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.
Understanding how the following variables affect the speed of a reaction is a very important activity for industrial and laboratory chemists.
The presence of a catalyst
This video shows four demonstrations that illustrate each of these variables.
Video: Rates of reactions
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
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.
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.
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