Catalysts — Rates of Reaction | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

Factors affecting rate and collision theory are high-frequency multi-mark questions.

What a catalyst is

A catalyst is a substance that speeds up a chemical reaction without being used up in it
The mass of the catalyst is the same at the end of the reaction as at the start
Only small amounts of catalyst are usually needed because each particle of catalyst can take part in many reactant turnovers
A catalyst is neither a reactant nor a product, so its formula is written next to the arrow itself rather than inside the balanced equation

Industrial examples of catalysts

Process	Catalyst	What it does
Haber process: N₂ + 3 H₂ ⇌ 2 NH₃	Iron	Speeds up the synthesis of ammonia
Contact process: 2 SO₂ + O₂ ⇌ 2 SO₃	Vanadium(V) oxide, V₂O₅	Step in the manufacture of sulfuric acid
Catalytic decomposition of H₂O₂	Manganese(IV) oxide, MnO₂	School demonstration of catalysis
Catalytic cracking of long-chain alkanes

Many catalysts are transition metals or their oxides — their variable oxidation states allow them to accept and donate electrons easily, which is what makes them catalytic
Enzymes are biological catalysts: protein molecules that accelerate specific reactions inside cells

How a catalyst works

A catalyst provides an alternative reaction pathway that has a lower than the uncatalysed route
More reactant particles now have at least the (lower) activation energy at any given temperature, so a larger proportion of every collision is successful
The catalyst is not consumed because it is regenerated at the end of the alternative pathway

Exam tip

Explain how a catalyst works (2-mark question)

What comes up: "Explain how a catalyst increases the rate of reaction."

Write (two marks): (1) The catalyst provides an alternative pathway (or route) for the reaction. (2) That pathway has a lower activation energy.

Watch out: general statements such as "the catalyst speeds up the reaction" or "the catalyst is not used up" — the mark scheme ignores these as they restate the question rather than explaining the mechanism. You must name the alternative pathway and the lower activation energy to score both marks.