Chemguide: Support for CIE A level Chemistry

Learning outcome 26: Reaction kinetics

26.2: Homogeneous and heterogeneous catalysts

Learning outcomes 26.2.1, 26.2.2 and 26.2.3

These statements are about examples of homogeneous and heterogeneous catalysts.

Read the statements in your copy of the syllabus.

Before we look at the specific examples mentioned in the syllabus, you should read the page about types of catalysis.

You can ignore the final section about autocatalysis. Don't worry about trying to remember the specific examples on this page, because some of CIE's examples are different. We will explore them below.

What matters for this first read-through is that you should:

  • Understand the difference between heterogeneous and homogeneous catalysts.

  • Understand that heterogeneous catalysts work by adsorbing the reactant molecules on to their surface, followed by reaction caused by bond weakening, and finally desorption of the products.

    The example of nickel catalysing the ethene-hydrogen reaction isn't on the syllabus, but a very similar case is. If you understand what happens when nickel catalyses the ethene-hydrogen reaction, you can apply the same principles to other similar reactions.

  • Understand that a homogeneous catalyst is changed during the first step of a reaction, but regenerated during a later step. The reaction between persulphate ions and iodide ions is a good example of this, and is included in the CIE syllabus.

Examples of heterogeneous catalysts required by CIE

Iron in the Haber Process

If you haven't come across it yet, read the very beginning of the page about the Haber Process. All you are interested in at the moment are the actual facts of the process. Ignore the main section exploring the conditions for the reaction.

So, solid iron is used to catalyse a reaction between nitrogen and hydrogen gases. The iron is a heterogeneous catalyst.

The process is very similar to ethene-hydrogen with a nickel catalyst.

The nitrogen and hydrogen are adsorbed on to the surface of the iron, and nitrogen-nitrogen and hydrogen-hydrogen bonds are broken and new ones made between the iron and the individual atoms of nitrogen and hydrogen.

Then bonds are made between the nitrogen and hydrogen to make ammonia, and the ammonia desorbs from the surface.

Catalytic converters

This is covered on the types of catalysis page. Once again, there will be a process of adsorption, making the molecules more reactive. This will be followed by reaction and desorption.

The equation given on that page is for carbon monoxide and nitrogen monoxide. If you were asked about the reaction between carbon monoxide and nitrogen dioxide, the products would be the same, but the equation would just need balancing differently.

Examples of homogeneous catalysts required by CIE

Oxides of nitrogen in the oxidation of atmospheric sulphur dioxide

Sulphur dioxide in the atmosphere is a cause of acid rain. It is oxidised to give sulphur trioxide which dissolves in rain water to give sulphuric acid.

The oxidation of the sulphur dioxide is catalysed by nitrogen dioxide (another atmospheric pollutant).

This is typical of a reaction involving a homogeneous catalyst. The catalyst reacts with one of the substances in the reaction it is catalysing in the first step, and is then regenerated in a later step.

Iron ions in the reaction between persulphate ions and iodide ions

This example is covered in detail on the page you have already read - types of catalysis.

Make sure that you understand what is going on so that you can quote the two steps of the reaction in whatever order you need them so that you can answer a particular question - depending on whether you were given iron(II) ions or iron(III) ions as the catalyst.

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© Jim Clark 2020