Catalyst Effect on Rate – IGCSE Chemistry Definition

A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed at the end of the reaction. It works by providing an alternative reaction pathway with a lower activation energy. This means that more reactant particles have enough energy to react in any given collision, so the proportion of successful collisions increases and the rate rises.

How it works at the particle level

Without a catalyst, particles must collide with at least the activation energy for the reaction to proceed. Only a fraction of collisions meet this threshold. A catalyst lowers this threshold by offering a different route from reactants to products. The lower activation energy means a larger fraction of all collisions are now energetic enough to succeed, so the rate increases even though the temperature has not changed.

What a catalyst does NOT do

A catalyst does not change the products of the reaction, the total amount of product formed, or the overall energy change (enthalpy change). It does not increase the temperature or the concentration. It does not shift the position of equilibrium in a reversible reaction. It only makes the reaction reach equilibrium faster by speeding up both the forward and backward reactions equally.

Industrial examples

Iron is the catalyst in the Haber process (N₂ + 3H₂ ⇌ 2NH₃). Vanadium(V) oxide (V₂O₅) catalyses the Contact process (2SO₂ + O₂ → 2SO₃). Manganese(IV) oxide (MnO₂) catalyses the decomposition of hydrogen peroxide (2H₂O₂ → 2H₂O + O₂). Nickel catalyses the hydrogenation of vegetable oils.

Biological catalysts are called enzymes. Yeast contains enzymes that catalyse the fermentation of glucose to produce ethanol and carbon dioxide.

Exam context

Paper 2 and Paper 4 frequently test the definition of a catalyst and its effect on activation energy. A common question asks candidates to explain how a catalyst increases the rate and to show its effect on an energy level diagram.

Worked exam question

Define the term catalyst and explain how it increases the rate of reaction. (3 marks)

A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed at the end [1]. It provides an alternative reaction pathway [1] with a lower activation energy, so more particles have sufficient energy to react / a greater proportion of collisions are successful [1].

Common mistakes

Candidates often write that a catalyst “gives particles more energy.” This is incorrect. A catalyst does not increase the kinetic energy of the particles --- it lowers the energy barrier they need to overcome. Another error is saying the catalyst is “not used” rather than “not chemically changed.” The catalyst does participate in the reaction; it is regenerated at the end.