--- title: "Adsorption theory of catalysis" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:adsorption-theory" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/adsorption-theory site: BCT Engineering --- # Adsorption theory of catalysis Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2070 Magh, 3 marks - Video: [Theory of intermediate compound formation and Adsorption theory | Theories of catalysis](https://www.youtube.com/watch?v=b7LqVwlpHts) — Chemistry Scholars - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/adsorption-theory - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Describe the adsorption theory of catalysis with a suitable example. ## Short answer According to adsorption theory, reactants adsorb on the catalyst surface, bonds weaken, and they react to form product which then desorbs, freeing active sites. Example: ethene and hydrogen adsorb on nickel, H-H breaks, surface reaction gives ethane which desorbs. ## Answer According to the **adsorption theory** (for heterogeneous catalysis), the catalyst works by adsorbing the reactant molecules on its surface. The steps are: 1. **Diffusion and adsorption:** reactant molecules diffuse to the catalyst surface and are adsorbed on the active sites, so their concentration at the surface becomes high. 2. **Activation:** adsorption weakens the bonds of the reactants (e.g. H–H breaks), forming activated, reactive species. Adsorption is exothermic, and the heat released helps to supply activation energy. 3. **Surface reaction:** the activated adsorbed molecules, now close together, react to form the product. 4. **Desorption:** the product molecules have little affinity for the surface; they leave the surface, which frees the active sites. 5. The freed sites adsorb new reactant molecules and the cycle repeats. ```text H₂ C₂H₄ H–H C=C H H ↓ ↓ ↓ ↓ | | ▒▒▒▒▒▒▒▒▒▒▒ → ▒▒▒▒▒▒▒▒▒▒▒ → H–C–C–H (leaves) Ni surface adsorbed, H–H | | (adsorption) bond broken H H ``` **Example:** hydrogenation of ethene on nickel: C₂H₄ + H₂ → C₂H₆ (Ni). H₂ is adsorbed on Ni and splits into H atoms, ethene is also adsorbed, the H atoms add to the double bond, and ethane is desorbed. The theory explains why a finely divided catalyst is more active (larger surface), why poisons stop catalysis (they occupy the active sites) and why promoters help (they increase the active surface). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the correct order of steps in the adsorption theory of catalysis? A. Activation, desorption, adsorption, surface reaction B. Adsorption, activation, surface reaction, desorption C. Surface reaction, adsorption, desorption, activation D. Adsorption, desorption, activation, surface reaction Answer: B. Reactants are adsorbed, activated, react on the surface, and the products desorb to free the sites. (Key point: Steps of adsorption theory) 2. How does adsorption help activate reactant molecules? A. It strengthens their bonds so they stay stable B. It lowers the concentration of reactants at the surface C. It weakens their bonds (e.g. H-H) and the heat released helps supply activation energy D. It raises the temperature of the whole reactor Answer: C. Adsorbed molecules have weakened bonds and the exothermic adsorption heat helps provide activation energy. (Key point: Activation by adsorption) 3. Why must the product desorb from the catalyst surface? A. So the catalyst is used up B. So the reactant is converted into a poison C. So the activation energy is raised D. So the active sites are freed to adsorb new reactant molecules Answer: D. Products have little affinity for the surface; leaving frees the sites and the cycle repeats. (Key point: Desorption step) 4. Which observation does the adsorption theory explain? A. A finely divided catalyst is more active because of its larger surface area B. Catalysts shift the equilibrium position C. Homogeneous catalysts work in solution D. Catalysts change the value of DeltaH Answer: A. A larger surface gives more active sites for adsorption. The theory also explains poisons (blocked sites) and promoters. 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