--- title: "Catalyst action; heterogeneous catalysis; adsorption theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:catalyst-heterogeneous-adsorption" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-heterogeneous-adsorption site: BCT Engineering --- # Catalyst action; heterogeneous catalysis; adsorption theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2073 Bhadra, 1+1+3 marks - Video: [Adsorption Theory of Heterogeneous Catalysis | Surface chemistry | JEE| NEET|](https://www.youtube.com/watch?v=vxLhXfXvh78) — 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/catalyst-heterogeneous-adsorption - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question How does a catalyst increase the speed of a reaction? Explain heterogeneous catalysis. Explain with an example the adsorption theory of catalysis. ## Short answer A catalyst lowers activation energy, providing a faster alternate path. Heterogeneous catalysis involves different phases, e.g., solid Fe for NH₃ synthesis. Adsorption theory: reactants adsorb on solid surface, activation weakens bonds, surface reaction occurs, product desorbs; e.g., C₂H₄ + H₂ on Ni. ## Answer #### How a catalyst increases the speed of a reaction A catalyst provides a new reaction path with a **lower activation energy** (Ea). More reacting molecules then have energy equal to or greater than the lowered threshold at the same temperature, so the number of effective collisions rises and the reaction becomes faster (rate constant k = A e^(−Ea/RT) increases). The catalyst is regenerated at the end. ```text Activation energy on the path R → P uncatalysed : ████████████ (high Ea) catalysed : ████ (low Ea) Catalyst lowers the barrier; R and P levels unchanged. ``` The energies of reactants (R) and products (P) are unchanged, so ΔH is the same; only the barrier is lowered. #### 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) #### Adsorption theory 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. How does a catalyst increase reaction speed? A. By raising the temperature of reactants B. By increasing the energy of products C. By providing a new path with lower activation energy D. By raising the activation energy Answer: C. A lower Ea means more molecules have enough energy, so there are more effective collisions. (Key point: Lower activation energy) 2. In heterogeneous catalysis the catalyst is: A. In the same phase as the reactants B. Always a gas C. In a different phase from the reactants, usually a solid with gaseous or liquid reactants D. Consumed during the reaction Answer: C. Heterogeneous catalysis has the catalyst in a separate phase, commonly a solid. (Key point: Heterogeneous catalysis definition) 3. 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) 4. 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. 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