Bond enthalpy

The bond enthalpy (also known as bond energy) is the average change in energy required to break one mole of a bond in the gaseous state, e.g. the bond enthalpy for breaking one mole of hydrogen-hydrogen bonds is:

$H_2(g)\rightarrow 2H(g)\; \; \; \; \; \; \; \Delta H_{be}=+436\: kJmol^{-1}$

Bond enthalpies are always positive as reaction systems absorb energy to break bonds.

The reason for including the term ‘average’ in the definition of bond enthalpy is that some bonds, e.g. the carbon-hydrogen bond, are present in different types of molecules:

Methane: H — CH₃

Methanol: H — CH₂OH

Methanoic acid: H — COOH

The bond dissociation energy, which is a specific value and not an average value, for the carbon-hydrogen bond in methane, is different from that in methanol, which is in turn different from that in methanoic acid. This is because the electron density in each carbon-hydrogen bond is influenced by its neighbouring atoms, which are different for each compound. Furthermore, each of the four carbon-hydrogen bonds in methane has a different bond dissociating energy if we were to remove the hydrogen atoms successively, since the electronic environment of the molecule changes from one fragment to another. The table below shows the various bond dissociation energies for one mole of each carbon-hydrogen bond for methane:

Bond	Bond dissociation energy (kJmol^-1)
H — CH₃	435
H — CH₂	444
H — CH	444
H — C	339

Therefore, the average bond dissociation energy for one mole of the carbon-hydrogen bond across a range of compounds is called the bond enthalpy of the carbon-hydrogen bond and is ΔH_be= +413 kJmol^-1. The table below shows bond enthalpies of some common bonds:

Bond	Bond enthalpy, (kJmol^-1)
$H-H$	435
$C-H$	416
$\sigma(C-C)$	348
$C= C$	612
$C\equiv C$	837
$C=O$	743
$\sigma(O-O)$	146
$O=O$	498
$O-H$	460
$Cl-Cl$	243
$H-Br$	366
$Br-Br$	192

Even though bond enthalpy, ΔH_be, is defined as the average change in energy required to break one mole of a bond in the gaseous state, the magnitude of the change in enthalpy to break one mole of a particular bond, $\left | \Delta H_{bb} \right |$ , is the same as the magnitude of the change in enthalpy to form one mole of the same bond, $\left | \Delta H_{bf} \right |$ . Hence, the data in the bond enthalpy table can be used to represent both $\left | \Delta H_{bb} \right |$ and $\left | \Delta H_{bf} \right |$ .

next article: Using bond enthalpies to Calculate the change in enthalpy of reactions

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