Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Did you know?

We also have Bitesize study guides covering many subjects at National 4 and National 5 on our Knowledge & Learning BETA website.

Home > Chemistry > Elements and reactions > Speed of reactions


Speed of reactions


This topic also builds on the work you have already done in S1 and S2. There are some simple terms and definitions that you need to remember.

Measuring the speed of a reaction

Understanding how the following variables affect the speed of a reaction is a very important activity for industrial and laboratory chemists.

  • Particle size

  • Concentration

  • Temperature

  • The presence of a catalyst

This video shows four demonstrations that illustrate each of these variables.

Video: Rates of reactions

In order to see this content you need to have both Javascript enabled and Flash installed.

Particle size

The smaller the particle size, the faster the reaction.

This means that powders always react faster than lumps. This video clip has demonstrations with charcoal, gunpowder and steel wool that show the effect of particle size and concentration on reaction rate.

Video: Particle size and concentration

In order to see this content you need to have both Javascript enabled and Flash installed.


Another common example is dust explosions in flour mills. Flour is a high-energy foodstuff which can burn if ignited. The Health and Safety Executive are always worried about very dry, hot days in these mills, as any small spark may cause an explosion between flour and oxygen. This is an example of a very, very fast reaction.


The higher the concentration of reactants, the faster the reaction.

This tells us that if two substances are to be reacted quickly, then high concentrations should be used. The reaction of dynamite is trinitrotoluene (TNT for short) with oxygen is:

TNT + oxygen→nitrogen + carbon dioxide + water

TNT can be burned slowly without great danger. An ignition spark will cause a fast reaction between the compound and the oxygen already attached.


  • man running in a cross country race

    Athletes need food and oxygen to get energy from respiration reactions. During a hard run, poisonous lactic acid builds up, causing tiredness and cramp.

    Oxygen is needed to remove lactic acid. After a hard run, pure oxygen is often given.


BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.