A catalyst is a substance that increases the rate of a reaction but is
chemically unchanged at the end of the reaction. Learn that definition
exactly — both halves get tested. Because catalysts are not consumed, a
small amount of catalyst can process a huge amount of reactant, and each
reaction has its own preferred catalyst: different reactions need different
catalysts.
The chapter's working example is the decomposition of
hydrogen-peroxide solution. Hydrogen peroxide is unstable and slowly
breaks down into water and oxygen:
2H₂O₂(aq) → 2H₂O(l) + O₂(g)
Left alone, it decomposes so slowly that only a trace of bubbles appears.
Add a pinch of black manganese(IV) oxide powder — MnO₂, also called
manganese dioxide — and oxygen streams off immediately. Lead(IV) oxide,
PbO₂, also catalyses this decomposition. Manganese(IV) oxide is what is
normally used in the lab for this reaction.
Notice what the equation does not contain: the catalyst. Catalysts are
never written into the equation, because they are chemically unchanged at
the end. If you want to show one, write its name or formula over the top
of the arrow.
Bombardier beetles run this chemistry for a living. The beetle defends
itself by spraying a hot, unpleasant liquid at attackers: it pumps hydrogen
peroxide into a chamber with the enzyme catalase, which decomposes it
into water and oxygen — almost explosively, releasing a lot of heat. An
enzyme is a biological catalyst, and the same catalase can be found in
potatoes and in liver tissue: drop a slice of either into hydrogen peroxide
and it fizzes.
Proving "chemically unchanged". Looking the same proves nothing — the
powder could have lost mass. The test is by weighing: weigh the
manganese(IV) oxide before adding it to the hydrogen peroxide, then
separate, dry and reweigh it at the end. Filter the mixture through a
weighed filter paper, allow the paper and residue to dry, and reweigh.
The mass of manganese(IV) oxide you recover equals the mass you started
with. Speeding up and unchanged: both halves of the definition
demonstrated.
Practical: comparing solid catalysts (spec 3.16). Measure how the
volume of oxygen produced changes with time, using a gas syringe on a
sealed flask:
- Measure 100 cm³ of 2 vol hydrogen peroxide and transfer it to a 250 cm³
conical flask.
- Weigh out 0.20 g of manganese(IV) oxide on a balance.
- Add the manganese(IV) oxide to the hydrogen peroxide and quickly replace
the bung, with the gas-syringe already attached. Swirl the reaction
mixture at a constant speed.
- Record the volume of oxygen produced every 20 seconds for 3 minutes and
plot volume of oxygen against time.
- Repeat with 0.20 g of lead(IV) oxide and 0.20 g of copper(II) oxide,
keeping everything else the same.
The results show manganese(IV) oxide is a very effective catalyst for
this decomposition — the volume–time curve climbs steeply to its final
level. Lead(IV) oxide is less effective: the same final volume of
oxygen arrives, but the curve is much shallower. Copper(II) oxide is not
a catalyst for this reaction at all — its curve barely leaves the axis.
Same equation, three very different helpers: catalytic activity is
substance-specific.
The decomposition itself is exothermic — the products (water and oxygen)
have lower energy than the reactants — so the flask warms slightly as the
oxygen pours off.
Key terms in this section
- catalyst
- — A substance that increases the rate of a reaction but is chemically unchanged at the end. It provides an alternative pathway with lower activation energy and is never written among the reactants or products.
- enzyme
- — A biological catalyst that speeds up reactions in living things. Catalase decomposes hydrogen peroxide to water and oxygen; the same enzyme occurs in potatoes and liver.
- hydrogen peroxide
- — H₂O₂, a compound that slowly decomposes into water and oxygen. Its catalysed decomposition — 2H₂O₂ → 2H₂O + O₂ — is the chapter's catalyst demonstration reaction.
- gas syringe
- — A sealed syringe attached to a reaction flask to measure the volume of gas produced as it forms — the standard apparatus for rate experiments that follow gas volume.