The activation energy in chemistry
is the energy needed by a
system to initiate a particular process. Activation energy is often used to denote
the minimum energy needed for a specific chemical
reaction to occur. For a reaction to occur between two colliding molecules
they must collide in the correct orientation and possess a certain minimum amount
of energy. As the molecules approach their electron clouds repel. This requires
energy - activation energy - and comes from the heat
of the system i.e., the translational, vibrational etc... energy of each molecule.
If there is enough energy available, this repulsion is overcome and the molecules
get close enough for attractions between the molecules to cause a rearrangement
of bonds. The Arrheniusequation
gives the quantitative basis of the relationship between the activation energy
and the rate at which a reaction proceeds. The study of reaction rates is termed
chemical kinetics.
The transition state in a reaction is the point at which the original
bonds have stretched to their limit. Transition states are only in existence
for extremely brief (10-15 sec)
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periods of time. The energy required to reach the transition state is equal
to the activation for that reaction. Multi-stage reactions involve a number
of transition points, here the activation energy is equal to the one requiring
the most energy. After this time either the molecules move apart again with
original bonds reforming, or the bonds break and new products form. This is
possible because both possibilities result in the release of energy (shown on
the enthalpy profile diagram, Fig. 1, as both positions lie below the
transition state). A substance that modifies the transition state to lower the
activation energy is termed a catalyst;
a biological catalyst
is termed an enzyme.
Fig 1. The enthalpy profile of a reaction between two hydrogen peroxide molecules to form water and oxygen. The Transition State is the point at which the original bonds have been stretched as far as they can without breaking (this is illustrated by the structural diagram below the graph.) The energy required to reach the transition state is equal to the activation energy of the reaction.
At low temperatures for a particular reaction most, but not all, molecules will not have enough energy to react. However there will nearly always be a certain number with enough energy at any temperature, because temperature is a measure of the average energy of the system - individual molecules can have more or less energy than the average. Increasing the temperature increases the proportion of molecules with more energy than the activation energy, and consequently the rate of reaction increases. Typically the activation energy is given as the energy in kilojoules needed for one mole of reactants to react.
