---
title: "Autocatalysis; positive/negative catalysis; poison"
subject: "Engineering Chemistry"
chapter: 2
key: "semester_2/engineering_chemistry:autocatalysis-poison"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/autocatalysis-poison
site: BCT Engineering
---

# Autocatalysis; positive/negative catalysis; poison

Engineering Chemistry · Chapter 2: Catalyst

- Asked in: 2073 Magh, 1+3+1 marks
- Video: [What is catalytic poisoning? | Autocatalysis | Promoters | Catalysis Part 2](https://www.youtube.com/watch?v=EMlqqiOWHSE) — Gate to Knowledge
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/autocatalysis-poison
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2

## Question

What is autocatalysis? Distinguish between positive and negative catalysis with examples. How does poison paralyze the activity of a catalyst?

## Short answer

Autocatalysis is a reaction where a product catalyzes the same reaction. A positive catalyst increases rate by lowering activation energy (e.g., MnO2 in KClO3 decomposition), while a negative catalyst decreases rate by raising it (e.g., glycerol in H2O2 decomposition). Poisons block active sites by strong adsorption, preventing reactant adsorption and deactivating the catalyst.

## Answer

#### Autocatalysis
**Autocatalysis:** a reaction in which one of the products acts as a catalyst for the same reaction. The rate is slow at first, rises as the catalyst product builds up, and then falls as reactants are used up.
- Oxidation of oxalic acid by KMnO₄ in acid: the Mn²⁺ formed catalyses the reaction.
- Hydrolysis of ethyl acetate: the acetic acid formed catalyses it.
- Decomposition of arsine (AsH₃): the arsenic formed acts as a catalyst.

#### Positive and negative catalysis
| Point | Positive catalyst | Negative catalyst (inhibitor) |
|---|---|---|
| Effect on rate | Increases the rate | Decreases the rate |
| Activation energy | Lowered | Raised |
| Mechanism | Gives an easier path | Blocks the path / destroys a chain carrier or a positive catalyst |
| Example | MnO₂ in 2KClO₃ → 2KCl + 3O₂; Fe in Haber's process | Glycerol or acetanilide slows the decomposition of H₂O₂; alcohol slows the oxidation of Na₂SO₃ |

```text
Energy barrier (Ea) on the path R → P

  negative catalyst  : ████████████  highest
  uncatalysed        : ████████      medium
  positive catalyst  : ████          lowest

  (reactant and product energies stay the same)
```

#### Action of poison
A poison is strongly adsorbed on the active centres of the catalyst surface (or combines with the catalyst to form an inactive compound). The active sites are blocked, so reactant molecules cannot be adsorbed and the catalyst loses its power. Examples: As₂O₃ on Pt in the contact process, H₂S on Fe in Haber's process, CO on Pt.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Autocatalysis is a reaction in which:
   A. One of the products acts as a catalyst for the same reaction
   B. An external catalyst is added at the start
   C. The catalyst is poisoned by the product
   D. The reaction runs without a catalyst
   Answer: A. The product catalyses its own formation. (Key point: Autocatalysis definition)

2. What is the rate pattern in an autocatalytic reaction?
   A. Fast at first, then constant
   B. Slow at first, rising as catalyst builds up, then falling as reactants are used
   C. Constant throughout
   D. Zero at first and then constant forever
   Answer: B. The rate rises as the product catalyst accumulates and falls when reactants run out. (Key point: Autocatalytic rate profile)

3. 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)

4. How does a poison make a catalyst inactive?
   A. It is strongly adsorbed on the active sites (or forms a stable compound), so reactants cannot be adsorbed
   B. It lowers the activation energy too much
   C. It dissolves the reactants
   D. It increases the surface area of the catalyst
   Answer: A. Blocked active sites mean no reactant adsorption and no catalysis. (Key point: Action of a poison)

## Related in Engineering Chemistry

- [Catalyst; positive and negative catalysis with examples](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-pos-neg-examples) — Engineering Chemistry, chapter 2
- [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
- [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

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