Sciences

Catalyst: What It Changes and What It Leaves Alone

A catalyst speeds a reaction by offering a route with lower activation energy, and is not used up. It changes how long the reaction takes, not the yield.

Catalyst

A catalyst is a substance that speeds up a chemical reaction by lowering its activation energy and is not used up or chemically changed by the reaction.

Aussi appelé: Enzyme, for a biological catalyst
Où les élèves le rencontrent: Grade 9 chemistry, in the unit on rates of reaction, and in Grade 9 biology where enzymes are introduced as biological catalysts made of protein.

La réponse en bref
A catalyst is a substance that speeds up a chemical reaction by providing an alternative route with a lower activation energy, and is chemically unchanged at the end. Because it is not used up, a small amount keeps working indefinitely. Enzymes are the catalysts that living cells use, and they work the same way.

Un exemple

2H₂O₂ → 2H₂O + O₂ (manganese(IV) oxide above the arrow)

Hydrogen peroxide decomposes into water and oxygen on its own, but so slowly that a bottle of it keeps for months. Add a spatula tip of black manganese(IV) oxide and it froths violently. Filter the mixture afterwards, dry the solid and weigh it: you get back what you put in. The same reaction happens in your own cells, where the enzyme catalase does the job instead.

What a lower activation energy means

For a collision between two particles to produce a reaction, they must meet with at least a certain amount of energy. That minimum is the activation energy, and on a reaction profile it is the hump between reactants and products. Most collisions fall short of it, which is why most reactions are slower than the collision rate alone would suggest.

A catalyst does not push particles harder. It offers a different route to the same destination, one with a smaller hump, so a much larger share of ordinary collisions now carries enough energy to get over it. On the profile the peak drops. The two ends do not move: the reactants start at the same level and the products finish at the same level, so the overall energy change is untouched.

Not used up, but not indestructible

Because a catalyst is regenerated in every cycle, the mass you weigh out at the start is the mass you recover at the end, and a tiny quantity can process an enormous amount of reactant. This is also why the catalyst is written above the arrow rather than in the equation, and why it never appears in a mole calculation.

Two honest limits go with that. Catalysts are specific — iron speeds up the Haber process and does nothing for hydrogen peroxide — so there is no such thing as a general-purpose one. And they can be poisoned, meaning a contaminant sticks to the surface and blocks it. Lead does this to the catalyst in a car's catalytic converter, which is the reason leaded petrol had to go.

The ones you are expected to name

One point catches people out in equilibrium questions: a catalyst changes the time taken, not the amount of product. It speeds up the forward and reverse reactions equally, so the position of equilibrium stays exactly where it was. You reach the same yield sooner, and no more of it.

Iron — the Haber process, making ammonia from nitrogen and hydrogen
Vanadium(V) oxide — the Contact process, on the way to sulfuric acid
Platinum, palladium and rhodium — the catalytic converter in a car exhaust
Nickel — adding hydrogen to unsaturated vegetable oils to harden them
Manganese(IV) oxide — the decomposition of hydrogen peroxide in the school lab
Enzymes — amylase, catalase, lipase and the rest, each shaped to fit one substrate

Questions fréquentes

Is a catalyst used up in a reaction?

No. It takes part, is changed during the process and is returned to its original chemical state by the end, so it can start again. That is why the same small mass keeps working and why it can be filtered off and reused. It is not consumed in the way a reactant is.

Does a catalyst increase the amount of product?

No, only the speed. The reaction reaches the same finishing point sooner. In a reversible reaction it speeds the forward and backward directions by the same factor, so the equilibrium yield is unchanged. If more product appears, something other than the catalyst changed too.

Are enzymes catalysts?

Yes — biological ones. Each is a protein folded into a shape with an active site that fits a particular substrate, which makes enzymes far more specific than industrial catalysts. They also work only within a narrow range of temperature and pH, because the shape that does the work is destroyed outside it.

Why is a catalyst written above the arrow?

Because everything written on the left of the arrow is used up and everything on the right is produced, and a catalyst is neither. Putting it above the arrow records that it was present and needed, without wrongly implying that some of it was consumed or that it has to be balanced.

Dernière mise à jour 21 août 2026