Higher and foundation tier

Rates of reaction and catalysts

Catalysts are amazing! Catalysts are substances that speed up a chemical reaction without being used up. Catalysts as usually transition metals or transition metal compounds. As an example of a catalyst at work consider the following decomposition reaction of hydrogen peroxide:

hydrogen peroxide(l) water(l) + oxygen(g)
2H2O2(l) 2H2O(g) + O2(g)

Hydrogen peroxide is a corrosive, colourless liquid. Its main uses are as bleach in the paper and textile industries and also as a disinfectant, although it can also be used as an oxidising agent and also as a propellant e.g. in submarine torpedoes. According to the equation above hydrogen peroxide decomposes to form water and also oxygen gas. However under normal conditions hydrogen peroxide is stable and you will be waiting a long long time to collect any oxygen gas or water from this reaction!

If the science lab technician had to prepare a class set of test-tubes full of oxygen gas for say a year 8 science lesson, they would use the decomposition reaction of hydrogen peroxide to prepare the test-tubes. So how would they do it? The lab technician would set-up the apparatus below:

apparatus to show decomposition of hydrogen peroxide using a catalyst

You may notice that in the conical flask is a dilute solution of hydrogen peroxide, it has been diluted in water, and one spatulas of a solid compound called manganese dioxide has been added. Manganese dioxide is a catalyst for the decomposition of hydrogen peroxide. The hydrogen peroxide will decompose violently and rapidly in the presence of manganese dioxide to form oxygen gas and water.

Once the lab technician has filled a class set full of test-tubes for use by students, it would be possible to filter out the manganese dioxide catalyst, remember catalyst do NOT get used up. It would also be possible to re-use the catalyst. It is worth mentioning that since the catalyst is not used up in the reaction only small amounts of it are needed.

Hydrogen peroxide is produced in the body as a by-product of cell activity. Since it is a corrosive substance it could kill the cells if its concentration rose to high levels. In the body there are enzymes, one of them, catalase will decompose hydrogen peroxide and quickly remove it from the cell. Enzyme are often called biological catalysts

How do catalyst work?

Before particles can react with each other they need to collide with enough energy to break the bonds in the reactants, this is called the activation energy. Catalyst are surface active agents, what this means is that reactions involving them take place on the surface of the metal catalyst. The reactants adsorb onto the surface of the catalyst and are altered or changed in some way or perhaps aligned in a fashion that allows them to react in a way that requires less activation energy from the reaction without a catalyst.
The catalyst basically gets from reactants to products via a route that requires less activation energy. More particles in the reactants are likely to have this new lower activation energy and therefore can react successful, so the rate of reaction will increase.

The energy profile diagram below is for an exothermic reaction. It shows how a catalyst speeds up a reaction by lowering the activation energy.

energy profile diagram for a catalysed reaction

Key points

Practice questions

Check your understanding - Questions on rates and catalysts