Introduction:
In many chemical reactions a catalyst is needed in order to get the reaction going. When MnO₂ is added to hydrogen peroxide, a very exothermic and rapid chemical reaction takes place. This reaction liberates water, oxygen and heat, as shown below.

Without the MnO₂ catalyst, this reaction does not occur at an appreciable rate. This is a very visual reaction that is carried out in a two liter plastic bottle.

Another very visual reaction is one involving the combustion of methanol vapor/ air. This reaction again liberates water, but as the reaction below shows, carbon dioxide is also given off. Without a spark or flame, this reaction does not go because of the high activation energy.

These two reactions can be discussed in terms of the activation energy and its effect on the rate of a chemical reaction. This concept can be related to many practical observations such as catalytic converters in automobiles exhaust systems and the fact that most things around us ate unstable with respect to combustion. In order to get these reactions started, either some energy or a catalyst is required.