How does a catalyst speed up a reaction without being consumed?
Explain how catalysts increase reaction rate by providing an alternative pathway with a lower activation energy, and represent this on an energy profile.
How a catalyst lowers activation energy by providing an alternative reaction pathway, how this is shown on an energy profile, and why catalysts are not consumed.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
You must explain what activation energy is, how a catalyst lowers it, and represent the catalysed and uncatalysed pathways on an energy profile.
Activation energy
Only collisions with energy are successful. The higher the barrier, the smaller the fraction of particles that can react, and the slower the reaction.
How a catalyst works
A catalyst provides a different reaction pathway (mechanism) that has a lower activation energy than the uncatalysed route. With a lower barrier, a larger proportion of colliding particles have enough energy to react, so the rate increases.
Catalysts are not consumed
A catalyst takes part in the mechanism but is regenerated by the end, so the overall amount of catalyst is unchanged. This is why a tiny amount of catalyst can process a large quantity of reactant.
The energy profile
On an energy diagram (energy vs reaction progress):
- the curve rises from reactants to a peak (the activated complex / transition state) then falls to products;
- the height from reactants to the peak is ;
- adding a catalyst lowers the peak (smaller ) but leaves the reactant and product energy levels - and therefore - unchanged.
Industrial importance
Catalysts let industrial reactions run fast at lower temperatures, saving energy and cost. Examples include iron in the Haber process and vanadium(V) oxide in the Contact process.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2022 SACE Stage 23 marksPlatinum is incorporated into the electrode of a fuel cell. The activation energies for the reaction with and without platinum are marked on a diagram showing the energy distribution of reactant particles. Explain how the diagram demonstrates that platinum is a catalyst for the reaction at electrode 2.Show worked answer →
Refer to the Maxwell-Boltzmann energy distribution and the activation energy.
With platinum, the marked activation energy (Ea platinum) is lower than the activation energy without platinum (Ea), showing platinum provides an alternative reaction pathway with a lower activation energy.
On the energy distribution curve, a greater proportion of particles now have energy equal to or greater than the lower Ea (a larger area under the curve lies to the right of Ea platinum).
Therefore a greater fraction of collisions are successful, so the reaction rate increases while platinum itself is not consumed. This is the definition of a catalyst. One mark each.
2024 SACE Stage 22 marksWrite one equation for a reaction in a catalytic converter that decreases the concentration of NO in the exhaust emissions.Show worked answer →
In a catalytic converter, NO is reduced to harmless nitrogen, usually by reacting with carbon monoxide:
2NO + 2CO -> N2 + 2CO2
One mark for correct reactants and products, one mark for correct balancing. (Accept 2NO -> N2 + O2 as an alternative decomposition.)
2022 SACE Stage 22 marksModern catalytic converters contain dispersed platinum nanoparticles that act as the catalyst. Explain one possible reason why the platinum catalyst is present as nanoparticles.Show worked answer →
Platinum is a heterogeneous catalyst, so the reaction occurs on its surface.
Dividing the platinum into nanoparticles gives a very large total surface area for a small mass of metal.
More surface area means more active sites are available for reactant gas molecules to adsorb and react, increasing the catalytic activity (and reducing the amount of expensive platinum needed). One mark for the large surface area, one for linking it to more reaction sites and efficiency.