Catalysts and Activation Energy

A catalyst speeds up a reaction without being used up. It provides a different route from reactants to products: the reactants bind to the catalyst, react there over a lower activation energy, and the product is released, leaving the catalyst free to be used again. The catalyst may be a solid surface, such as a metal, or an enzyme. First compare the same reaction without and with a catalyst, then step through how the catalyst works, and finally test your knowledge. Work through the three stages in order.

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Catalyst:

Temperature

Catalyst

Remember

A catalyst provides an alternative route with a lower activation energy, E_a. More collisions then have enough energy to react, so the rate increases.
The catalyst is not used up: it is released unchanged at the end of each cycle and can be used again.
A catalyst does not change the energies of the reactants or the products, so ΔH (and ΔG) of the reaction stay the same. It cannot make a non-spontaneous reaction go; it only makes a possible reaction faster.
Enzymes are biological catalysts. The reactants (substrates) bind at the active site, whose shape fits only particular molecules, so enzymes are highly specific. Most unfold (denature) above about 45–50 °C and stop working.
Solid catalysts, such as palladium or nickel, work at their surface, so a finely divided catalyst with a large surface area is more effective. They tolerate high temperatures but can be poisoned by impurities, such as sulfur compounds, that bind strongly to the surface.
Catalysts can also be dissolved molecules or ions, such as acids or metal complexes; they work in the same way, by providing a route with a lower activation energy.
In the boxes the barriers are made much lower than in a real reaction, so that reactions happen while you watch.