Chemguide: Support for CIE A level Chemistry

Learning outcome 8.3

Reaction kinetics

Homogeneous and heterogeneous catalysts

These statements are about catalysts and the way they affect rates of reaction.

Before you go on, you should find and read the statements in your copy of the syllabus.

Statements 8.3.1(a), 8.3.1(b) and 8.3.1(c)

You will find most of this explained on the page about the effect of catalysts on rates of reaction.

Although there isn't a specific statement about this, the title of the section talks about the terms homogeneous and heterogeneous as applied to catalysts. That isn't covered in the page you have just read, but you need to know the meaning of the terms.

Before you can understand these terms, you need to have met the term "phase". You should read the first part of the page types of catalysis. Just read the bit about "What is a phase?". The rest of the page involves examples which are more complicated than you need for the moment.

You need to understand that, although we often use the word as if it was the same as physical state, it could also be any other case where you can see a boundary between two (or more) parts of the system.

  • A homogeneous catalyst is one which is in the same phase as the reactants. For example a gas catalysing a gas reaction, or a liquid catalysing a liquid reaction where the whole mixture was just a single liquid.

  • A heterogeneous catalyst is one which is in a different phase from the reactants. For example a solid catalysing a gas reaction or a liquid reaction.

There are two other words that aren't specifically used on the Chemguide page.

Catalysis is just the process of speeding up a reaction using a catalyst. For example, using iron in the manufacture of ammonia is an example of catalysis.

A mechanism is a detailed step-by-step description of how a reaction happens. In the Chemguide page, I talk about a catalysed reaction "going by an alternative route". I could equally have said "going by an alternative mechanism". You will meet various mechanisms for reactions later in the course.

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© Jim Clark 2020