Autocatalysis is a reaction in which:

An external catalyst is added at the start

The catalyst is poisoned by the product

The reaction runs without a catalyst

What is the rate pattern in an autocatalytic reaction?

Slow at first, rising as catalyst builds up, then falling as reactants are used

Zero at first and then constant forever

Which statement distinguishes a negative catalyst from a positive one?

A negative catalyst decreases the rate by raising the activation energy

A negative catalyst lowers the activation energy

A positive catalyst decreases the rate

Both lower the activation energy

How does a poison make a catalyst inactive?

It is strongly adsorbed on the active sites (or forms a stable compound), so reactants cannot be adsorbed

It lowers the activation energy too much

It increases the surface area of the catalyst

2073 Magh · 1+3+1 marks

Autocatalysis; positive/negative catalysis; poison

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.

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

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

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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