58 Bond Energy and Enthalpy
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Bond Energy
Bond energy is the measure of bond strength. In order to turn one mole of a molecule into its constituent atoms, an amount of heat equal to the bond energy needs to be put into the system.
LEARNING OBJECTIVES
Identify the relationship between bond energy and strength of chemical bonds
KEY TAKEAWAYS
Key Points
- Values listed in tables of bond energy and bond length are averages taken over a variety of compounds that contain a specific atom pair.
- A plot of the potential energy of a two-atom system and the distance between the atoms reveals a distance at which the energy is at its minimum. This distance is the bond length between the atoms.
- The higher the bond energy associated with a specific atom pair, the stronger the bond is said to be, and the smaller the distance between the two atoms.
Key Terms
- equilibrium bond length: The average distance between two atoms when they are bonded to each other.
- Morse curve: A plot showing the dependence of the energy associated with a system of two atoms on the distance between them (referred to as the ‘internuclear distance’).
- enthalpy: In thermodynamics, a measure of the heat content of a chemical or physical system, measured under conditions of constant pressure.
- bond energy: A measure of a chemical bond’s strength. It is experimentally determined by measuring the heat (or enthalpy) required to break a mole of molecules into their constituent individual atoms.
The Energy Associated with a Chemical Bond
Bond energy is a measure of a chemical bond‘s strength, meaning that it tells us how likely a pair of atoms is to remain bonded in the presence of energy perturbations. Alternatively, it can be thought of as a measure of the stability gained when two atoms bond to each other, as opposed to their free or unbound states.
Bond energy is determined by measuring the heat required to break one mole of molecules into their individual atoms, and it represents the average energy associated with breaking the individual bonds of a molecule. The higher the bond energy, the ‘stronger’ we say the bond is between the two atoms, and the distance between them (bond length) is smaller.
For instance, the HO-H bond in a water molecule requires 493 kJ/mol to break and generate the hydroxide ion ([latex]\text{OH}^-[/latex]). Breaking the O-H bond in the hydroxide ion requires an additional 424 kJ/mol. Therefore, the bond energy of the covalent O-H bonds in water is reported to be the average of the two values, or 458.9 kJ/mol. These energy values (493 and 424 kJ/mol) required to break successive O-H bonds in the water molecule are called ‘bond dissociation energies,’ and they are different from the bond energy. The bond energy is the average of the bond dissociation energies in a molecule.
The exact properties of a specific kind of bond are determined in part by the nature of the other bonds in the molecule; for example, the energy and length of the C–H bond will va