In general, the transition state of a reaction is always at a higher energy level than the reactants or products, such that ext E_{ ext A}E
A
E, start subscript, A, end subscript always has a positive value – independent of whether the reaction is endergonic or exergonic overall. The activation energy shown in the diagram below is for the forward reaction (reactants
ightarrow→right arrow products), which is exergonic. If the reaction were to proceed in the reverse direction (endergonic), the transition state would remain the same, but the activation energy would be larger. This is because the product molecules are lower-energy and would thus need more energy added to reach the transition state at the top of the reaction “hill.” (An activation energy arrow for the reverse reaction would extend from the products up to the transition state.)