Final answer:
A permanent change in a molecule is achieved by breaking covalent bonds, with the required energy known as bond energy or bond dissociation energy. Stronger bonds require more energy to break, reflecting the stability and reactivity of the molecule containing them.
Step-by-step explanation:
To achieve a permanent change in a molecule, covalent bonds must be broken. This process requires energy because breaking a bond is an endothermic process. The energy needed is referred to as bond energy or bond dissociation energy, and it varies for different types of bonds. Covalent bonds are measured by this energy, with a stronger bond requiring more energy to break. The energy liberated when atoms form a chemical bond is equal to the bond dissociation energy required to break it later.
For example, the bond energy of a single carbon-carbon bond (C-C) is about 80 kcal/mol, whereas a double carbon-carbon bond (C=C) has a bond energy of approximately 145 kcal/mol, indicating that it is stronger. Molecules with higher bond energies are more stable and less reactive. Conversely, molecules that are more reactive have bonds with lower bond energies.
In ionic compounds, interactions are described by lattice energy, but our focus here is on covalent bonds within molecules. Breaking these bonds, whether it's separating a bond between fluorine and boron or disrupting a hydrogen molecule at high temperatures, always requires energy. This is because bonded atoms are more stable than individual atoms due to their lower potential energy and tendency to achieve stable electronic configurations.