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.
- Energy here means free energy rather than normal energy