Catalyst – IGCSE Chemistry Definition

A catalyst speeds up a chemical reaction without being consumed. It is one of the five factors affecting rate of reaction on the 0620 syllabus and appears in questions about energy level diagrams, industrial processes, and experimental design. The precise definition carries marks on every paper type.

The 0620 definition

A catalyst is a substance that increases the rate of a chemical reaction without itself being chemically changed at the end of the reaction. It provides an alternative reaction pathway with a lower activation energy.

How catalysts work

1. The catalyst provides an alternative pathway for the reaction
2. This alternative pathway has a lower activation energy
3. More particles have enough energy to react (more collisions exceed the lower energy barrier)
4. The rate of reaction increases
5. The catalyst is not used up — it can be recovered unchanged at the end

Key points about catalysts

Statement	True or false
Catalysts speed up reactions	True
Catalysts are used up	False — they are unchanged at the end
Catalysts lower activation energy	True
Catalysts change the products	False — same products, same energy change
Catalysts increase the yield of product	False — same yield, just reached faster
Catalysts are specific	True — each catalyst works for specific reactions

Important catalysts in the 0620 syllabus

Reaction	Catalyst
Decomposition of H₂O₂	Manganese(IV) oxide, MnO₂
Haber process (N₂ + H₂ → NH₃)	Iron
Contact process (SO₂ → SO₃)	Vanadium(V) oxide, V₂O₅
Hydrogenation of oils	Nickel
Catalytic cracking of hydrocarbons	Aluminium oxide / silicon dioxide (zeolite)

Biological catalysts

Enzymes are biological catalysts — proteins that catalyse reactions in living organisms. They are specific to one reaction and work best at a particular temperature and pH.

Worked exam question

Hydrogen peroxide decomposes to form water and oxygen. The reaction is slow at room temperature. (a) Name a catalyst for this reaction. (1) (b) Define the term catalyst. (2) (c) Explain, in terms of activation energy, how the catalyst increases the rate. (2)

Mark scheme

(a) Manganese(IV) oxide / MnO₂ [1]

(b) A substance that increases the rate of a chemical reaction [1]; without being (chemically) changed / used up at the end of the reaction [1]

(c) The catalyst provides an alternative (reaction) pathway [1]; with lower activation energy, so more particles have enough energy to react / more successful collisions per unit time [1]

Common exam mistakes

• Defining a catalyst as something that “speeds up a reaction” without adding “without being used up”. Both parts are needed for full marks.
• Saying a catalyst “lowers the energy of the reaction”. It lowers the activation energy, not the overall energy change.
• Claiming a catalyst increases the amount of product. A catalyst increases the rate — the reaction reaches completion faster, but the same amount of product forms in the end.