Question

Calculate the bond energy for breaking all the bonds in a mole of methane, CH4 and the bond energy for forming all the bonds in a mole of water molecules, H2O. Answer is in kJ/mol.

Answer 1

Concepts and reason

Bond energy is the measurement of strength of a chemical bond. A chemical reaction involves breaking and making of bonds. The amount of energy required for breaking of bond and the amount of energy released for bond making is also represented as bond energy.

Fundamentals

The bond energy depends on the following factors:

When the Ionic character increases, bond energy will also increase.

The bond energy depends on the bond length. As bond length increases, bond energy decreases. If bond length decreases, bond energy increases. In multiple bond molecules, the bond length will be shorter but it has the highest bond energy which is due to the attraction of electrons.

Example:

Bond	Average bond length (pm)	Average bond energy (kJ/mol)
${\rm{C - C}}$	154	347
${\rm{C = C}}$	134	614

The average bond energies were calculated at the temperature 298K.

$\begin{array}{l}\\{\rm{Average}}\,\,{\rm{bond}}\,\,{\rm{energy}}\,\,{\rm{of}}\,\,{\rm{C - H}}\,\,{\rm{ = }}\,\,{\rm{414}}\,{\rm{kJ/mol}}\\\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{Bond energy of C}}{{\rm{H}}_{\rm{4}}}{\rm{ = }}\,\,{\rm{4}}\,{\rm{ \times }}\,{\rm{414}}\\\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{Bond energy of C}}{{\rm{H}}_{\rm{4}}}{\rm{ = }}\,{\rm{1656}}\,{\rm{kJ/mol}}\\\end{array}$

The bond energy for breaking all the bonds in methane is ${\rm{1656}}\,{\rm{kJ/mol}}$.

$\begin{array}{l}\\\,\,\,\,\,\,\,{\rm{Average}}\,\,{\rm{bond}}\,\,{\rm{energy}}\,\,{\rm{of}}\,\,{\rm{O - H}}\,\,{\rm{ = }}\,\,{\rm{464}}\,{\rm{kJ/mol}}\\\\{\rm{The}}\,{\rm{Energy}}\,{\rm{for}}\,{\rm{Formation}}\,\,{\rm{of}}\,{\rm{bond}}\,{\rm{ = }}\,\,{\rm{ - 464}}\,{\rm{kJ/mol}}\\\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{Bond energy of }}{{\rm{H}}_{\rm{2}}}{\rm{O}}\,{\rm{ = }}\,\,{\rm{2}}\,{\rm{ \times }}\,{\rm{ - 464}}\,{\rm{kJ/mol}}\\\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{Bond energy of }}{{\rm{H}}_{\rm{2}}}{\rm{O}}\,{\rm{ = }}\,\,{\rm{ - 928}}\,\,{\rm{kJ/mol}}\\\end{array}$

The bond energy for the formation of water molecule is${\rm{ - 928}}\,\,{\rm{kJ/mol}}$.

Ans:

The bond energy for breaking all the bonds in methane is${\rm{1656}}\,{\rm{kJ/mol}}$.

The bond energy for the formation of water molecule is${\rm{ - 928}}\,\,{\rm{kJ/mol}}$.