Positive catalyst

A positive catalyst is a catalyst that speeds a reaction up by providing an alternative reaction path with a lower activation energy. Malay: mangkin positif · Chinese: 正催化剂.

The word “positive” here means the catalyst increases the rate. It does this not by giving the particles more energy, but by opening a different route from reactants to products, one whose energy barrier is lower. Because the activation energy of this alternative path is smaller, a larger fraction of the colliding particles now has enough energy to react. The frequency of effective collisions therefore rises, and the reaction goes faster. Like any catalyst, a positive catalyst is not used up and its mass is unchanged at the end.

Example. Manganese(IV) oxide is a positive catalyst for the decomposition of hydrogen peroxide: with it added, oxygen is released much faster. On an energy profile diagram, the catalysed reaction is drawn with a lower peak than the uncatalysed one, while the reactant and product levels stay exactly the same.

Confusion to avoid. Do not say a positive catalyst gives the particles more energy or raises the temperature; it does neither. It only lowers the activation energy by offering an alternative path. Also, in SPM the everyday word “catalyst” almost always means a positive catalyst, but naming it “positive” makes the direction of the effect clear.

The positive catalyst is the type you meet most often in this chapter, and understanding that it works by lowering the activation energy links it straight to the collision theory and the energy profile diagram in SPM Chemistry. A common question asks you to explain, in collision-theory terms, why the rate rises: the lower barrier means a greater fraction of collisions is now effective, so effective collisions become more frequent even though the temperature is unchanged.