phase reactants are in what catalyst class?
Compare with the explanation
Homogeneous
Catalyst and reactants share one phase.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which phase comparison defines a homogeneous catalyst and a heterogeneous catalyst?
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.