Q.v. What is the activation energy of a reaction?

Activation energy, $E_a$, is the minimum kinetic energy that colliding reactant molecules must together possess for a reaction to occur -- graphically, it is the height of the potential-energy barrier that must be climbed between reactants and the fleeting, weakly-bonded activated complex before the system can roll down to products. Only collisions with combined kinetic energy $\geq E_a$ can surmount this barrier and go on to form products; the fraction of molecules meeting this threshold at temperature $T$ is $f=e^{-E_a/RT}$. For an endothermic reaction, the forward activation energy exceeds the reverse activation energy ($E_f>E_r$), since $\Delta H=E_f-E_r$ is positive. A catal …