键能和键长

Bond energy is the energy required to break a specific covalent bond in one mole of gaseous molecules, expressed in kJ/mol. It depends on the type of bonded atoms and the number of shared electron pairs. Bond energy is typically reported as an average value across the same bond in different compounds, providing a useful estimate for predicting reaction behavior.

Bond strength is indirectly proportional to bond length: stronger bonds are shorter, while weaker bonds are longer. Triple bonds are stronger and shorter than double bonds between the same atoms; double bonds are stronger and shorter than single bonds. This relationship holds because atoms with multiple bonds are held more tightly together, increasing molecular stability.

Bond breaking is endothermic (requires energy input), while bond formation is exothermic (releases energy). If bonds in products are stronger than bonds in reactants, the reaction releases more energy than it consumes, making it exothermic. Conversely, if product bonds are weaker, the reaction is endothermic, absorbing net energy.

The strength of a bond increases as the number of electron pairs in the bond increases. Covalent bonding and bond multiplicity directly affect both bond strength and bond length. Triple bonds contain more electron pairs than double bonds, making them stronger and shorter, which also increases molecular stability and the energy required to dissociate them.

The enthalpy change equals the sum of energy required to break all bonds in reactants plus the energy released when all bonds form in products. According to Hess's law, the overall enthalpy change is the sum of enthalpy changes for reactants and products. This calculation provides a rough estimate when exact formation enthalpies are unavailable.

Average bond energies are used because the same bond type requires different energy to break in different molecules. For example, the first C–H bond in methane requires 439 kJ/mol, but subsequent bonds require less energy. The average value of 415 kJ/mol represents a useful estimate across multiple compounds for predicting reaction outcomes.

When one atom bonds to various atoms in a group, bond strength typically decreases moving down the group. For example, C–F bonds are stronger (439 kJ/mol) than C–Cl bonds (330 kJ/mol), which are stronger than C–Br bonds (275 kJ/mol). This trend reflects how atomic size and orbital overlap affect the stability of bonds formed.