--- title: "Catalysis, equilibrium effect, promoter" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:catalysis-equilibrium-promoter" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalysis-equilibrium-promoter site: BCT Engineering --- # Catalysis, equilibrium effect, promoter Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2071 Magh, 1+2+1+1 marks - Video: [Catalysts: Homogeneous & Heterogeneous | A-level Chemistry | OCR, AQA, Edexcel](https://www.youtube.com/watch?v=xhGfFtn0Xug) — Launchpad Learning - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalysis-equilibrium-promoter - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question What is catalysis? How a catalyst alters the rate of reaction? What happens to the equilibrium position of a reaction by the application of a catalyst? What is promoter? ## Short answer A catalyst is a substance that changes reaction rate without being consumed, by providing a lower activation energy path. It does not alter equilibrium position because it speeds up forward and reverse reactions equally. A promoter (e.g., Mo in Haber process) is a substance that increases catalyst activity without being a catalyst itself. ## Answer #### 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. #### How a catalyst alters the rate 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. #### Effect on equilibrium A catalyst does **not** change the position of equilibrium or the equilibrium constant. It speeds up the forward and the backward reactions by the same factor (it lowers the activation energy of both by the same amount), so the equilibrium is reached faster but the equilibrium composition is unchanged. For example, in N₂ + 3H₂ ⇌ 2NH₃, Fe makes equilibrium arrive quickly but does not increase the final percentage of NH₃. #### Promoter **Promoters** (activators) are substances that, though not themselves catalysts, increase the activity of a catalyst when added in a small quantity. They may increase the active surface or change the crystal structure (creating more defects/active sites), or they may improve the adsorption of the reactants. - In Haber's process, Mo (or Al₂O₃ + K₂O) is a promoter for the iron catalyst. - In the water-gas shift reaction, Cr₂O₃ promotes Fe₂O₃. - Cu promotes Ni in some hydrogenation reactions. ## 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. What is the effect of a catalyst on the position of equilibrium? A. It shifts equilibrium to the products B. It shifts equilibrium to the reactants C. No change in the equilibrium position or constant D. It increases the equilibrium constant Answer: C. The catalyst only speeds up the approach to equilibrium. (Key point: Effect on equilibrium) 3. Why does a catalyst not change the equilibrium composition? A. It speeds up the forward and backward reactions by the same factor B. It speeds up only the forward reaction C. It slows the backward reaction only D. It changes DeltaH Answer: A. Ea is lowered by the same amount for both directions, so the ratio of rates is unchanged. (Key point: Why equilibrium is unchanged) 4. What is a promoter? A. A substance that destroys catalyst activity B. A substance, not itself a catalyst, that increases a catalyst's activity when added in small amount C. A substance that decreases the reaction rate D. A reactant that is consumed in the reaction Answer: B. Promoters (activators) boost catalyst activity, e.g. by more active sites or better adsorption. 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