The activation energy is a concept in chemistry that plays an important role in reactions. So, to discuss what activation energy is, we'll first take a look at reactions.

In a chemical reaction, one or more molecules (the reactants) are transformed into other molecules (the product). These reactions often do not occur spontaneously: you generally do not see a bushel of wood just turn into carbon dioxide, water and ashes, which is energetically1 favorable, and thus something Nature wants to occur. So, something must be inhibiting the reaction. This "something" is the activation energy.

In order to transform the molecules, an intermediate state has to be formed. This intermediate state is not favorable energetically: this is why it is not the normal state in which the molecules occur. Think of it as an energy mountain, which the molecules have to climb before they can slide off on the other side. This mountain is called the activation energy, and it is this energy that determines how fast a reaction goes. Note that the size of the activation energy has little bearing on the net reaction needed for the reaction, because the decay of the intermediate state into the products releases energy again.

All matter contains a certain amount of thermal energy, which is proportional with temperature. Thermal energy is statistically distributed over the particles in this matter, with some having more than average, and others less than the average amount. Well above the average amount, the amount of particles with a certain energy drops exponentially as a function of increasing energy. Hence, the amount of particles which can trigger a reaction, because their energy is greater than the activation energy, increases exponentially with temperature. This means that heating greatly increases reaction speed, which is something that is used extensively.

A second way of increasing reaction speed is using catalysis. A catalyst is a substance that is not consumed in a reaction, but does decrease the activation energy, increasing reaction speed.

In summary, the activation energy can be thought of as the energy that needs to be spend initially to get a reaction going. It has no bearing on the net reaction energy, but it does have a huge impact on reaction speed.

  1. Energy here means free energy rather than normal energy

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