Catalysts

What is a catalyst and how does it increase the rate of reaction?

A catalyst is a substance that increases the rate of a chemical reaction without being consumed (used up) in the process. It works by providing an alternative reaction pathway with a lower activation energy than the uncatalysed route. Because Ea is lower, a greater proportion of collisions have sufficient energy to react, so the rate increases.

What is the difference between a homogeneous and a heterogeneous catalyst, with an example of each?

A homogeneous catalyst is in the same phase (state) as the reactants. Example: concentrated sulfuric acid (a liquid) catalysing the esterification reaction between a liquid alcohol and a liquid carboxylic acid.

A heterogeneous catalyst is in a different phase from the reactants. Example: iron (solid) used in the Haber process to catalyse the reaction between nitrogen and hydrogen gases; or nickel (solid) used in the hydrogenation of alkenes (gas/liquid).

How does a heterogeneous catalyst work at the surface level?

Reactant molecules adsorb (stick) onto the surface of the solid catalyst at active sites. This weakens the bonds within the reactant molecules, lowering the energy needed to break them — effectively reducing the activation energy. The other reactant then collides with the adsorbed species on the surface, and the reaction proceeds more easily. Once the products form, they desorb (leave the surface), freeing the active sites for more reactant molecules to adsorb. For this reason, heterogeneous catalysts are often used as fine powders or meshes to maximise the available surface area.

How do you show the effect of a catalyst on an enthalpy profile diagram?

What are the economic and environmental benefits of using catalysts?

Catalysts allow reactions to proceed at lower temperatures and pressures, which reduces the energy demand — less fuel needs to be burned, so fewer fossil fuels are consumed and less CO₂ is emitted. This improves sustainability and reduces costs. Catalysts can also improve selectivity — they favour the desired product over unwanted side products, reducing waste and improving atom economy. Although some catalysts are toxic (e.g. heavy metals), they do not need frequent replacement as they are not consumed, limiting their environmental impact.