---
title: "Catalyst; positive and negative catalysis with examples"
subject: "Engineering Chemistry"
chapter: 2
key: "semester_2/engineering_chemistry:catalyst-pos-neg-examples"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-pos-neg-examples
site: BCT Engineering
---

# Catalyst; positive and negative catalysis with examples

Engineering Chemistry · Chapter 2: Catalyst

- Asked in: 2067 Chaitra, 1+2+2 marks
- Video: [Catalyst and Types of Catalyst - Positive Catalyst and Negative Catalyst, Explained with Graphs](https://www.youtube.com/watch?v=HUIc_HXmhd8) — Continuous Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-pos-neg-examples
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2

## Question

What is catalyst? Explain positive and negative catalysis with two examples for each.

## Short answer

A catalyst changes reaction rate without being consumed. Positive catalysts like MnO₂ in KClO₃ decomposition and Fe in ammonia synthesis increase rate by lowering activation energy. Negative catalysts like glycerol in H₂O₂ decomposition and alcohol in Na₂SO₃ oxidation decrease rate.

## Answer

#### Catalyst
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.

#### Positive catalysis
The catalyst increases the rate of the reaction by lowering the activation energy.
1. Decomposition of KClO₃: 2KClO₃ → 2KCl + 3O₂, catalyst MnO₂.
2. Haber's process: N₂ + 3H₂ → 2NH₃, catalyst Fe.

#### Negative catalysis
The catalyst (inhibitor) decreases the rate of the reaction.
1. Decomposition of H₂O₂ is slowed by glycerol or acetanilide.
2. Oxidation of sodium sulphite solution by air is slowed by alcohol.

| Point | Positive | Negative |
|---|---|---|
| Rate | Increased | Decreased |
| Activation energy | Lowered | Raised |

## 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 statement distinguishes a negative catalyst from a positive one?
   A. A negative catalyst lowers the activation energy
   B. A positive catalyst decreases the rate
   C. A negative catalyst decreases the rate by raising the activation energy
   D. Both lower the activation energy
   Answer: C. Positive catalysts lower Ea and increase rate; negative catalysts (inhibitors) raise Ea and decrease rate. (Key point: Positive vs negative catalyst)

3. Which pair is correct?
   A. Positive: glycerol in H2O2; negative: MnO2 in KClO3
   B. Positive: MnO2 in KClO3 decomposition; negative: glycerol in H2O2 decomposition
   C. Positive: alcohol in Na2SO3 oxidation; negative: Fe in Haber's process
   D. Positive: acetanilide in H2O2; negative: V2O5 in contact process
   Answer: B. Glycerol, acetanilide and alcohol act as inhibitors; MnO2, Fe and V2O5 are positive catalysts. (Key point: Examples of catalysis types)

4. How does a negative catalyst (inhibitor) act?
   A. It gives an easier path to the products
   B. It blocks the path or destroys a chain carrier or a positive catalyst
   C. It adds more active sites
   D. It lowers the activation energy
   Answer: B. Inhibitors work by blocking or removing the species that carry the reaction. (Key point: Inhibitor mechanism)

## Related in Engineering Chemistry

- [Positive/negative catalysis; promoter; catalyst criteria](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apos-neg-promoter-criteria) — Engineering Chemistry, chapter 2
- [Autocatalysis; positive/negative catalysis; poison](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aautocatalysis-poison) — Engineering Chemistry, chapter 2
- [Positive/negative catalyst; intermediate compound theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apos-neg-intermediate) — Engineering Chemistry, chapter 2
- [Catalyst action; heterogeneous catalysis; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-heterogeneous-adsorption) — Engineering Chemistry, chapter 2
- [Catalyst, catalysis; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-definition-adsorption) — Engineering Chemistry, chapter 2

## Related in other subjects

- [Effective mass can be positive or negative](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aeffective-mass-positive-negative) — Electrical Engineering Material, chapter 1, asked 4×
- [Positive XOR equals negative XNOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-xor-negative-xnor-define) — Digital Logic, chapter 2, asked 3×
- [Prove positive XOR equals negative XNOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-xor-negative-xnor-prove) — Digital Logic, chapter 2, asked 3×
- [Positive and negative logic](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-negative-logic) — Digital Logic, chapter 2, asked 2×