Chemguide: Support for CIE A level Chemistry


Learning outcomes 8.3(a), 8.3(b) and 8.3(c)

These statements are an introduction to catalysts and how the Boltzmann distribution and activation energy relate to how catalysts work.

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


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


Statement 8.3(b) introduces the terms homogeneous and heterogeneous as applied to catalysts. That isn't covered in the page you have just read, but in another one which you mostly won't need until the second half of the course.

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.


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© Jim Clark 2011 (last modified April 2014)