Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide into water and oxygen: 2H2O2(aq) → 2H2O(l) + O2(g); the catalyst speeds up the reaction without being used up.
Hydrogen peroxide decomposes slowly on its own into water and oxygen, but the reaction is very slow at room temperature. Adding manganese(IV) oxide as a catalyst makes it fast enough to study easily, which is why it is the standard experiment for showing how a catalyst affects the rate of reaction in the Rate of Reaction chapter of 4541.
The balanced equation
Hydrogen peroxide decomposes into water and oxygen. The manganese(IV) oxide is written above the arrow because it is a catalyst, not a reactant. With correct formulae and state symbols:
2H2O2(aq) → 2H2O(l) + O2(g)
Two molecules of hydrogen peroxide give two molecules of water and one molecule of oxygen gas. The manganese(IV) oxide (MnO2) takes part by providing a surface that lowers the activation energy, but it is recovered unchanged at the end, so it does not appear in the balanced equation as a reactant or product.
Conditions required
The reaction is carried out at room temperature; no heating is needed once the catalyst is added. A measured volume of hydrogen peroxide solution is placed in a flask, and a known mass of manganese(IV) oxide powder is added to start the fast reaction. Other substances such as fresh liver (which contains the enzyme catalase) can also catalyse the same decomposition, and the syllabus sometimes uses this biological example.
What you observe
As soon as the catalyst is added, you see brisk effervescence, bubbles of a colourless gas, oxygen, rising through the solution, and the flask feels slightly warm because the reaction is exothermic. The gas is tested by lowering a glowing wooden splint into the mouth of the tube: the splint relights, confirming oxygen. At the end, the black manganese(IV) oxide powder is still present; when filtered, dried and weighed, its mass is unchanged, proving it acted as a catalyst.
Where it appears in the SPM exam
In 4541/1 you identify oxygen from the glowing-splint test or recognise the catalyst. In 4541/2 you write the balanced equation, explain that the catalyst speeds up the reaction by providing an alternative path with a lower activation energy, and state that the catalyst is unchanged in mass and chemical composition. In the practical paper 4541/3, this is a classic rate experiment: you collect the oxygen over time and plot volume against time, comparing the gradient with and without the catalyst.
How we teach it
Our teachers make sure you say exactly why a catalyst works, it lowers the activation energy, providing an alternative pathway, without being consumed, and that you give the oxygen test correctly as a glowing splint that relights, never a lighted splint with a “pop”, which is the hydrogen test. Students most often lose marks by writing manganese(IV) oxide as a reactant, or by mixing up the oxygen and hydrogen tests. Keeping the catalyst idea and the gas test precise makes this an easy source of marks.
Quick summary
Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide into water and oxygen, speeding it up without being used up. Learn the balanced equation with the catalyst over the arrow, the effervescence and glowing-splint test for oxygen, the proof that the catalyst is unchanged, and the activation-energy explanation, and this reliable rate topic is fully covered.
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