Catalysts

A catalyst speeds up a reaction without being used up. It is not one of the reactants, so it isn't written in the overall equation (it may be written over the arrow).

Catalysts work by providing a different reaction pathway with a lower activation energy. The particles collide just as often, but a greater proportion of the collisions now have enough energy to be successful, so the rate increases.

On a reaction profile, a catalyst lowers the peak, but the energy of the reactants and products, and so the overall energy change, stays the same.

Different reactions need different catalysts. Many are transition metals or their compounds:

• iron in the Haber process for making ammonia
• manganese(IV) oxide to decompose hydrogen peroxide into water and oxygen
• nickel to add hydrogen to vegetable oils

Enzymes are biological catalysts. They are proteins that speed up reactions in living things, such as the enzymes in yeast that ferment sugar into ethanol.

Catalysts save energy and money in industry, because reactions can run fast enough at lower temperatures. Because they are not used up, only small amounts are needed.

Manganese(IV) oxide speeds up the decomposition of hydrogen peroxide into water and oxygen. To show it is a catalyst:

1. Weigh some manganese(IV) oxide.
2. Add it to hydrogen peroxide and measure the volume of oxygen given off over time. Compare this with the reaction without it.
3. When the reaction stops, filter off the solid, dry it and weigh it again.

The oxygen is given off much faster, and the mass of the solid is unchanged: it speeds up the reaction without being used up. You can compare other solids in the same way to find the best catalyst.

A reaction profile shows the energy of the reactants and products, with a hump between them. The height of the hump above the reactants is the activation energy.

With a catalyst, the hump is lower, but:

• the reactants start at the same energy
• the products end at the same energy
• so the overall energy change is the same