--- title: "Homogeneous/heterogeneous catalysis; intermediate compound theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:homo-hetero-intermediate" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/homo-hetero-intermediate site: BCT Engineering --- # Homogeneous/heterogeneous catalysis; intermediate compound theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2072 Asoj, 2+3 marks - Video: [Catalysis by Intermediate Formation theory or Homogeneous Catalysis theory](https://www.youtube.com/watch?v=Ikh-xaBKXRg) — Shivam Educations - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/homo-hetero-intermediate - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Show your acquaintance with homogeneous and heterogeneous catalysis. Describe the intermediate compound formation theory of catalysis. ## Short answer Homogeneous catalysis has catalyst and reactants in the same phase; heterogeneous has different phases. The intermediate compound formation theory says the catalyst forms an unstable intermediate with one reactant, which then reacts with the other to give the product and regenerate the catalyst, providing a lower activation energy path. Example: NO forms NO2 and regenerates in the lead chamber process. ## Answer #### Homogeneous catalysis **Homogeneous catalysis:** the catalyst and the reactants are in the same phase (all gases or all in one solution). - 2SO₂(g) + O₂(g) → 2SO₃(g), catalyst NO(g) (lead chamber process; all gaseous) - CH₃COOCH₃(l) + H₂O(l) → CH₃COOH + CH₃OH, catalyst H⁺ (acid hydrolysis of ester, all in solution) - C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆, catalyst H⁺ (inversion of cane sugar) #### Heterogeneous catalysis **Heterogeneous catalysis:** the catalyst is in a different phase from the reactants; usually a solid catalyst with gaseous or liquid reactants. - N₂(g) + 3H₂(g) → 2NH₃(g), catalyst Fe(s) (Haber's process) - 2SO₂(g) + O₂(g) → 2SO₃(g), catalyst V₂O₅(s) or Pt (contact process) - Vegetable oil + H₂ → vegetable ghee, catalyst Ni(s) (hydrogenation) - 4NH₃ + 5O₂ → 4NO + 6H₂O, catalyst Pt gauze (Ostwald's process) #### 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. In homogeneous catalysis, the catalyst and the reactants are: A. In different phases, with a solid catalyst B. In the same phase, forming one homogeneous mixture C. Both solids mixed together D. Always gases only, never in solution Answer: B. Homogeneous catalysis has catalyst and reactants in a single phase (all gas or all in one solution). (Key point: Homogeneous catalysis definition) 2. Which is an example of heterogeneous catalysis? A. Haber's process: N2(g) + 3H2(g) -> 2NH3(g) over solid Fe B. Acid hydrolysis of methyl acetate with H+ C. Inversion of cane sugar with H+ D. Lead chamber process with NO(g) Answer: A. Solid Fe with gaseous reactants are different phases. The others have catalyst and reactants in the same phase. (Key point: Example of heterogeneous catalysis) 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. What is a limitation of the intermediate compound formation theory? A. It is not suitable for many heterogeneous cases where no stable intermediate is found B. It cannot explain any homogeneous catalysis C. It requires the catalyst to be consumed D. It predicts a higher activation energy Answer: A. It works well for homogeneous catalysis but fails where no intermediate can be identified. 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