Bond Enthalpy

 

Learning Objective

  • Describe the changes in enthalpy accompanying the breaking or formation of a bond

Key Points

    • Breaking a bond requires the input of energy (positive change in enthalpy); energy is released (negative change in enthalpy) when forming a bond.
    • Bond enthalpy, or dissociation energy, is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero).

Terms

  • endothermicA chemical reaction that absorbs heat energy from its surroundings.
  • enthalpyA measure of the heat energy content in a thermodynamic system.
  • exothermicA chemical reaction that releases heat energy to its surroundings.
  • homolysisThe symmetrical breaking of a chemical bond within a molecule so that each of the resulting fragments retains one of the originally bonded electrons.

Energy Transfer During Breaking or Formation of Bonds

Change in Enthalpy

Enthalpy is a measure of the total heat energy content in a thermodynamic system, and it is practically used to describe energy transfer during chemical or physical processes in which the pressure remains constant.

The total enthalpy, H, of a system cannot be measured directly. Thus, the change in enthalpy, [latex]\Delta H[/latex], is a more useful quantity than its absolute value. The change ([latex]\Delta H[/latex]) is positive in endothermic reactions because the products of the reaction have a greater enthalpy than the reactants, and heat is absorbed by the system from its surroundings. The change in enthalpy is negative in exothermic processes, because energy is released from the system into its surroundings.

Generally, a positive change in enthalpy is required to break a bond, while a negative change in enthalpy is accompanied by the formation of a bond. In other words, breaking a bond is an endothermic process, while the formation of bonds is exothermic.

Bond Enthalpy or Dissociation Energy

Bond enthalpy, also known as bond dissociation energy, is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero).

Homolysis of a chemical bondA two-electron covalent bond is equally split when bond breaking, with each resulting fragment having one electron from the original shared pair. Notice that the products are free-radicals.

For instance, the bond enthalpy, or bond-dissociation energy, for one of the C-H bonds in ethane (C2H6) is defined by the process:

[latex]CH_3CH_2-H \rightarrow CH_3CH_2 \cdot + H \cdot[/latex]

[latex]\Delta H = 101.1 \frac{kcal}{mol} (423.0 \frac{kJ}{mol})[/latex]

The strength of bonds between different atoms varies across the periodic table and is well documented.

Bond dissociation energyBond dissociation energies for different element pairings are listed. It is evident that bond strength varies significantly for different combinations of elements in the periodic table.

Each bond in a molecule has its own bond dissociation energy, so a molecule with four bonds will require more energy to break the bonds than a molecule with one bond. As each successive bond is broken, the bond dissociation energy required for the other bonds changes slightly.