--- title: "Positive/negative catalyst; intermediate compound theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:pos-neg-intermediate" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/pos-neg-intermediate site: BCT Engineering --- # Positive/negative catalyst; intermediate compound theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2069 Poush, 2+3 marks - Video: [Theories of catalysis p 1Intermediate compound formation theory](https://www.youtube.com/watch?v=9bdbWq8jLsA) — Professor Nazir ahmad Marwat - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/pos-neg-intermediate - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Explain the following with examples: a) Positive catalyst b) Negative catalyst Explain the action of catalyst in the light of intermediate compound formation 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. ## Answer #### 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. ```text Uncatalysed: A + B ───────────→ AB (high Ea) Catalysed : A + C → [AC] (fast, low Ea) [AC] + B → AB + C (fast, low Ea) (C = catalyst, regenerated) ``` **Examples** 1. Lead chamber process (NO as catalyst): 2NO + O₂ → 2NO₂ (intermediate) NO₂ + SO₂ → SO₃ + NO (catalyst regenerated) 2. Preparation of diethyl ether from ethanol with conc. H₂SO₄: C₂H₅OH + H₂SO₄ → C₂H₅HSO₄ (ethyl hydrogen sulphate) + H₂O C₂H₅HSO₄ + C₂H₅OH → C₂H₅OC₂H₅ + H₂SO₄ 3. 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement distinguishes a negative catalyst from a positive one? A. A negative catalyst lowers the activation energy B. A positive catalyst decreases the rate C. A negative catalyst decreases the rate by raising the activation energy D. Both lower the activation energy Answer: C. Positive catalysts lower Ea and increase rate; negative catalysts (inhibitors) raise Ea and decrease rate. (Key point: Positive vs negative catalyst) 2. Which pair is correct? A. Positive: glycerol in H2O2; negative: MnO2 in KClO3 B. Positive: MnO2 in KClO3 decomposition; negative: glycerol in H2O2 decomposition C. Positive: alcohol in Na2SO3 oxidation; negative: Fe in Haber's process D. Positive: acetanilide in H2O2; negative: V2O5 in contact process Answer: B. Glycerol, acetanilide and alcohol act as inhibitors; MnO2, Fe and V2O5 are positive catalysts. (Key point: Examples of catalysis types) 3. According to the intermediate compound formation theory, the catalyst: A. Adsorbs all reactants permanently on its surface B. Is used up completely forming the product C. Forms an unstable intermediate with a reactant, which reacts further and regenerates the catalyst D. Only raises the temperature of the reaction Answer: C. The intermediate provides a lower-Ea path and releases the catalyst again when the product forms. (Key point: Intermediate compound theory) 4. In the lead chamber process, what is the intermediate formed by the NO catalyst? A. SO3 formed directly from NO B. NO2, formed from 2NO + O2 C. N2O5 D. V2O4 Answer: B. 2NO + O2 -> 2NO2, then NO2 + SO2 -> SO3 + NO regenerates NO. 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