This is the case for the reaction of diamond with oxygen for example. Similarly, the breaking of the N-N triple bond and the H-H single bond in the Haber process are spontaneous at high temperature but too slow to be economically useful. In fact all reactions which use a catalyst do so in order to overcome the activation energy (Ea).
For combustion and other very exothermic reactions only a few particles need to overcome the activation barrier in order to provoke a runaway effect, with energy released supplying the Ea for further reaction:
| Don't be deceived into thinking
activation energy is extra energy which affects enthalpy changes. Whatever
energy is lost in getting to the transition state is gained going down the
other side to the products. Activation energy has no net effect on enthalpy
change of reaction |
/ activated complex
Runaway effectThe changing height represents the relative energy of the reactants and products.
Clearly, products will have greater thermal (kinetic) energy which they will pass on through collisions to more reactants.