--- title: "Catalytic poisoning; intermediate compound theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:catalytic-poisoning" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalytic-poisoning site: BCT Engineering --- # Catalytic poisoning; intermediate compound theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2074 Bhadra, 2+3 marks - Video: [INTERMEDIATE COMPOUND FORMATION THEORY/U-10/SURFACE CHEMISTRY/XII EM/DAILY](https://www.youtube.com/watch?v=N1utLS4OJpY) — KSK CHEMISTRY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalytic-poisoning - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question What is meant by catalytic poisoning? Explain intermediate compound formation theory of catalysis referring suitable example. ## Short answer Catalytic poisoning is deactivation by trace impurities that strongly adsorb on active sites, e.g., As₂O₃ poisoning Pt in the contact process. The intermediate compound formation theory: catalyst first forms an unstable intermediate with one reactant, which reacts with the second reactant, regenerating the catalyst, e.g., NO in lead chamber process. ## Answer #### Catalytic poisoning **Catalytic poisoning:** a small amount of some substance (the **poison**) greatly reduces or destroys the activity of a catalyst. Poisons do not take part in the main reaction. - As₂O₃ poisons platinum in the contact process (2SO₂ + O₂ → 2SO₃). - H₂S, CO and compounds of S or P poison iron in the Haber process. - CO poisons platinum in the reaction 2H₂ + O₂ → 2H₂O and in hydrogenation reactions; lead compounds poison the Pt catalyst of a car's catalytic converter. **How a poison acts:** the poison is strongly adsorbed on the active sites of the catalyst surface (or forms a stable compound with the catalyst). The sites are then unavailable for the reactant molecules, so the surface area available for reaction decreases and the catalyst becomes inactive. Poisoning may be temporary (poison removed by heating/washing) or permanent. For this reason the reactants are purified before use in industry. #### 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. Catalytic poisoning is: A. Increase of catalyst activity by a promoter B. Loss of catalyst activity caused by a small amount of a foreign substance C. Complete consumption of the catalyst in the reaction D. Loss of catalyst by melting only Answer: B. A small amount of poison greatly reduces or destroys activity. (Key point: Poisoning definition) 2. How does a poison make a catalyst inactive? A. It is strongly adsorbed on the active sites (or forms a stable compound), so reactants cannot be adsorbed B. It lowers the activation energy too much C. It dissolves the reactants D. It increases the surface area of the catalyst Answer: A. Blocked active sites mean no reactant adsorption and no catalysis. (Key point: Action of a poison) 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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