Energy of activation

The energy of activation is the quantity of energy necessary to initiate a chemical process, generally a reaction. Indeed, to start a process, one must often face an energy barrier (i.e. to bring a minimum of energy for starting). This quantity is expressed, according to the international system of units, in kJ/mol (kilojoules by mole of reagent).

A simplified explanation of this barrier of energy can be given by the following mechanism.

So that two molecules can react, it is necessary that their electronic clouds can come into contact. For that, it is necessary for them to overcome their electric repulsion. This requires that they run up with a sufficient dash. This kinetic energy constitutes the energy of activation (representing energy coarsely required to pass the " barrier of énergie" to cross in first part of the way on the diagram of the reaction).

This energy of activation can be decreased, by using a Catalyseur.

Two sources of the energy of activation

External

Says itself of an energy of activation which is external with the combustive/combustible contact.

Example: I have paper in the air and it is necessary me to bring energy (a flame for example) so that combustion starts.

Induced

Says itself of an energy of activation which is induced by the combustive/combustible contact.

Example: I have a Hydrocarbure (gasoline) and I put it in contact with the nitric acid concentrated (combustive). The contact of the one with the other generates an heat emission (energy) which is sufficiently important to start combustion (explosion).

It is the principle of the incompatible reactions.

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