Chemists say an uncatalyzed reaction is slow because its activation energy is high. What does activation energy actually measure?

Core Concept

Activation energy is the height of the energy barrier separating reactants from the transition state, the unstable arrangement in which old bonds are partially broken and new ones partially formed. Only collisions carrying at least this much energy can proceed to products, so a taller barrier means a smaller successful fraction of collisions and a slower reaction. It is not the overall energy released or absorbed; that difference between reactants and products is separate thermodynamic bookkeeping.