A catalyst is a substance that:
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.
|Iron||Haber process (making ammonia)|
|Vanadium (V) oxide||Contact process (a stage in making sulfuric acid)|
|Manganese dioxide||Decomposition of hydrogen peroxide (produces water and oxygen)|
Notice that these catalysts are transition metals or compounds of transition metals.
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.
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.