# The bond "energies"

## Definition

The energy of a chemical bond (enthalpy of dissociation of a bond) is the variation of enthalpy of the reaction which dissociates one mole of such bonds..

### Example

At $25^oC$ and 1 bar $436\;kJ$ heat must be given to one mole of hydrogen gas to dissociate it to 2 moles of single hydrogen atoms :
$H_2(g)$ $\longrightarrow$ $2H(g)$
$\Delta H$ $=$ $+436\; kJ$
The energy of the $H-H$ bond (= the enthalpy of the dissociation reaction of $H_2$) = $436\;kJ$ !!

The same type of bond is often found in numerous different molecules, the $C-H$ bond for instance is present in the molecules
$CH_4$, $C_2H_6$, $C_{10}H_{22}$..
The environment in all those molecules is slightly different, so that the enthalpy of dissociation will also be slightly different.
See the → Table
of averages of the usual energies of a bond .

## Approximate forecast of reaction heats

Many bond energies have been determined.
They allow to calculate approximately most "reaction heats"
$(= -\Delta H_{reaction})$
and "transformation heats"
$(= -\Delta H_{transformation})$
if the bonds present in reagents and products are known.

Let us calculate approximately the heat of the following reaction :
$2H_2O(g)$ $+$ $2Cl_2(g)$ $\longrightarrow$ $4HCl(g)$ $+$ $O_2(g)$

- In the orange box are written the single atoms
- The transformations $1\rightarrow 3$ and $2\rightarrow 3$ are dissociation reactions of chemical bonds !
$\Delta H_{12}$
$=\Delta H_{13}$ $+$ $\Delta H_{32}$
$=\Delta H_{13}$ $-$ $\Delta H_{23}$
$=(4\cdot E_{bond}(H-O)$ $+$ $2\cdot E_{bond}(Cl-Cl))$ $-$ $(4\cdot E_{bond}(H-Cl)$ $+$ $E_{bond}(O=O))$
$=4\cdot 465$ $+$ $2\cdot 243 $ $- $ $(4\cdot 431$ $ +$ $494)$
$=128\;kJ$
Note that this result is slightly different from the (more correct) value found with standard enthalpies of formation ($114.4\; kJ)$!

## The "strenght" of chemical bonds

The bond energy is a good measure for the "strength" of bonds ( = the tear resistance), for instance in the hydrogen peroxide molecule, where the O-O is weak: