Activation Energy
Definition
The energy hump reactants must get over to become products, like pushing a boulder over a hill before it rolls down. Catalysts lower this hump.
Symbol: · Unit: kJ/mol
Technical definition
Activation energy is the minimum energy barrier separating reactants from products along the reaction coordinate, corresponding to the transition state. It determines the rate constant through the Arrhenius equation but does not affect the enthalpy change or the equilibrium position.
Example
A spark provides the activation energy to start gasoline burning. Once over the hump, the exothermic reaction sustains itself.
Activation energy is not the reaction's energy change. A very exothermic reaction can still be extremely slow if its activation barrier is high, like diamond turning to graphite.