Chemical Energetics (3) Enthalpies of Combustion of Alcohols and Bond Energies

Chemical Energetics (3) The enthalpy of combustion of alcohols and bond energies.

Let’s examine in this blog the implications that follow from measuring the enthalpy of combustion of the alcohols

Here is the data we might have collected:

Name of Alcohol	Formula of alcohol	Enthalpy of combustion ΔHoc     kJ/mol
Methanol	CH3OH	–726
Ethanol	CH3CH2OH	–1367.3
Propan-1-ol	CH3CH2CH2OH	–2021.0
Butan-1-ol	CH3CH2CH2CH2OH	–2675.6
Pentan-1-ol	CH3(CH2)4OH	–3328.7
Hexan-1-ol	CH3(CH2)5OH	–3983.8

Let’s plot this data in the form of standard enthalpy of combustion vs number of carbon atoms per alcohol.

As you can see the plot is linear.

Why?

It seems that adding a —CH₂— group to each alcohol increases the enthalpy of combustion by the same amount.

This feature suggests that the additional bonds contribute a specific amount of energy to the overall combustion of an alcohol. 

In fact, we could probably predict the enthalpy of combustion of heptan-1-ol from this data.

The implication is that bonds require a specific energy value to break a mole of them. 

So what do we mean by bond energy?

Sometimes bond energy is termed bond enthalpy or bond strength or even bond dissociation enthalpy!! (I’m making a collection of these terms !!!)

All four terms though refer to the same thing. 

Bond enthalpy is not merely the energy required to break one mole of covalent bonds.

You need to define the states of the particles and the type of species involved

Here is a definition from a well-known book of data:

“E(X—Y) Bond Energy defined

a)   for X₂ molecules as the molar enthalpy change for the process X₂(g)  =  2X(g)

b)   for XY_n molecules as the molar enthalpy change for the process

1/n XY_n(g)   =   1/n X(g)   +   Y(g)

Both processes are at 298K with individual species pressures of 1 atm. 

A looser definition is here:

The bond dissociation enthalpy is the energy needed to break one mole of the bond to give separated atoms - everything being in the gas state.

Or this from Avogadro.com:

The Bond Enthalpy is the energy required to break a chemical bond. It is usually expressed in units of kJ mol^-1, measured at 298 K. The exact bond enthalpy of a particular chemical bond depends upon the molecular environment in which the bond exists.

Because bonds only exist in elements or compounds the context or chemical environment affects the bond energy.

The energy of a mole of C—H bonds in methane is slightly different then from that of a mole of the same bonds in alcohol.

So tables of bond energies usually caution you to remember that these are AVERAGE values taken over several molecular environments. 

Breaking bonds requires energy to pull the atoms apart so all bond enthalpies are endothermic values.

Now the point of bond enthalpies is to use them to calculate enthalpy changes in different reactions especially in what are called Hess Cycles.

I will blog about Hess Cycles here later.