What is a Catalyst ?

Definition of a catalyst:

A catalyst is a substance, meaning an element or compound, that changes (increases) the rate of a chemical reaction, but is not used-up* by the reaction.

* When the reaction has stopped the mass of the catalyst is the same as before the reaction started, even though the mass of reactants (starting chemicals) is reduced, and the mass of products (formed by the reaction) increased.

  • The process by which a catalyst changes the rate of a reaction is called catalysis.
  • Relatively small quantities of catalysts can catalyze relatively large masses of reactants. This is possible because the catalyst is not used-up by the reaction and so the small quantity of catalyst can be used over and over again to catalyze the conversion of more reactants into reaction products.
  • Catalysts are usually specific to particular reactions. That is, a chemical that works as a catalyst for one reaction will not necessarily catalyze a different reaction, not even if the reactants are similar or one of the reactants is common to both reactions.

Do any catalysts slow-down (rather than speed-up) reactions ?

A chemical used in the same way as a catalyst but to slow-down a reaction is called an inhibitor.
Inhibitors are used in certain medical and pharmaceutical applications.

How do catalysts work ? (Briefly)

Chemical reactions occur when chemical bonds change (that is, some chemical bonds are broken and other chemical bonds are formed) when atoms, ions or molecules collide with each other and interact.

One way to increase the rate of a chemical reaction, i.e. to speed-up the reaction, is to increase the frequency of collisions between reactants e.g. by increasing the concentration of the reactants and increasing the temperature. For more about this read about simple collision theory.


  • Catalysts do not increase the frequency of collisions.
  • They increase the speed of reactions by increasing the success rate of the collisions.

Catalysts can increase the success rate of collisions by influencing reactants to encounter each other at ideal orientations. This enables collisions between reactants with less kinetic energy than would otherwise be necessary for a reaction to occur to result in chemical changes (chemical bones broken and new bonds formed). Therefore, a larger number of individual chemical changes takes place per unit of time, i.e. the overall rate of the reaction is increased.

More about how catalysts work:

Another way to express the above is in terms of the energy needed for chemical reactions to happen. Chemical reactions need a certain amount of energy in order to start. This is called the 'activation energy' of the reaction and is different for different reactions.

A catalyst is not always needed in order for a chemical reaction to occur.
Some reactions have a relatively high 'activation energy' and so might not happen at all under certain physical conditions, that is - not unless additional energy were supplied by heating, applying pressure to gasses, or in other ways appropriate for that particular reaction. However, the presence of a catalyst for such a reaction whose 'activation energy' is relatively high can reduce the activation energy for the reaction, increasing the possibility of the reaction occurring and increasing the efficiency of it.

Catalysts generally work by either changing the structure of one or more of the reactants or by bonding to reactants in such a way as to cause them to combine, react, and release a product, products or energy. Simple descriptions of catalysts sometimes state that they 'do not participate' in the reaction. However, it is more accurate to state that catalysts 'are not used-up by the reaction', or 'are not consumed by the reaction'. This is because they may take part in a stage or stages of the reaction process, after which they are released in their original form or in parts that revert back to their original form. Further detail about the role played by catalysts in reactions vary with the specific reaction and catalyst.

Where are catalysts used ?

    • Biochemical reactions, i.e. chemical reactions that occur within biological systems e.g. humans and other animals.
      The most common catalysts in biological systems are enzymes, which are specialized proteins that accelerate specific chemical reactions e.g. the enzyme salivary amalyse is present in human saliva and aids the digestive process by breaking starch down into maltose and dextrin.
    • Scientific laboratories
    • Manufacturing and industrial processes, e.g. the Haber process in which ammonia (NH3) is formed from hydrogen (H2) and nitrogen (N2) gases.
    • Automobiles, a common example being the use of catalysts in the catalytic converters used in modern cars to increase the efficiency of fuel consumption.

Examples of catalysts

Catalysts are specific to particular reactions.
The following examples are often included in school chemistry.

  • The Haber Process
    ( Catalyst: Iron )

  • Fermentation of glucose to form ethanol
    ( Catalyst: Specific enzymes in yeast )

  • Hydration of ethene to form ethanol
    ( Catalyst: Phosphoric Acid )

  • Hydrogenation of unsaturated fats (to harden oils in the manufacture of margarine)
    ( Catalyst: Nickel, Ni )

Further examples are included on the pages about the role of enzymes in the human digestive system.

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