5.3.5Unit 5. Transition Metals and Organic Nitrogen Chemistry
Homogeneous Catalysis — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 25 of 43 Unit quiz
Part 1 of 6The Uncatalysed Reaction
Equation
- S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂ is thermodynamically feasible but very slow.
Reason for slowness
- Both reactants are negative ions, so repulsion between them makes collisions with enough energy very rare.
Observation
- Iodine forms slowly, turning the solution brown, and a starch indicator gives a blue-black colour.
Part 2 of 6Catalysis by Iron Ions
1
Add a few drops of Fe²⁺(aq) or Fe³⁺(aq), and the reaction rate increases markedly.
2
Step 1: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺, which involves a positive and negative ion that attract rather than repel.
3
Step 2: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂, again involving oppositely charged ions.
4
Iron(II) is regenerated, so the catalyst appears at the start and end and is not consumed overall.
5
Adding the two steps and cancelling Fe²⁺ and Fe³⁺ gives the original equation.
Part 3 of 6Using E⦵ Values to Check
Using E⦵ Values to Check
Step 1
- E⦵(S₂O₈²⁻/SO₄²⁻) = +2.01 V and E⦵(Fe³⁺/Fe²⁺) = +0.77 V, giving E⦵cell = +1.24 V.
Step 2
- E⦵(Fe³⁺/Fe²⁺) = +0.77 V and E⦵(I₂/I⁻) = +0.54 V, giving E⦵cell = +0.23 V.
Conclusion
- Both steps are feasible, which confirms that iron ions can act as an effective catalyst.
Part 4 of 6Key Features
Key Features
Same phase
- The catalyst and reactants are all in aqueous solution, so mixing is complete.
Intermediate
- The catalyst changes oxidation state during the reaction, which is why transition metals are well suited.
Separation
- A drawback is that homogeneous catalysts can be harder to remove from the products.
Part 5 of 6Worked Example
Question Check step 1 of the iron-catalysed reaction is feasible using E⦵ values of +2.01 V and +0.77 V.
- S₂O₈²⁻/SO₄²⁻ has the more positive E⦵, so it is reduced.
- E⦵cell = +2.01 - (+0.77) = +1.24 V.
Answer E⦵cell = +1.24 V, so step 1 is feasible
Part 6 of 6Exam Checklist & Tips
5.3.5 – Key Points for the Exam
- Homogeneous catalyst A catalyst in the same phase as the reactants.
- Intermediate A species formed in one step and consumed in another.
- Peroxodisulfate The S₂O₈²⁻ ion, a strong oxidising agent.
- Starch Indicator giving a blue-black colour with iodine.
Common Mistakes
- Saying the catalyst is absent from the mechanism.
- Believing that Fe³⁺ must be added rather than either ion of iron.
- Ignoring repulsion between negative ions in the uncatalysed reaction.
- Forgetting to cancel intermediates when adding the steps.
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