Activation Energy
Activation energy is the minimum energy input required to start a chemical reaction — the height of the energy barrier between reactants and products. Most thermodynamically favorable reactions (products have lower energy than reactants) still do not proceed at room temperature because few molecules have enough energy to cross this barrier spontaneously.
The barrier exists because reactants must pass through a high-energy transition state before rearranging into products. Molecules that lack sufficient kinetic energy bounce off each other rather than reacting.
Catalysis as Barrier Reduction
Enzymes (biological catalysts) lower the activation energy barrier without changing the reaction's thermodynamics — the start and end energies remain the same, but the hill between them shrinks. Enzymes achieve this by:
- Physically binding substrates and straining their bonds
- Orienting reactants optimally in the active site
- Providing an alternative reaction pathway with a lower-energy transition state
The result: reactions that would take hours or years at body temperature proceed in milliseconds. Life is possible because enzymes make chemistry fast.
Connections
- enzyme-catalysis — the biological mechanism that lowers activation energy
- atomic-hypothesis — activation energy is an emergent property of atomic interactions