Final answer:
Only molecules with high energy could break bonds to form new ones, as breaking bonds is an endothermic process that requires bond dissociation energy. Stronger bonds are more stable and less reactive, requiring more energy to break.
Step-by-step explanation:
The missing word in the sentence 'Only molecules with high energy could break the bonds to form new ones' refers to the fact that breaking chemical bonds is an endothermic process, which requires an input of energy. When we talk about the energy needed to break a bond, we refer to it as the bond dissociation energy, which is specific to each type of bond. For example, the covalent bond in a molecule of hydrogen (H₂) is quite strong and requires 436 kJ per mole to break. Therefore, when considering molecular stability and reactivity, molecules with stronger bonds requiring higher bond dissociation energies tend to be more stable and less reactive.
Energy is needed not only to overcome the bond strength but also to make the molecules absorb sufficient energy from the surroundings so that the bonds can be broken. It's interesting to note that at a high enough temperature, even stable molecules like H₂ will dissociate into their atoms. However, very high temperatures, much higher than what would be needed for pure thermal excitation, would be required for such dissociation.