5.3.4Unit 5. Transition Metals and Organic Nitrogen Chemistry
Catalysts — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 24 of 43 Unit quiz
Section 1 of 6Catalyst Basics
Mechanism
- A catalyst provides an alternative reaction pathway with a lower activation energy, so more molecules react in a given time.
Not consumed
- It is regenerated at the end of the reaction, so it does not appear in the overall equation.
Equilibrium
- A catalyst speeds up the forward and reverse reactions equally, so it does not change the position of equilibrium or the yield.
Types
- Heterogeneous catalysts are in a different phase from the reactants, whereas homogeneous catalysts are in the same phase.
Section 2 of 6Why Transition Metals
Why Transition Metals
Variable oxidation states
- They can accept and donate electrons, forming intermediates and then returning to the original state.
Surface
- Partially filled d and 4s orbitals allow reactant molecules to be adsorbed and held in the right orientation.
Weakening bonds
- Adsorption weakens bonds in the reactant, lowering the activation energy.
Section 3 of 6Heterogeneous Examples
1
Haber process: iron catalyses N₂ + 3H₂ ⇌ 2NH₃, with sulfur compounds acting as poisons.
2
Contact process: vanadium(V) oxide catalyses 2SO₂ + O₂ ⇌ 2SO₃.
3
Catalytic converters: platinum, palladium and rhodium on a ceramic honeycomb convert 2CO + 2NO → 2CO₂ + N₂.
4
Hydrogenation: nickel catalyses the addition of hydrogen to the C=C bond of unsaturated oils.
Section 4 of 6Surface Catalysis Steps
Surface Catalysis Steps
Adsorption
- Reactant molecules bond weakly to active sites on the catalyst surface.
Reaction
- Bonds in the adsorbed molecules weaken and new bonds form between the neighbouring particles.
Desorption
- Products leave the surface, freeing the active sites for more reactants.
Poisoning
- Impurities bond too strongly to the surface and block active sites, as lead does in catalytic converters.
Section 5 of 6Worked Example
Question State what happens to the activation energy and the enthalpy change when a catalyst is added.
- The catalyst offers an alternative route with a lower Ea.
- The reactants and products are unchanged, so the enthalpy change ΔH is the same.
Answer Ea decreases; ΔH is unchanged
Section 6 of 6Exam Checklist & Tips
5.3.4 – Key Points for the Exam
- Catalyst A substance that increases rate without being used up.
- Heterogeneous The catalyst is in a different phase from the reactants.
- Adsorption Attachment of molecules to a surface.
- Catalyst poison A substance that blocks the active sites.
Common Mistakes
- Saying that a catalyst changes the yield at equilibrium.
- Claiming that a catalyst is used up in the reaction.
- Forgetting that surface area matters for solid catalysts.
- Saying catalysts provide energy to the reaction.
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