Catalysts

A catalyst is a substance that:

  • speeds up the rate of a reaction
  • does not alter the products of the reaction
  • is unchanged chemically and in mass at the end of the reaction

Only a very small mass of catalyst is needed to increase the rate of a reaction. However, not all reactions have suitable catalysts.

Catalysts only affect the rate of reaction - they do not affect the yield of the reaction. A catalysed reaction produces the same amount of product as an uncatalysed reaction but it produces the product at a faster rate.

Different substances catalyse different reactions. The table describes three common catalysts.

CatalystReaction catalysed
IronHaber process (making ammonia)
Vanadium (V) oxideContact process (a stage in making sulfuric acid)
Manganese dioxideDecomposition of hydrogen peroxide (produces water and oxygen)

Notice that these catalysts are transition metals or compounds of transition metals.

How catalysts work

A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because a greater proportion of collisions now exceeds this lower activation energy.

The effect of a catalyst on the activation energy is shown on a chart called a reaction profile. This shows how the energy of the reactants and products change during a reaction.

A reaction profile for a reaction with and without a catalystA reaction profile for a reaction with and without a catalyst

Enzymes

An enzyme is a biological catalyst. Enzymes are important for controlling reactions in cells. They are also important in industry. The use of enzymes allows some industrial reactions to happen at lower temperatures and pressures than traditionally needed.

Yeast is a single-celled fungus. The enzymes in yeast are used to produce wine, beer and other alcoholic drinks by fermentation of sugars.