Collisions and reactions


It is hard to imagine that during a bush fire it is not the wood that burns but the smoke. Heat causes the wood to produce smoke which ignites in flames causing more smoke to form.


We have always thought of fires as wood burning but shown on the right is a video of how the smoke ignites and not the wood.

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Consider the reaction shown on the left. It looks like all reactant particles will react to form the product. But this is not true, if the reactant particles are not travelling fast enough then molecules simply bounce off each other when they collide and no reaction is possible. For a reaction to occur, molecules must collide with enough force so as to break apart.

The individual atoms then recombine to form a totally new product. This new product is usually very strong in holding its atoms together. Some times a little heat is supplied to get the molecules traveling at speeds high enough to cause energetic collisions.

This heat is called activation energy. Petrol and oxygen for example can mix freely with each other without reacting. When activation energy is provided, in the form of a spark, the reaction proceeds with a huge release of energy and force. The spark has caused molecules of oxygen and petrol to increase in speed and collide with a force big enough to break the molecules apart. This reaction releases heat and fuels more collisions and the reaction soon becomes explosive.

Hydrogen and oxygen are used to fuel the Space Shuttle. They burn to produce a great deal of heat, as shown on the left. Air contains huge amounts of oxygen and hydrogen but lighting a match in the open does not cause a huge explosion. Explain WHY?

Candle wax does not burn but it will melt. However the wax vapour will ignite. Have a look at the 120kb movie and explain why wax vapour ignites while solid wax does not burn.

Particles of a particular substance have more energy when they form a vapour than when they are in the solid or liquid state. Greater energy means more forceful collisions which are the basis of chemical reactions.

Will the smoke from burning wood that has just been extinguished ignite? Click to see a 120kb video and explain why?

Air bags use an explosive mixture to create gas. This gas quickly fills a bag which cushions the driver or passenger from impact with the metal frame of the vehicle. a
What must particles do before a reaction can take place?
What is more combustible a gas, solid or liquid?
What is activation energy?
Why is it that wax vapour ignites but solid or liquid wax does not?
Click to see a video of wood vapour burning. What must be in the vapour?