Listed in the table are the energies of selected chemical bonds. Which of these is the strongest bond?
The energies listed for the five chemical bonds in the table are bond energies. Bond energy, also called bond enthalpy, is the average amount of energy required to break a specific bond in one mole of gaseous particles. The units of bond energy are kilojoules per mole. The amount of energy required to break just one bond in one molecule is incredibly small. Therefore, it is far more useful to report bond energy in kilojoules per mole of molecules because in chemical reactions, moles of molecules are reacting, not just one molecule.
Breaking bonds is an endothermic process, which means that energy must be absorbed from the surroundings to break bonds in molecules. Chemists use the convention of a positive sign for enthalpy changes when energy is absorbed from the surroundings. Therefore, all of the bond energy values in the table are positive.
Let’s take a look at an example. Breaking all the hydrogen-to-hydrogen single bonds in one mole of gaseous hydrogen molecules requires the absorption of 432 kilojoules of energy. The result of breaking all the bonds is two moles of hydrogen atoms. The larger a bond energy value is, the greater the amount of energy needed to be absorbed to break the bond.
Bond energy is also related to the strength of bonds. Stronger bonds require more energy to break. The largest bond energy value in the table is 942 kilojoules per mole, which is the bond energy of the nitrogen-to-nitrogen triple bond. In general, triple bonds are stronger and require more energy to break than double bonds and single bonds. While this relationship is always true when we’re comparing bonds composed of the same atoms, it is true most of the time even if we are not.
By examining the data in the table, we notice that the nitrogen-to-nitrogen triple bond is stronger than both of the double bonds, which are stronger than both of the single bonds. The nitrogen-to-nitrogen triple bond has the greatest bond energy value. Therefore, it requires the most energy to break and is the strongest bond.