- 2069 Poush · 2+3 marks
Positive/negative catalyst; intermediate compound theory
Short answer
A positive catalyst, like MnO₂ in KClO₃ decomposition, increases reaction rate by lowering activation energy. A negative catalyst, like glycerol in H₂O₂ decomposition, decreases rate. The intermediate theory states the catalyst forms an unstable intermediate with a reactant, lowering the energy path, then regenerates, as in NO catalyzing SO₂ oxidation.
Explain the following with examples: a) Positive catalyst b) Negative catalyst Explain the action of catalyst in the light of intermediate compound formation theory.
a) Positive catalyst
A positive catalyst increases the rate of a reaction by lowering the activation energy. Examples: MnO₂ in 2KClO₃ → 2KCl + 3O₂; Fe in N₂ + 3H₂ → 2NH₃; V₂O₅ in the contact process.
b) Negative catalyst
A negative catalyst (inhibitor) decreases the rate of a reaction. Examples: glycerol or acetanilide retards the decomposition of H₂O₂; alcohol retards the oxidation of Na₂SO₃ solution; tetraethyl lead is added to petrol to stop knocking.
Action in the light of intermediate compound formation theory
According to the intermediate compound formation theory, the catalyst takes part in the reaction by forming an unstable intermediate compound with one of the reactants. The intermediate has a lower activation energy path and reacts with the other reactant to give the product and regenerate the catalyst.
Uncatalysed: A + B ───────────→ AB (high Ea)
Catalysed : A + C → [AC] (fast, low Ea)
[AC] + B → AB + C (fast, low Ea)
(C = catalyst, regenerated)
Examples
- Lead chamber process (NO as catalyst): 2NO + O₂ → 2NO₂ (intermediate) NO₂ + SO₂ → SO₃ + NO (catalyst regenerated)
- Preparation of diethyl ether from ethanol with conc. H₂SO₄: C₂H₅OH + H₂SO₄ → C₂H₅HSO₄ (ethyl hydrogen sulphate) + H₂O C₂H₅HSO₄ + C₂H₅OH → C₂H₅OC₂H₅ + H₂SO₄
- Contact process with V₂O₅: V₂O₅ + SO₂ → V₂O₄ + SO₃, then V₂O₄ + ½O₂ → V₂O₅.
Limitation: it explains homogeneous catalysis well, but is not suitable for many heterogeneous cases where no stable intermediate is found.
✓ Answer checked by Claude Opus 5.5 (high effort)
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