Transition-metal Catalysis
Learn and apply Transition-metal Catalysis in the published Chemistry course sequence.
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The core idea
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Transition-metal Catalysis: Orientation
Catalysis questions are mostly “explain the pathway”: heterogeneous catalysis uses adsorption on a surface, while homogeneous catalysis often uses variable oxidation states and complex formation in a cycle (catalyst regenerated).
This chapter depends on periodic trends, so keep The Periodic Table (A Level) nearby and use the Transition Elements hub for the full sequence.
Definitions (Must Know)
A. Catalyst
A catalyst increases reaction rate without being consumed overall, by providing an alternative pathway with lower activation energy.
B. Heterogeneous vs homogeneous catalysis
- Heterogeneous: catalyst is in a different phase from reactants (often solid catalyst, gas reactants).
- Homogeneous: catalyst and reactants are in the same phase (often all in solution).
Detailed Explanations
A. Why transition metals are good catalysts (the key causal chain)
Because transition metals have variable oxidation states and can form complex ions/intermediates with reactants, therefore they can provide alternative pathways with lower activation energy and be regenerated at the end.
B. Heterogeneous catalysis (surface mechanism, what to write)
Typical steps:
- reactants adsorb on catalyst surface
- bonds weaken and/or new bonds form on surface
- products desorb (leave surface)
- catalyst surface is regenerated
Example contexts (name + role):
- iron catalyst in the Haber process
- nickel catalyst for hydrogenation of alkenes
- vanadium(V) oxide in the Contact process (often treated as heterogeneous even though the cycle involves oxidation states)
C. Workflow: answering a heterogeneous catalyst question (exam method)
- Use the keyword adsorption (reactants temporarily bond to the surface).
- State “adsorption brings reactants close together” and “weakens bonds”.
- State “products desorb” and “surface is regenerated”.
Mini example: Powdered Ni has more surface area → more adsorption sites → higher rate.
D. Homogeneous catalysis (redox cycle example)
A common syllabus example is the catalysis of the reaction between S₂O₈²⁻ and I⁻ by Fe²⁺/Fe³⁺:
Step 1: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
Step 2: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
Add the two steps (catalyst cancels) to get the overall reaction: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
E. What Section 13 adds
Transition elements can provide a lower-energy route because variable oxidation states permit successive electron-transfer steps and ligand binding permits temporary intermediates. In a homogeneous cycle, write each step and verify that the transition-metal species is regenerated. In heterogeneous catalysis, describe adsorption, bond weakening/reorientation, surface reaction and desorption rather than merely saying “surface area increases”.
F. The prescribed catalysis contexts from Reaction Kinetics
| Context | Mode and decisive explanation |
|---|---|
| Haber process | heterogeneous: nitrogen and hydrogen adsorb on iron, bonds weaken, ammonia forms and desorbs |
| removal of nitrogen oxides from car exhaust | heterogeneous: gases adsorb on a solid catalytic surface; nitrogen oxides are reduced while carbon monoxide or hydrocarbons are oxidised |
| atmospheric oxidation of sulfur dioxide | homogeneous nitrogen-oxide cycle: NO is oxidised to NO₂, then NO₂ oxidises SO₂ to SO₃ and regenerates NO |
| I⁻/S₂O₈²⁻ reaction | homogeneous Fe²⁺/Fe³⁺ cycle: each step transfers electrons between compatible couples and regenerates Fe²⁺ |
For every example, identify the alternative pathway and prove regeneration or surface release. Naming the catalyst alone does not explain its action.
Worked Examples
Modelled example 1
Why Transition Metals Catalyse
Problem
Study the worked solution
Use variable oxidation states
Method
Allow repeated electron-transfer steps.Reason
The metal can move between accessible oxidation states and be regenerated.Working
Redox cycling supplies an alternative pathway.Use bonding or surfaces
Method
Form intermediate complexes or adsorb reactants on a surface.Reason
Temporary interactions can weaken bonds and lower the pathway’s activation energy.Working
Any two valid reasons score.
Common misconception 2
Why the Catalyst Cancels
Learner claim
Track consumption and regeneration
View solution step by step
Follow the steps
Method
Consume the catalyst in one step and regenerate it later.Reason
It participates chemically in the alternative pathway.Working
Catalyst → intermediate → catalyst.Add equations
Method
Cancel the regenerated catalyst.Reason
It appears on opposite sides of component steps and is unchanged overall.Working
Absent from net equation, present in mechanism.
Challenge 3
Powder versus Lump Catalyst
Physical-form transfer
Link geometry to reaction rate
Hints
Hint 1: geometry
Hint 2: mechanism
View solution step by step
Compare exposed area
Method
Give powder the larger surface area.Reason
Many small particles expose more surface for the same mass.Working
Powder area > lump area.Connect to rate
Method
Provide more adsorption sites and more successful surface events per time.Reason
More reactant can use the catalytic pathway simultaneously.Working
Larger area → more active sites → higher rate.
Mind Stretchers
Mind stretcher 1Extension
Suggest why some catalysts become less effective over time (catalyst poisoning).
Show Hint
Catalyst poisoning removes available active sites or prevents a required intermediate from forming.
Show Answer
Mark scheme:
- Impurities bind strongly to active sites on the catalyst surface or deactivate the catalyst in solution.
- This reduces the number of available sites / stops the catalytic cycle.
Mind stretcher 2: Testing a proposed catalytic cycleExtension
Question. A proposed soluble catalyst M reacts with A to form M³⁺ and product P. In a second step, M³⁺ reacts with B to regenerate M and form Q. State two checks needed before accepting this as a catalytic pathway.
Show Hint
Add the proposed steps and check both regeneration and the activation-energy claim.
Show Answer
Adding the steps must cancel M and M³⁺ to give the known overall reaction between A and B; this shows that the catalyst is regenerated and the intermediate is not a net reagent. The two steps must also provide a kinetically accessible alternative pathway—normally one with a lower activation-energy bottleneck than the uncatalysed reaction. Regeneration alone does not prove that the route increases rate.