School of Chemistry, UNSW

How does a catalyst speed up a chemical reaction?

To understand how a catalyst works you need to understand what happens in a chemical reaction without any catalyst being present. In the process of converting reactant molecules into product molecules some chemical bonds in the reactants will be broken and new chemical bonds will be formed. Think about the formation of water from hydrogen and oxygen.

2H₂(g) + O₂(g) -> 2H₂O(g)

If you were to follow two hydrogen molecules and an oxygen molecule as they combined to eventually form two water molecules you would see two H-H bonds break, and an O-O bond would need to be broken, and four O-H bonds would be formed. Overall this reaction produces heat - it is exothermic. But some energy is needed to get it started by breaking some H-H and O-O bonds.

So one way to speed up a reaction would be to weaken those bonds which need to break. This means the reactant molecules would need to pick up less energy in collisions with other molecules in order to break these bonds and this will lead to a faster reaction. This is one way that a catalyst can work. By bonding to a reactant molecule and, in doing so, weakening a critical bond which must break for the reaction to proceed, the catalyst speeds up the reaction.

For example, metals such as nickel and palladium are known to function as catalysts in reactions involving hydrogen gas. What happens is that hydrogen molecules bond with the surface metal atoms. Once the metal-H₂ bonds form, the H-H bond becomes weaker so that eventually separated hydrogen atoms form on the metal surface. These can easily form bonds to other molecules which collide with the surface. Compare this with gaseous hydrogen at room temperature where the strength of the H-H bond makes it very unlikely that many H₂ molecules will break up into atoms.

In summary, a catalyst bonds with the reactants to allow the reaction to take place via a different sequence of steps, requiring less energy to start the reaction, than if the catalyst were not present.

If you would like to make any comments about this article, please e-mail the author, Dr Ron Haines.

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How does a catalyst speed up a chemical reaction?