--- title: "Catalyst, catalysis; adsorption theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:catalyst-definition-adsorption" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-definition-adsorption site: BCT Engineering --- # Catalyst, catalysis; adsorption theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2068 Magh, 1.5+3.5 marks - Video: [Principles of Heterogeneous Catalysis](https://www.youtube.com/watch?v=evBiIO6fuHI) — Professor Dave Explains - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-definition-adsorption - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Define catalyst and catalysis. Describe the adsorption theory of heterogeneous catalysis with a suitable example. ## Short answer A catalyst alters reaction rate without being consumed, as in MnO₂ accelerating KClO₃ decomposition. The adsorption theory for heterogeneous catalysis involves reactant adsorption on active sites, bond weakening, surface reaction, and product desorption, like H₂ and C₂H₄ reacting on Ni to form C₂H₆. ## Answer #### Catalyst and catalysis A **catalyst** is a substance that changes the rate of a chemical reaction without being consumed in the reaction (its mass and chemical nature are the same at the end). The phenomenon is called **catalysis**. Example: MnO₂ speeds up the decomposition of KClO₃ to O₂. #### Adsorption theory of heterogeneous catalysis 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. A catalyst is a substance that: A. Changes the rate of a reaction without being consumed B. Is consumed to form the product C. Changes only the equilibrium constant D. Always increases the rate Answer: A. A catalyst's mass and chemical nature are the same at the end; catalysts can also slow reactions (negative). (Key point: Catalyst definition) 2. Which is a correct catalyst example? A. MnO2 slows the decomposition of KClO3 B. KClO3 is the catalyst for MnO2 C. MnO2 in the decomposition of KClO3 to give O2 D. Fe in the decomposition of KClO3 as a poison Answer: C. MnO2 speeds up decomposition of KClO3 and is not consumed. (Key point: Catalyst example) 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. (Key point: Activation by adsorption) ## Related in Engineering Chemistry - [Catalyst action; heterogeneous catalysis; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-heterogeneous-adsorption) — Engineering Chemistry, chapter 2 - [Heterogeneous catalysis; adsorption theory; criteria](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ahetero-adsorption-criteria) — Engineering Chemistry, chapter 2 - [Promoters, adsorption theory, catalyst criteria](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apromoters-adsorption) — Engineering Chemistry, chapter 2 - [Adsorption theory of catalysis](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aadsorption-theory) — Engineering Chemistry, chapter 2 - [Homogeneous/heterogeneous catalysis; intermediate compound theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ahomo-hetero-intermediate) — Engineering Chemistry, chapter 2 - [Heterogeneous catalysis and its theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aheterogeneous-catalysis-theory) — Engineering Chemistry, chapter 2