In Chemistry, an amount of energy has to be supplied for a reaction to occur. This energy, known as the "activation energy", breaks up the bonds between molecules in the substance. It is equivalent to the total bond energy of the reactants.
In chemistry activation energy is not the energy required to break a bond. Instead, the activation energy is the difference in energy between a transition state and the reactants.
A product bond can begin to form before the reactant bond is completely broken.
A transition state may be stabilized by a catalyst, lowering the activation energy. This is critical to all life, as many biochemical reactions would not proceed at necessary rates without enzyme catalysis.
I learnt that the energy required to start a nuclear reaction is the difference between the binding energy of the reactants and the binding energy of the products.
As for nuclear reactions, the activation energy is not the difference between the binding energy of the reactants and the binding energy of the products. That would be the negative of the energy released by the reaction. For activation energy of a nuclear reaction, the energy difference between a transition state and the reactants should be considered.