The relationships between energies of reactants, the activated state, and products are in the drawing for the NO₂ + CO reaction shown opposite(fig 12).

The activated state, which the molecules must reach before either the forward or the reverse reaction is possible, is 31.6 kcal mole^-1 higher in energy (enthalpy) than the NO₂ and CO reactants, and 85.7 kcal mole-1 higher in energy than the NO and CO₂ products.

The difference between the forward and reverse E_a�s is the thermodynamic enthalpy of reaction (DH⁰= -54.1 kcal mole^-1). In the collision theory of reaction, the activation energy is simply a barrier that the colliding molecules must surmount before they can react.

The transition-state theory proposes that this activated state is an intermediate arrangement that the molecules go through, which has a real physical existence although it cannot be isolated and studied at leisure.

In the NO₂ + CO reaction, this transition state, or activated complex, would be one in which the O to be transferred has not yet been completely lost by NO, nor completely acquired by CO. If the two molecules approached end-on, the activated complex would be something like

The dots indicate attractions between atoms that are longer than normal covalent bonds, and quantum mechanical calculations suggest that they might be some 30% longer.