Activation energy
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
The minimum energy a colliding set of reactants must possess for a reaction to proceed.
Activation energy in plain English
Even reactions that 'want' to happen usually need a shove to get started — that shove is the activation energy. It's the minimum jolt of energy the reactants need when they collide so they can break their old bonds and rearrange into something new. Think of a match: the chemicals are eager to burn and release energy, but they just sit there until you strike them and give that first burst of heat. That initial burst you have to supply is the activation energy, and lowering it (which is what catalysts do) makes reactions go faster.
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