Chemguide: Support for CIE A level Chemistry

Learning outcome 8.1(a)

This statement is about the use of the term rate or reaction.

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

Important background work

You should read the first half of the page about collision theory. This is an important introduction to the topic and you will refer back to this page again in later statements in this section.

And, although it is very trivial and you will certainly have met this previously, have a quick look at the page about the effect of surface area on reaction rates. This isn't mentioned by the syllabus, but you will find it crops up now and then throughout the Chemguide pages you will look at later.

Rate of reaction

There are all sorts of ways of measuring a rate of reaction. For example, in a reaction which gives off a gas you could measure it in terms of the volume of gas being given off per second. Obviously this (and any other measure of rate of reaction) is likely to change during a reaction as the reactants get used up and the reaction slows down.

In a reaction in which a reasonably dense gas is given off, and allowed to escape, you could also measure it in terms of how fast the mass of the mixture fell (in grams per second) as the reaction was happening.

But the standard way of measuring rates of reaction is in terms of the rate of fall in concentration of one of the reactants.

Concentration is measured in moles per cubic decimetre (moles per litre). If the concentration of one of the reactants is falling by, say, 0.01 moles per cubic decimetre every second at some point in a reaction, then the rate of reaction at that point would be expressed as:

Rate = 0.01 mol dm-3 s-1

. . . which you read as 0.01 moles per cubic decimetre per second.

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© Jim Clark 2011 (last modified April 2014)