activation energy
/ ack-tih-VAY-shun energy /
Even a reaction that releases energy overall often refuses to start on its own. A pile of paper next to oxygen does not burst into flame; you need a match. Activation energy is the reason: it is the minimum energy hump that reacting molecules must climb over before they can turn into products, the energy cost of getting to the transition state at the top of the hill.
Quantitatively, the activation energy (often written Ea) is the energy gap between the reactants and the highest transition state on the path. Molecules are always jostling with a spread of energies; only those that happen to have enough energy, and collide in the right orientation, can make it over the barrier. That is why heating a reaction speeds it up: warmer molecules move faster and a larger fraction clear the hump. A high barrier means a slow reaction; a low barrier means a fast one. Crucially, activation energy is about how fast (kinetics), and is completely separate from how favorable the reaction is overall (thermodynamics, the difference between reactant and product energies).
Activation energy is the lever that controls reaction rates and the target that catalysts aim at. A catalyst speeds a reaction by providing an alternative pathway with a lower barrier, so more molecules can clear it at a given temperature, without altering the energies of the reactants, products, or the equilibrium position. This is why a small amount of catalyst, or a modest temperature increase, can dramatically change how fast a reaction runs while leaving its final yield at equilibrium untouched.
Hydrogen and oxygen can sit together unreacted for years because the activation energy for forming water is high; a spark supplies that energy and the reaction then races to completion.
A favorable reaction can still be slow if the activation barrier is tall; the spark gets molecules over the hump.
Activation energy governs speed (kinetics), not whether a reaction is favorable (thermodynamics); a strongly exothermic reaction can be immeasurably slow if its barrier is high.